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<!DOCTYPE html> <!--[if IE 8]><html class="no-js lt-ie9" lang="en" > <![endif]--> <!--[if gt IE 8]><!--> <html class="no-js" lang="en" > <!--<![endif]--> <head> <meta charset="utf-8"> <meta name="viewport" content="width=device-width, initial-scale=1.0"> <title>Disk Images — QEMU qemu-kvm-6.2.0-53.module+el8.10.0+2055+8eb7870b.4 documentation</title> <link rel="shortcut icon" href="../_static/qemu_32x32.png"/> <link rel="stylesheet" href="../_static/css/theme.css" type="text/css" /> <link rel="stylesheet" href="../_static/pygments.css" type="text/css" /> <link rel="index" title="Index" href="../genindex.html" /> <link rel="search" title="Search" href="../search.html" /> <link rel="next" title="QEMU virtio-net standby (net_failover)" href="virtio-net-failover.html" /> <link rel="prev" title="QEMU Monitor" href="monitor.html" /> <script src="../_static/js/modernizr.min.js"></script> </head> <body class="wy-body-for-nav"> <div class="wy-grid-for-nav"> <nav data-toggle="wy-nav-shift" 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</nav> <section data-toggle="wy-nav-shift" class="wy-nav-content-wrap"> <nav class="wy-nav-top" aria-label="top navigation"> <i data-toggle="wy-nav-top" class="fa fa-bars"></i> <a href="../index.html">QEMU</a> </nav> <div class="wy-nav-content"> <div class="rst-content"> <div role="navigation" aria-label="breadcrumbs navigation"> <ul class="wy-breadcrumbs"> <li><a href="../index.html">Docs</a> »</li> <li><a href="index.html">System Emulation</a> »</li> <li>Disk Images</li> <li class="wy-breadcrumbs-aside"> <a href="https://gitlab.com/qemu-project/qemu/blob/master/docs/system/images.rst" class="fa fa-gitlab"> Edit on GitLab</a> </li> </ul> <hr/> </div> <div role="main" class="document" itemscope="itemscope" itemtype="http://schema.org/Article"> <div itemprop="articleBody"> <div class="section" id="disk-images"> <span id="id1"></span><h1>Disk Images<a class="headerlink" href="#disk-images" title="Permalink to this headline">¶</a></h1> <p>QEMU supports many disk image formats, including growable disk images (their size increase as non empty sectors are written), compressed and encrypted disk images.</p> <div class="section" id="quick-start-for-disk-image-creation"> <span id="disk-005fimages-005fquickstart"></span><h2>Quick start for disk image creation<a class="headerlink" href="#quick-start-for-disk-image-creation" title="Permalink to this headline">¶</a></h2> <p>You can create a disk image with the command:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">qemu</span><span class="o">-</span><span class="n">img</span> <span class="n">create</span> <span class="n">myimage</span><span class="o">.</span><span class="n">img</span> <span class="n">mysize</span> </pre></div> </div> <p>where myimage.img is the disk image filename and mysize is its size in kilobytes. You can add an <code class="docutils literal notranslate"><span class="pre">M</span></code> suffix to give the size in megabytes and a <code class="docutils literal notranslate"><span class="pre">G</span></code> suffix for gigabytes.</p> <p>See the <code class="docutils literal notranslate"><span class="pre">qemu-img</span></code> invocation documentation for more information.</p> </div> <div class="section" id="snapshot-mode"> <span id="disk-005fimages-005fsnapshot-005fmode"></span><h2>Snapshot mode<a class="headerlink" href="#snapshot-mode" title="Permalink to this headline">¶</a></h2> <p>If you use the option <code class="docutils literal notranslate"><span class="pre">-snapshot</span></code>, all disk images are considered as read only. When sectors in written, they are written in a temporary file created in <code class="docutils literal notranslate"><span class="pre">/tmp</span></code>. You can however force the write back to the raw disk images by using the <code class="docutils literal notranslate"><span class="pre">commit</span></code> monitor command (or C-a s in the serial console).</p> </div> <div class="section" id="vm-snapshots"> <span id="vm-005fsnapshots"></span><h2>VM snapshots<a class="headerlink" href="#vm-snapshots" title="Permalink to this headline">¶</a></h2> <p>VM snapshots are snapshots of the complete virtual machine including CPU state, RAM, device state and the content of all the writable disks. In order to use VM snapshots, you must have at least one non removable and writable block device using the <code class="docutils literal notranslate"><span class="pre">qcow2</span></code> disk image format. Normally this device is the first virtual hard drive.</p> <p>Use the monitor command <code class="docutils literal notranslate"><span class="pre">savevm</span></code> to create a new VM snapshot or replace an existing one. A human readable name can be assigned to each snapshot in addition to its numerical ID.</p> <p>Use <code class="docutils literal notranslate"><span class="pre">loadvm</span></code> to restore a VM snapshot and <code class="docutils literal notranslate"><span class="pre">delvm</span></code> to remove a VM snapshot. <code class="docutils literal notranslate"><span class="pre">info</span> <span class="pre">snapshots</span></code> lists the available snapshots with their associated information:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">(</span><span class="n">qemu</span><span class="p">)</span> <span class="n">info</span> <span class="n">snapshots</span> <span class="n">Snapshot</span> <span class="n">devices</span><span class="p">:</span> <span class="n">hda</span> <span class="n">Snapshot</span> <span class="nb">list</span> <span class="p">(</span><span class="kn">from</span> <span class="nn">hda</span><span class="p">):</span> <span class="n">ID</span> <span class="n">TAG</span> <span class="n">VM</span> <span class="n">SIZE</span> <span class="n">DATE</span> <span class="n">VM</span> <span class="n">CLOCK</span> <span class="mi">1</span> <span class="n">start</span> <span class="mi">41</span><span class="n">M</span> <span class="mi">2006</span><span class="o">-</span><span class="mi">08</span><span class="o">-</span><span class="mi">06</span> <span class="mi">12</span><span class="p">:</span><span class="mi">38</span><span class="p">:</span><span class="mi">02</span> <span class="mi">00</span><span class="p">:</span><span class="mi">00</span><span class="p">:</span><span class="mf">14.954</span> <span class="mi">2</span> <span class="mi">40</span><span class="n">M</span> <span class="mi">2006</span><span class="o">-</span><span class="mi">08</span><span class="o">-</span><span class="mi">06</span> <span class="mi">12</span><span class="p">:</span><span class="mi">43</span><span class="p">:</span><span class="mi">29</span> <span class="mi">00</span><span class="p">:</span><span class="mi">00</span><span class="p">:</span><span class="mf">18.633</span> <span class="mi">3</span> <span class="n">msys</span> <span class="mi">40</span><span class="n">M</span> <span class="mi">2006</span><span class="o">-</span><span class="mi">08</span><span class="o">-</span><span class="mi">06</span> <span class="mi">12</span><span class="p">:</span><span class="mi">44</span><span class="p">:</span><span class="mi">04</span> <span class="mi">00</span><span class="p">:</span><span class="mi">00</span><span class="p">:</span><span class="mf">23.514</span> </pre></div> </div> <p>A VM snapshot is made of a VM state info (its size is shown in <code class="docutils literal notranslate"><span class="pre">info</span> <span class="pre">snapshots</span></code>) and a snapshot of every writable disk image. The VM state info is stored in the first <code class="docutils literal notranslate"><span class="pre">qcow2</span></code> non removable and writable block device. The disk image snapshots are stored in every disk image. The size of a snapshot in a disk image is difficult to evaluate and is not shown by <code class="docutils literal notranslate"><span class="pre">info</span> <span class="pre">snapshots</span></code> because the associated disk sectors are shared among all the snapshots to save disk space (otherwise each snapshot would need a full copy of all the disk images).</p> <p>When using the (unrelated) <code class="docutils literal notranslate"><span class="pre">-snapshot</span></code> option (<a class="reference internal" href="#disk-005fimages-005fsnapshot-005fmode"><span class="std std-ref">Snapshot mode</span></a>), you can always make VM snapshots, but they are deleted as soon as you exit QEMU.</p> <p>VM snapshots currently have the following known limitations:</p> <ul class="simple"> <li>They cannot cope with removable devices if they are removed or inserted after a snapshot is done.</li> <li>A few device drivers still have incomplete snapshot support so their state is not saved or restored properly (in particular USB).</li> </ul> </div> <div class="section" id="disk-image-file-formats"> <h2>Disk image file formats<a class="headerlink" href="#disk-image-file-formats" title="Permalink to this headline">¶</a></h2> <p>QEMU supports many image file formats that can be used with VMs as well as with any of the tools (like <code class="docutils literal notranslate"><span class="pre">qemu-img</span></code>). This includes the preferred formats raw and qcow2 as well as formats that are supported for compatibility with older QEMU versions or other hypervisors.</p> <p>Depending on the image format, different options can be passed to <code class="docutils literal notranslate"><span class="pre">qemu-img</span> <span class="pre">create</span></code> and <code class="docutils literal notranslate"><span class="pre">qemu-img</span> <span class="pre">convert</span></code> using the <code class="docutils literal notranslate"><span class="pre">-o</span></code> option. This section describes each format and the options that are supported for it.</p> <dl class="option"> <dt id="cmdoption-image-formats-arg-raw"> <code class="descname">raw</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-image-formats-arg-raw" title="Permalink to this definition">¶</a></dt> <dd><p>Raw disk image format. This format has the advantage of being simple and easily exportable to all other emulators. If your file system supports <em>holes</em> (for example in ext2 or ext3 on Linux or NTFS on Windows), then only the written sectors will reserve space. Use <code class="docutils literal notranslate"><span class="pre">qemu-img</span> <span class="pre">info</span></code> to know the real size used by the image or <code class="docutils literal notranslate"><span class="pre">ls</span> <span class="pre">-ls</span></code> on Unix/Linux.</p> <p>Supported options:</p> <dl class="option"> <dt id="cmdoption-raw-arg-preallocation"> <code class="descname">preallocation</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-raw-arg-preallocation" title="Permalink to this definition">¶</a></dt> <dd><p>Preallocation mode (allowed values: <code class="docutils literal notranslate"><span class="pre">off</span></code>, <code class="docutils literal notranslate"><span class="pre">falloc</span></code>, <code class="docutils literal notranslate"><span class="pre">full</span></code>). <code class="docutils literal notranslate"><span class="pre">falloc</span></code> mode preallocates space for image by calling <code class="docutils literal notranslate"><span class="pre">posix_fallocate()</span></code>. <code class="docutils literal notranslate"><span class="pre">full</span></code> mode preallocates space for image by writing data to underlying storage. This data may or may not be zero, depending on the storage location.</p> </dd></dl> </dd></dl> <dl class="option"> <dt id="cmdoption-image-formats-arg-qcow2"> <code class="descname">qcow2</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-image-formats-arg-qcow2" title="Permalink to this definition">¶</a></dt> <dd><p>QEMU image format, the most versatile format. Use it to have smaller images (useful if your filesystem does not supports holes, for example on Windows), zlib based compression and support of multiple VM snapshots.</p> <p>Supported options:</p> <dl class="option"> <dt id="cmdoption-qcow2-arg-compat"> <code class="descname">compat</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-qcow2-arg-compat" title="Permalink to this definition">¶</a></dt> <dd><p>Determines the qcow2 version to use. <code class="docutils literal notranslate"><span class="pre">compat=0.10</span></code> uses the traditional image format that can be read by any QEMU since 0.10. <code class="docutils literal notranslate"><span class="pre">compat=1.1</span></code> enables image format extensions that only QEMU 1.1 and newer understand (this is the default). Amongst others, this includes zero clusters, which allow efficient copy-on-read for sparse images.</p> </dd></dl> <dl class="option"> <dt id="cmdoption-qcow2-arg-backing-file"> <code class="descname">backing_file</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-qcow2-arg-backing-file" title="Permalink to this definition">¶</a></dt> <dd><p>File name of a base image (see <code class="docutils literal notranslate"><span class="pre">create</span></code> subcommand)</p> </dd></dl> <dl class="option"> <dt id="cmdoption-qcow2-arg-backing-fmt"> <code class="descname">backing_fmt</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-qcow2-arg-backing-fmt" title="Permalink to this definition">¶</a></dt> <dd><p>Image format of the base image</p> </dd></dl> <dl class="option"> <dt id="cmdoption-qcow2-arg-encryption"> <code class="descname">encryption</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-qcow2-arg-encryption" title="Permalink to this definition">¶</a></dt> <dd><p>This option is deprecated and equivalent to <code class="docutils literal notranslate"><span class="pre">encrypt.format=aes</span></code></p> </dd></dl> <dl class="option"> <dt id="cmdoption-qcow2-arg-encrypt-format"> <code class="descname">encrypt.format</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-qcow2-arg-encrypt-format" title="Permalink to this definition">¶</a></dt> <dd><p>If this is set to <code class="docutils literal notranslate"><span class="pre">luks</span></code>, it requests that the qcow2 payload (not qcow2 header) be encrypted using the LUKS format. The passphrase to use to unlock the LUKS key slot is given by the <code class="docutils literal notranslate"><span class="pre">encrypt.key-secret</span></code> parameter. LUKS encryption parameters can be tuned with the other <code class="docutils literal notranslate"><span class="pre">encrypt.*</span></code> parameters.</p> <p>If this is set to <code class="docutils literal notranslate"><span class="pre">aes</span></code>, the image is encrypted with 128-bit AES-CBC. The encryption key is given by the <code class="docutils literal notranslate"><span class="pre">encrypt.key-secret</span></code> parameter. This encryption format is considered to be flawed by modern cryptography standards, suffering from a number of design problems:</p> <ul class="simple"> <li>The AES-CBC cipher is used with predictable initialization vectors based on the sector number. This makes it vulnerable to chosen plaintext attacks which can reveal the existence of encrypted data.</li> <li>The user passphrase is directly used as the encryption key. A poorly chosen or short passphrase will compromise the security of the encryption.</li> <li>In the event of the passphrase being compromised there is no way to change the passphrase to protect data in any qcow images. The files must be cloned, using a different encryption passphrase in the new file. The original file must then be securely erased using a program like shred, though even this is ineffective with many modern storage technologies.</li> </ul> <p>The use of this is no longer supported in system emulators. Support only remains in the command line utilities, for the purposes of data liberation and interoperability with old versions of QEMU. The <code class="docutils literal notranslate"><span class="pre">luks</span></code> format should be used instead.</p> </dd></dl> <dl class="option"> <dt id="cmdoption-qcow2-arg-encrypt-key-secret"> <code class="descname">encrypt.key-secret</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-qcow2-arg-encrypt-key-secret" title="Permalink to this definition">¶</a></dt> <dd><p>Provides the ID of a <code class="docutils literal notranslate"><span class="pre">secret</span></code> object that contains the passphrase (<code class="docutils literal notranslate"><span class="pre">encrypt.format=luks</span></code>) or encryption key (<code class="docutils literal notranslate"><span class="pre">encrypt.format=aes</span></code>).</p> </dd></dl> <dl class="option"> <dt id="cmdoption-qcow2-arg-encrypt-cipher-alg"> <code class="descname">encrypt.cipher-alg</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-qcow2-arg-encrypt-cipher-alg" title="Permalink to this definition">¶</a></dt> <dd><p>Name of the cipher algorithm and key length. Currently defaults to <code class="docutils literal notranslate"><span class="pre">aes-256</span></code>. Only used when <code class="docutils literal notranslate"><span class="pre">encrypt.format=luks</span></code>.</p> </dd></dl> <dl class="option"> <dt id="cmdoption-qcow2-arg-encrypt-cipher-mode"> <code class="descname">encrypt.cipher-mode</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-qcow2-arg-encrypt-cipher-mode" title="Permalink to this definition">¶</a></dt> <dd><p>Name of the encryption mode to use. Currently defaults to <code class="docutils literal notranslate"><span class="pre">xts</span></code>. Only used when <code class="docutils literal notranslate"><span class="pre">encrypt.format=luks</span></code>.</p> </dd></dl> <dl class="option"> <dt id="cmdoption-qcow2-arg-encrypt-ivgen-alg"> <code class="descname">encrypt.ivgen-alg</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-qcow2-arg-encrypt-ivgen-alg" title="Permalink to this definition">¶</a></dt> <dd><p>Name of the initialization vector generator algorithm. Currently defaults to <code class="docutils literal notranslate"><span class="pre">plain64</span></code>. Only used when <code class="docutils literal notranslate"><span class="pre">encrypt.format=luks</span></code>.</p> </dd></dl> <dl class="option"> <dt id="cmdoption-qcow2-arg-encrypt-ivgen-hash-alg"> <code class="descname">encrypt.ivgen-hash-alg</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-qcow2-arg-encrypt-ivgen-hash-alg" title="Permalink to this definition">¶</a></dt> <dd><p>Name of the hash algorithm to use with the initialization vector generator (if required). Defaults to <code class="docutils literal notranslate"><span class="pre">sha256</span></code>. Only used when <code class="docutils literal notranslate"><span class="pre">encrypt.format=luks</span></code>.</p> </dd></dl> <dl class="option"> <dt id="cmdoption-qcow2-arg-encrypt-hash-alg"> <code class="descname">encrypt.hash-alg</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-qcow2-arg-encrypt-hash-alg" title="Permalink to this definition">¶</a></dt> <dd><p>Name of the hash algorithm to use for PBKDF algorithm Defaults to <code class="docutils literal notranslate"><span class="pre">sha256</span></code>. Only used when <code class="docutils literal notranslate"><span class="pre">encrypt.format=luks</span></code>.</p> </dd></dl> <dl class="option"> <dt id="cmdoption-qcow2-arg-encrypt-iter-time"> <code class="descname">encrypt.iter-time</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-qcow2-arg-encrypt-iter-time" title="Permalink to this definition">¶</a></dt> <dd><p>Amount of time, in milliseconds, to use for PBKDF algorithm per key slot. Defaults to <code class="docutils literal notranslate"><span class="pre">2000</span></code>. Only used when <code class="docutils literal notranslate"><span class="pre">encrypt.format=luks</span></code>.</p> </dd></dl> <dl class="option"> <dt id="cmdoption-qcow2-arg-cluster-size"> <code class="descname">cluster_size</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-qcow2-arg-cluster-size" title="Permalink to this definition">¶</a></dt> <dd><p>Changes the qcow2 cluster size (must be between 512 and 2M). Smaller cluster sizes can improve the image file size whereas larger cluster sizes generally provide better performance.</p> </dd></dl> <dl class="option"> <dt id="cmdoption-qcow2-arg-preallocation"> <code class="descname">preallocation</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-qcow2-arg-preallocation" title="Permalink to this definition">¶</a></dt> <dd><p>Preallocation mode (allowed values: <code class="docutils literal notranslate"><span class="pre">off</span></code>, <code class="docutils literal notranslate"><span class="pre">metadata</span></code>, <code class="docutils literal notranslate"><span class="pre">falloc</span></code>, <code class="docutils literal notranslate"><span class="pre">full</span></code>). An image with preallocated metadata is initially larger but can improve performance when the image needs to grow. <code class="docutils literal notranslate"><span class="pre">falloc</span></code> and <code class="docutils literal notranslate"><span class="pre">full</span></code> preallocations are like the same options of <code class="docutils literal notranslate"><span class="pre">raw</span></code> format, but sets up metadata also.</p> </dd></dl> <dl class="option"> <dt id="cmdoption-qcow2-arg-lazy-refcounts"> <code class="descname">lazy_refcounts</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-qcow2-arg-lazy-refcounts" title="Permalink to this definition">¶</a></dt> <dd><p>If this option is set to <code class="docutils literal notranslate"><span class="pre">on</span></code>, reference count updates are postponed with the goal of avoiding metadata I/O and improving performance. This is particularly interesting with <code class="xref std std-option docutils literal notranslate"><span class="pre">cache=writethrough</span></code> which doesn’t batch metadata updates. The tradeoff is that after a host crash, the reference count tables must be rebuilt, i.e. on the next open an (automatic) <code class="docutils literal notranslate"><span class="pre">qemu-img</span> <span class="pre">check</span> <span class="pre">-r</span> <span class="pre">all</span></code> is required, which may take some time.</p> <p>This option can only be enabled if <code class="docutils literal notranslate"><span class="pre">compat=1.1</span></code> is specified.</p> </dd></dl> <dl class="option"> <dt id="cmdoption-qcow2-arg-nocow"> <code class="descname">nocow</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-qcow2-arg-nocow" title="Permalink to this definition">¶</a></dt> <dd><p>If this option is set to <code class="docutils literal notranslate"><span class="pre">on</span></code>, it will turn off COW of the file. It’s only valid on btrfs, no effect on other file systems.</p> <p>Btrfs has low performance when hosting a VM image file, even more when the guest on the VM also using btrfs as file system. Turning off COW is a way to mitigate this bad performance. Generally there are two ways to turn off COW on btrfs:</p> <ul class="simple"> <li>Disable it by mounting with nodatacow, then all newly created files will be NOCOW.</li> <li>For an empty file, add the NOCOW file attribute. That’s what this option does.</li> </ul> <p>Note: this option is only valid to new or empty files. If there is an existing file which is COW and has data blocks already, it couldn’t be changed to NOCOW by setting <code class="docutils literal notranslate"><span class="pre">nocow=on</span></code>. One can issue <code class="docutils literal notranslate"><span class="pre">lsattr</span> <span class="pre">filename</span></code> to check if the NOCOW flag is set or not (Capital ‘C’ is NOCOW flag).</p> </dd></dl> </dd></dl> <dl class="option"> <dt id="cmdoption-image-formats-arg-qed"> <code class="descname">qed</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-image-formats-arg-qed" title="Permalink to this definition">¶</a></dt> <dd><p>Old QEMU image format with support for backing files and compact image files (when your filesystem or transport medium does not support holes).</p> <p>When converting QED images to qcow2, you might want to consider using the <code class="docutils literal notranslate"><span class="pre">lazy_refcounts=on</span></code> option to get a more QED-like behaviour.</p> <p>Supported options:</p> <dl class="option"> <dt id="cmdoption-qed-arg-backing-file"> <code class="descname">backing_file</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-qed-arg-backing-file" title="Permalink to this definition">¶</a></dt> <dd><p>File name of a base image (see <code class="docutils literal notranslate"><span class="pre">create</span></code> subcommand).</p> </dd></dl> <dl class="option"> <dt id="cmdoption-qed-arg-backing-fmt"> <code class="descname">backing_fmt</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-qed-arg-backing-fmt" title="Permalink to this definition">¶</a></dt> <dd><p>Image file format of backing file (optional). Useful if the format cannot be autodetected because it has no header, like some vhd/vpc files.</p> </dd></dl> <dl class="option"> <dt id="cmdoption-qed-arg-cluster-size"> <code class="descname">cluster_size</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-qed-arg-cluster-size" title="Permalink to this definition">¶</a></dt> <dd><p>Changes the cluster size (must be power-of-2 between 4K and 64K). Smaller cluster sizes can improve the image file size whereas larger cluster sizes generally provide better performance.</p> </dd></dl> <dl class="option"> <dt id="cmdoption-qed-arg-table-size"> <code class="descname">table_size</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-qed-arg-table-size" title="Permalink to this definition">¶</a></dt> <dd><p>Changes the number of clusters per L1/L2 table (must be power-of-2 between 1 and 16). There is normally no need to change this value but this option can between used for performance benchmarking.</p> </dd></dl> </dd></dl> <dl class="option"> <dt id="cmdoption-image-formats-arg-qcow"> <code class="descname">qcow</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-image-formats-arg-qcow" title="Permalink to this definition">¶</a></dt> <dd><p>Old QEMU image format with support for backing files, compact image files, encryption and compression.</p> <p>Supported options:</p> <blockquote> <div><dl class="option"> <dt id="cmdoption-qcow-arg-backing-file"> <code class="descname">backing_file</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-qcow-arg-backing-file" title="Permalink to this definition">¶</a></dt> <dd><p>File name of a base image (see <code class="docutils literal notranslate"><span class="pre">create</span></code> subcommand)</p> </dd></dl> <dl class="option"> <dt id="cmdoption-qcow-arg-encryption"> <code class="descname">encryption</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-qcow-arg-encryption" title="Permalink to this definition">¶</a></dt> <dd><p>This option is deprecated and equivalent to <code class="docutils literal notranslate"><span class="pre">encrypt.format=aes</span></code></p> </dd></dl> <dl class="option"> <dt id="cmdoption-qcow-arg-encrypt-format"> <code class="descname">encrypt.format</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-qcow-arg-encrypt-format" title="Permalink to this definition">¶</a></dt> <dd><p>If this is set to <code class="docutils literal notranslate"><span class="pre">aes</span></code>, the image is encrypted with 128-bit AES-CBC. The encryption key is given by the <code class="docutils literal notranslate"><span class="pre">encrypt.key-secret</span></code> parameter. This encryption format is considered to be flawed by modern cryptography standards, suffering from a number of design problems enumerated previously against the <code class="docutils literal notranslate"><span class="pre">qcow2</span></code> image format.</p> <p>The use of this is no longer supported in system emulators. Support only remains in the command line utilities, for the purposes of data liberation and interoperability with old versions of QEMU.</p> <p>Users requiring native encryption should use the <code class="docutils literal notranslate"><span class="pre">qcow2</span></code> format instead with <code class="docutils literal notranslate"><span class="pre">encrypt.format=luks</span></code>.</p> </dd></dl> <dl class="option"> <dt id="cmdoption-qcow-arg-encrypt-key-secret"> <code class="descname">encrypt.key-secret</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-qcow-arg-encrypt-key-secret" title="Permalink to this definition">¶</a></dt> <dd><p>Provides the ID of a <code class="docutils literal notranslate"><span class="pre">secret</span></code> object that contains the encryption key (<code class="docutils literal notranslate"><span class="pre">encrypt.format=aes</span></code>).</p> </dd></dl> </div></blockquote> </dd></dl> <dl class="option"> <dt id="cmdoption-image-formats-arg-luks"> <code class="descname">luks</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-image-formats-arg-luks" title="Permalink to this definition">¶</a></dt> <dd><p>LUKS v1 encryption format, compatible with Linux dm-crypt/cryptsetup</p> <p>Supported options:</p> <dl class="option"> <dt id="cmdoption-luks-arg-key-secret"> <code class="descname">key-secret</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-luks-arg-key-secret" title="Permalink to this definition">¶</a></dt> <dd><p>Provides the ID of a <code class="docutils literal notranslate"><span class="pre">secret</span></code> object that contains the passphrase.</p> </dd></dl> <dl class="option"> <dt id="cmdoption-luks-arg-cipher-alg"> <code class="descname">cipher-alg</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-luks-arg-cipher-alg" title="Permalink to this definition">¶</a></dt> <dd><p>Name of the cipher algorithm and key length. Currently defaults to <code class="docutils literal notranslate"><span class="pre">aes-256</span></code>.</p> </dd></dl> <dl class="option"> <dt id="cmdoption-luks-arg-cipher-mode"> <code class="descname">cipher-mode</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-luks-arg-cipher-mode" title="Permalink to this definition">¶</a></dt> <dd><p>Name of the encryption mode to use. Currently defaults to <code class="docutils literal notranslate"><span class="pre">xts</span></code>.</p> </dd></dl> <dl class="option"> <dt id="cmdoption-luks-arg-ivgen-alg"> <code class="descname">ivgen-alg</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-luks-arg-ivgen-alg" title="Permalink to this definition">¶</a></dt> <dd><p>Name of the initialization vector generator algorithm. Currently defaults to <code class="docutils literal notranslate"><span class="pre">plain64</span></code>.</p> </dd></dl> <dl class="option"> <dt id="cmdoption-luks-arg-ivgen-hash-alg"> <code class="descname">ivgen-hash-alg</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-luks-arg-ivgen-hash-alg" title="Permalink to this definition">¶</a></dt> <dd><p>Name of the hash algorithm to use with the initialization vector generator (if required). Defaults to <code class="docutils literal notranslate"><span class="pre">sha256</span></code>.</p> </dd></dl> <dl class="option"> <dt id="cmdoption-luks-arg-hash-alg"> <code class="descname">hash-alg</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-luks-arg-hash-alg" title="Permalink to this definition">¶</a></dt> <dd><p>Name of the hash algorithm to use for PBKDF algorithm Defaults to <code class="docutils literal notranslate"><span class="pre">sha256</span></code>.</p> </dd></dl> <dl class="option"> <dt id="cmdoption-luks-arg-iter-time"> <code class="descname">iter-time</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-luks-arg-iter-time" title="Permalink to this definition">¶</a></dt> <dd><p>Amount of time, in milliseconds, to use for PBKDF algorithm per key slot. Defaults to <code class="docutils literal notranslate"><span class="pre">2000</span></code>.</p> </dd></dl> </dd></dl> <dl class="option"> <dt id="cmdoption-image-formats-arg-vdi"> <code class="descname">vdi</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-image-formats-arg-vdi" title="Permalink to this definition">¶</a></dt> <dd><p>VirtualBox 1.1 compatible image format.</p> <p>Supported options:</p> <dl class="option"> <dt id="cmdoption-vdi-arg-static"> <code class="descname">static</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-vdi-arg-static" title="Permalink to this definition">¶</a></dt> <dd><p>If this option is set to <code class="docutils literal notranslate"><span class="pre">on</span></code>, the image is created with metadata preallocation.</p> </dd></dl> </dd></dl> <dl class="option"> <dt id="cmdoption-image-formats-arg-vmdk"> <code class="descname">vmdk</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-image-formats-arg-vmdk" title="Permalink to this definition">¶</a></dt> <dd><p>VMware 3 and 4 compatible image format.</p> <p>Supported options:</p> <dl class="option"> <dt id="cmdoption-image-formats-arg-backing-file"> <code class="descname">backing_file</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-image-formats-arg-backing-file" title="Permalink to this definition">¶</a></dt> <dd><p>File name of a base image (see <code class="docutils literal notranslate"><span class="pre">create</span></code> subcommand).</p> </dd></dl> <dl class="option"> <dt id="cmdoption-image-formats-arg-compat6"> <code class="descname">compat6</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-image-formats-arg-compat6" title="Permalink to this definition">¶</a></dt> <dd><p>Create a VMDK version 6 image (instead of version 4)</p> </dd></dl> <dl class="option"> <dt id="cmdoption-image-formats-arg-hwversion"> <code class="descname">hwversion</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-image-formats-arg-hwversion" title="Permalink to this definition">¶</a></dt> <dd><p>Specify vmdk virtual hardware version. Compat6 flag cannot be enabled if hwversion is specified.</p> </dd></dl> <dl class="option"> <dt id="cmdoption-image-formats-arg-subformat"> <code class="descname">subformat</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-image-formats-arg-subformat" title="Permalink to this definition">¶</a></dt> <dd><p>Specifies which VMDK subformat to use. Valid options are <code class="docutils literal notranslate"><span class="pre">monolithicSparse</span></code> (default), <code class="docutils literal notranslate"><span class="pre">monolithicFlat</span></code>, <code class="docutils literal notranslate"><span class="pre">twoGbMaxExtentSparse</span></code>, <code class="docutils literal notranslate"><span class="pre">twoGbMaxExtentFlat</span></code> and <code class="docutils literal notranslate"><span class="pre">streamOptimized</span></code>.</p> </dd></dl> </dd></dl> <dl class="option"> <dt id="cmdoption-image-formats-arg-vpc"> <code class="descname">vpc</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-image-formats-arg-vpc" title="Permalink to this definition">¶</a></dt> <dd><p>VirtualPC compatible image format (VHD).</p> <p>Supported options:</p> <dl class="option"> <dt id="cmdoption-vpc-arg-subformat"> <code class="descname">subformat</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-vpc-arg-subformat" title="Permalink to this definition">¶</a></dt> <dd><p>Specifies which VHD subformat to use. Valid options are <code class="docutils literal notranslate"><span class="pre">dynamic</span></code> (default) and <code class="docutils literal notranslate"><span class="pre">fixed</span></code>.</p> </dd></dl> </dd></dl> <dl class="option"> <dt id="cmdoption-image-formats-arg-vhdx"> <code class="descname">VHDX</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-image-formats-arg-vhdx" title="Permalink to this definition">¶</a></dt> <dd><p>Hyper-V compatible image format (VHDX).</p> <p>Supported options:</p> <dl class="option"> <dt id="cmdoption-vhdx-arg-subformat"> <code class="descname">subformat</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-vhdx-arg-subformat" title="Permalink to this definition">¶</a></dt> <dd><p>Specifies which VHDX subformat to use. Valid options are <code class="docutils literal notranslate"><span class="pre">dynamic</span></code> (default) and <code class="docutils literal notranslate"><span class="pre">fixed</span></code>.</p> <dl class="option"> <dt id="cmdoption-vhdx-arg-block-state-zero"> <code class="descname">block_state_zero</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-vhdx-arg-block-state-zero" title="Permalink to this definition">¶</a></dt> <dd><p>Force use of payload blocks of type ‘ZERO’. Can be set to <code class="docutils literal notranslate"><span class="pre">on</span></code> (default) or <code class="docutils literal notranslate"><span class="pre">off</span></code>. When set to <code class="docutils literal notranslate"><span class="pre">off</span></code>, new blocks will be created as <code class="docutils literal notranslate"><span class="pre">PAYLOAD_BLOCK_NOT_PRESENT</span></code>, which means parsers are free to return arbitrary data for those blocks. Do not set to <code class="docutils literal notranslate"><span class="pre">off</span></code> when using <code class="docutils literal notranslate"><span class="pre">qemu-img</span> <span class="pre">convert</span></code> with <code class="docutils literal notranslate"><span class="pre">subformat=dynamic</span></code>.</p> </dd></dl> <dl class="option"> <dt id="cmdoption-vhdx-arg-block-size"> <code class="descname">block_size</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-vhdx-arg-block-size" title="Permalink to this definition">¶</a></dt> <dd><p>Block size; min 1 MB, max 256 MB. 0 means auto-calculate based on image size.</p> </dd></dl> <dl class="option"> <dt id="cmdoption-vhdx-arg-log-size"> <code class="descname">log_size</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-vhdx-arg-log-size" title="Permalink to this definition">¶</a></dt> <dd><p>Log size; min 1 MB.</p> </dd></dl> </dd></dl> </dd></dl> </div> <div class="section" id="read-only-formats"> <h2>Read-only formats<a class="headerlink" href="#read-only-formats" title="Permalink to this headline">¶</a></h2> <p>More disk image file formats are supported in a read-only mode.</p> <dl class="option"> <dt id="cmdoption-image-formats-arg-bochs"> <code class="descname">bochs</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-image-formats-arg-bochs" title="Permalink to this definition">¶</a></dt> <dd><p>Bochs images of <code class="docutils literal notranslate"><span class="pre">growing</span></code> type.</p> </dd></dl> <dl class="option"> <dt id="cmdoption-image-formats-arg-cloop"> <code class="descname">cloop</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-image-formats-arg-cloop" title="Permalink to this definition">¶</a></dt> <dd><p>Linux Compressed Loop image, useful only to reuse directly compressed CD-ROM images present for example in the Knoppix CD-ROMs.</p> </dd></dl> <dl class="option"> <dt id="cmdoption-image-formats-arg-dmg"> <code class="descname">dmg</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-image-formats-arg-dmg" title="Permalink to this definition">¶</a></dt> <dd><p>Apple disk image.</p> </dd></dl> <dl class="option"> <dt id="cmdoption-image-formats-arg-parallels"> <code class="descname">parallels</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-image-formats-arg-parallels" title="Permalink to this definition">¶</a></dt> <dd><p>Parallels disk image format.</p> </dd></dl> </div> <div class="section" id="using-host-drives"> <h2>Using host drives<a class="headerlink" href="#using-host-drives" title="Permalink to this headline">¶</a></h2> <p>In addition to disk image files, QEMU can directly access host devices. We describe here the usage for QEMU version >= 0.8.3.</p> <div class="section" id="linux"> <h3>Linux<a class="headerlink" href="#linux" title="Permalink to this headline">¶</a></h3> <p>On Linux, you can directly use the host device filename instead of a disk image filename provided you have enough privileges to access it. For example, use <code class="docutils literal notranslate"><span class="pre">/dev/cdrom</span></code> to access to the CDROM.</p> <dl class="docutils"> <dt>CD</dt> <dd>You can specify a CDROM device even if no CDROM is loaded. QEMU has specific code to detect CDROM insertion or removal. CDROM ejection by the guest OS is supported. Currently only data CDs are supported.</dd> <dt>Floppy</dt> <dd>You can specify a floppy device even if no floppy is loaded. Floppy removal is currently not detected accurately (if you change floppy without doing floppy access while the floppy is not loaded, the guest OS will think that the same floppy is loaded). Use of the host’s floppy device is deprecated, and support for it will be removed in a future release.</dd> <dt>Hard disks</dt> <dd>Hard disks can be used. Normally you must specify the whole disk (<code class="docutils literal notranslate"><span class="pre">/dev/hdb</span></code> instead of <code class="docutils literal notranslate"><span class="pre">/dev/hdb1</span></code>) so that the guest OS can see it as a partitioned disk. WARNING: unless you know what you do, it is better to only make READ-ONLY accesses to the hard disk otherwise you may corrupt your host data (use the <code class="docutils literal notranslate"><span class="pre">-snapshot</span></code> command line option or modify the device permissions accordingly).</dd> </dl> </div> <div class="section" id="windows"> <h3>Windows<a class="headerlink" href="#windows" title="Permalink to this headline">¶</a></h3> <dl class="docutils"> <dt>CD</dt> <dd><p class="first">The preferred syntax is the drive letter (e.g. <code class="docutils literal notranslate"><span class="pre">d:</span></code>). The alternate syntax <code class="docutils literal notranslate"><span class="pre">\\.\d:</span></code> is supported. <code class="docutils literal notranslate"><span class="pre">/dev/cdrom</span></code> is supported as an alias to the first CDROM drive.</p> <p class="last">Currently there is no specific code to handle removable media, so it is better to use the <code class="docutils literal notranslate"><span class="pre">change</span></code> or <code class="docutils literal notranslate"><span class="pre">eject</span></code> monitor commands to change or eject media.</p> </dd> <dt>Hard disks</dt> <dd><p class="first">Hard disks can be used with the syntax: <code class="docutils literal notranslate"><span class="pre">\\.\PhysicalDriveN</span></code> where <em>N</em> is the drive number (0 is the first hard disk).</p> <p class="last">WARNING: unless you know what you do, it is better to only make READ-ONLY accesses to the hard disk otherwise you may corrupt your host data (use the <code class="docutils literal notranslate"><span class="pre">-snapshot</span></code> command line so that the modifications are written in a temporary file).</p> </dd> </dl> </div> <div class="section" id="mac-os-x"> <h3>Mac OS X<a class="headerlink" href="#mac-os-x" title="Permalink to this headline">¶</a></h3> <p><code class="docutils literal notranslate"><span class="pre">/dev/cdrom</span></code> is an alias to the first CDROM.</p> <p>Currently there is no specific code to handle removable media, so it is better to use the <code class="docutils literal notranslate"><span class="pre">change</span></code> or <code class="docutils literal notranslate"><span class="pre">eject</span></code> monitor commands to change or eject media.</p> </div> </div> <div class="section" id="virtual-fat-disk-images"> <h2>Virtual FAT disk images<a class="headerlink" href="#virtual-fat-disk-images" title="Permalink to this headline">¶</a></h2> <p>QEMU can automatically create a virtual FAT disk image from a directory tree. In order to use it, just type:</p> <pre class="literal-block"> qemu-kvm linux.img -hdb fat:/my_directory </pre> <p>Then you access access to all the files in the <code class="docutils literal notranslate"><span class="pre">/my_directory</span></code> directory without having to copy them in a disk image or to export them via SAMBA or NFS. The default access is <em>read-only</em>.</p> <p>Floppies can be emulated with the <code class="docutils literal notranslate"><span class="pre">:floppy:</span></code> option:</p> <pre class="literal-block"> qemu-kvm linux.img -fda fat:floppy:/my_directory </pre> <p>A read/write support is available for testing (beta stage) with the <code class="docutils literal notranslate"><span class="pre">:rw:</span></code> option:</p> <pre class="literal-block"> qemu-kvm linux.img -fda fat:floppy:rw:/my_directory </pre> <p>What you should <em>never</em> do:</p> <ul class="simple"> <li>use non-ASCII filenames</li> <li>use “-snapshot” together with “:rw:”</li> <li>expect it to work when loadvm’ing</li> <li>write to the FAT directory on the host system while accessing it with the guest system</li> </ul> </div> <div class="section" id="nbd-access"> <h2>NBD access<a class="headerlink" href="#nbd-access" title="Permalink to this headline">¶</a></h2> <p>QEMU can access directly to block device exported using the Network Block Device protocol.</p> <pre class="literal-block"> qemu-kvm linux.img -hdb nbd://my_nbd_server.mydomain.org:1024/ </pre> <p>If the NBD server is located on the same host, you can use an unix socket instead of an inet socket:</p> <pre class="literal-block"> qemu-kvm linux.img -hdb nbd+unix://?socket=/tmp/my_socket </pre> <p>In this case, the block device must be exported using <code class="docutils literal notranslate"><span class="pre">qemu-nbd</span></code>:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">qemu</span><span class="o">-</span><span class="n">nbd</span> <span class="o">--</span><span class="n">socket</span><span class="o">=/</span><span class="n">tmp</span><span class="o">/</span><span class="n">my_socket</span> <span class="n">my_disk</span><span class="o">.</span><span class="n">qcow2</span> </pre></div> </div> <p>The use of <code class="docutils literal notranslate"><span class="pre">qemu-nbd</span></code> allows sharing of a disk between several guests:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">qemu</span><span class="o">-</span><span class="n">nbd</span> <span class="o">--</span><span class="n">socket</span><span class="o">=/</span><span class="n">tmp</span><span class="o">/</span><span class="n">my_socket</span> <span class="o">--</span><span class="n">share</span><span class="o">=</span><span class="mi">2</span> <span class="n">my_disk</span><span class="o">.</span><span class="n">qcow2</span> </pre></div> </div> <p>and then you can use it with two guests:</p> <pre class="literal-block"> qemu-kvm linux1.img -hdb nbd+unix://?socket=/tmp/my_socket qemu-kvm linux2.img -hdb nbd+unix://?socket=/tmp/my_socket </pre> <p>If the <code class="docutils literal notranslate"><span class="pre">nbd-server</span></code> uses named exports (supported since NBD 2.9.18, or with QEMU’s own embedded NBD server), you must specify an export name in the URI:</p> <pre class="literal-block"> qemu-kvm -cdrom nbd://localhost/debian-500-ppc-netinst qemu-kvm -cdrom nbd://localhost/openSUSE-11.1-ppc-netinst </pre> <p>The URI syntax for NBD is supported since QEMU 1.3. An alternative syntax is also available. Here are some example of the older syntax:</p> <pre class="literal-block"> qemu-kvm linux.img -hdb nbd:my_nbd_server.mydomain.org:1024 qemu-kvm linux2.img -hdb nbd:unix:/tmp/my_socket qemu-kvm -cdrom nbd:localhost:10809:exportname=debian-500-ppc-netinst </pre> </div> <div class="section" id="iscsi-luns"> <h2>iSCSI LUNs<a class="headerlink" href="#iscsi-luns" title="Permalink to this headline">¶</a></h2> <p>iSCSI is a popular protocol used to access SCSI devices across a computer network.</p> <p>There are two different ways iSCSI devices can be used by QEMU.</p> <p>The first method is to mount the iSCSI LUN on the host, and make it appear as any other ordinary SCSI device on the host and then to access this device as a /dev/sd device from QEMU. How to do this differs between host OSes.</p> <p>The second method involves using the iSCSI initiator that is built into QEMU. This provides a mechanism that works the same way regardless of which host OS you are running QEMU on. This section will describe this second method of using iSCSI together with QEMU.</p> <p>In QEMU, iSCSI devices are described using special iSCSI URLs. URL syntax:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">iscsi</span><span class="p">:</span><span class="o">//</span><span class="p">[</span><span class="o"><</span><span class="n">username</span><span class="o">></span><span class="p">[</span><span class="o">%<</span><span class="n">password</span><span class="o">></span><span class="p">]</span><span class="o">@</span><span class="p">]</span><span class="o"><</span><span class="n">host</span><span class="o">></span><span class="p">[:</span><span class="o"><</span><span class="n">port</span><span class="o">></span><span class="p">]</span><span class="o">/<</span><span class="n">target</span><span class="o">-</span><span class="n">iqn</span><span class="o">-</span><span class="n">name</span><span class="o">>/<</span><span class="n">lun</span><span class="o">></span> </pre></div> </div> <p>Username and password are optional and only used if your target is set up using CHAP authentication for access control. Alternatively the username and password can also be set via environment variables to have these not show up in the process list:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">export</span> <span class="n">LIBISCSI_CHAP_USERNAME</span><span class="o">=<</span><span class="n">username</span><span class="o">></span> <span class="n">export</span> <span class="n">LIBISCSI_CHAP_PASSWORD</span><span class="o">=<</span><span class="n">password</span><span class="o">></span> <span class="n">iscsi</span><span class="p">:</span><span class="o">//<</span><span class="n">host</span><span class="o">>/<</span><span class="n">target</span><span class="o">-</span><span class="n">iqn</span><span class="o">-</span><span class="n">name</span><span class="o">>/<</span><span class="n">lun</span><span class="o">></span> </pre></div> </div> <p>Various session related parameters can be set via special options, either in a configuration file provided via ‘-readconfig’ or directly on the command line.</p> <p>If the initiator-name is not specified qemu will use a default name of ‘iqn.2008-11.org.linux-kvm[:<uuid>’] where <uuid> is the UUID of the virtual machine. If the UUID is not specified qemu will use ‘iqn.2008-11.org.linux-kvm[:<name>’] where <name> is the name of the virtual machine.</p> <p>Setting a specific initiator name to use when logging in to the target:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o">-</span><span class="n">iscsi</span> <span class="n">initiator</span><span class="o">-</span><span class="n">name</span><span class="o">=</span><span class="n">iqn</span><span class="o">.</span><span class="n">qemu</span><span class="o">.</span><span class="n">test</span><span class="p">:</span><span class="n">my</span><span class="o">-</span><span class="n">initiator</span> </pre></div> </div> <p>Controlling which type of header digest to negotiate with the target:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o">-</span><span class="n">iscsi</span> <span class="n">header</span><span class="o">-</span><span class="n">digest</span><span class="o">=</span><span class="n">CRC32C</span><span class="o">|</span><span class="n">CRC32C</span><span class="o">-</span><span class="n">NONE</span><span class="o">|</span><span class="n">NONE</span><span class="o">-</span><span class="n">CRC32C</span><span class="o">|</span><span class="n">NONE</span> </pre></div> </div> <p>These can also be set via a configuration file:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="n">iscsi</span><span class="p">]</span> <span class="n">user</span> <span class="o">=</span> <span class="s2">"CHAP username"</span> <span class="n">password</span> <span class="o">=</span> <span class="s2">"CHAP password"</span> <span class="n">initiator</span><span class="o">-</span><span class="n">name</span> <span class="o">=</span> <span class="s2">"iqn.qemu.test:my-initiator"</span> <span class="c1"># header digest is one of CRC32C|CRC32C-NONE|NONE-CRC32C|NONE</span> <span class="n">header</span><span class="o">-</span><span class="n">digest</span> <span class="o">=</span> <span class="s2">"CRC32C"</span> </pre></div> </div> <p>Setting the target name allows different options for different targets:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="p">[</span><span class="n">iscsi</span> <span class="s2">"iqn.target.name"</span><span class="p">]</span> <span class="n">user</span> <span class="o">=</span> <span class="s2">"CHAP username"</span> <span class="n">password</span> <span class="o">=</span> <span class="s2">"CHAP password"</span> <span class="n">initiator</span><span class="o">-</span><span class="n">name</span> <span class="o">=</span> <span class="s2">"iqn.qemu.test:my-initiator"</span> <span class="c1"># header digest is one of CRC32C|CRC32C-NONE|NONE-CRC32C|NONE</span> <span class="n">header</span><span class="o">-</span><span class="n">digest</span> <span class="o">=</span> <span class="s2">"CRC32C"</span> </pre></div> </div> <p>How to use a configuration file to set iSCSI configuration options:</p> <pre class="literal-block"> cat >iscsi.conf <<EOF [iscsi] user = "me" password = "my password" initiator-name = "iqn.qemu.test:my-initiator" header-digest = "CRC32C" EOF qemu-kvm -drive file=iscsi://127.0.0.1/iqn.qemu.test/1 \ -readconfig iscsi.conf </pre> <p>How to set up a simple iSCSI target on loopback and access it via QEMU: this example shows how to set up an iSCSI target with one CDROM and one DISK using the Linux STGT software target. This target is available on Red Hat based systems as the package ‘scsi-target-utils’.</p> <pre class="literal-block"> tgtd --iscsi portal=127.0.0.1:3260 tgtadm --lld iscsi --op new --mode target --tid 1 -T iqn.qemu.test tgtadm --lld iscsi --mode logicalunit --op new --tid 1 --lun 1 \ -b /IMAGES/disk.img --device-type=disk tgtadm --lld iscsi --mode logicalunit --op new --tid 1 --lun 2 \ -b /IMAGES/cd.iso --device-type=cd tgtadm --lld iscsi --op bind --mode target --tid 1 -I ALL qemu-kvm -iscsi initiator-name=iqn.qemu.test:my-initiator \ -boot d -drive file=iscsi://127.0.0.1/iqn.qemu.test/1 \ -cdrom iscsi://127.0.0.1/iqn.qemu.test/2 </pre> </div> <div class="section" id="glusterfs-disk-images"> <h2>GlusterFS disk images<a class="headerlink" href="#glusterfs-disk-images" title="Permalink to this headline">¶</a></h2> <p>GlusterFS is a user space distributed file system.</p> <p>You can boot from the GlusterFS disk image with the command:</p> <p>URI:</p> <pre class="literal-block"> qemu-kvm -drive file=gluster[+TYPE]://[HOST}[:PORT]]/VOLUME/PATH [?socket=...][,file.debug=9][,file.logfile=...] </pre> <p>JSON:</p> <pre class="literal-block"> qemu-kvm 'json:{"driver":"qcow2", "file":{"driver":"gluster", "volume":"testvol","path":"a.img","debug":9,"logfile":"...", "server":[{"type":"tcp","host":"...","port":"..."}, {"type":"unix","socket":"..."}]}}' </pre> <p><em>gluster</em> is the protocol.</p> <p><em>TYPE</em> specifies the transport type used to connect to gluster management daemon (glusterd). Valid transport types are tcp and unix. In the URI form, if a transport type isn’t specified, then tcp type is assumed.</p> <p><em>HOST</em> specifies the server where the volume file specification for the given volume resides. This can be either a hostname or an ipv4 address. If transport type is unix, then <em>HOST</em> field should not be specified. Instead <em>socket</em> field needs to be populated with the path to unix domain socket.</p> <p><em>PORT</em> is the port number on which glusterd is listening. This is optional and if not specified, it defaults to port 24007. If the transport type is unix, then <em>PORT</em> should not be specified.</p> <p><em>VOLUME</em> is the name of the gluster volume which contains the disk image.</p> <p><em>PATH</em> is the path to the actual disk image that resides on gluster volume.</p> <p><em>debug</em> is the logging level of the gluster protocol driver. Debug levels are 0-9, with 9 being the most verbose, and 0 representing no debugging output. The default level is 4. The current logging levels defined in the gluster source are 0 - None, 1 - Emergency, 2 - Alert, 3 - Critical, 4 - Error, 5 - Warning, 6 - Notice, 7 - Info, 8 - Debug, 9 - Trace</p> <p><em>logfile</em> is a commandline option to mention log file path which helps in logging to the specified file and also help in persisting the gfapi logs. The default is stderr.</p> <p>You can create a GlusterFS disk image with the command:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">qemu</span><span class="o">-</span><span class="n">img</span> <span class="n">create</span> <span class="n">gluster</span><span class="p">:</span><span class="o">//</span><span class="n">HOST</span><span class="o">/</span><span class="n">VOLUME</span><span class="o">/</span><span class="n">PATH</span> <span class="n">SIZE</span> </pre></div> </div> <p>Examples</p> <pre class="literal-block"> qemu-kvm -drive file=gluster://1.2.3.4/testvol/a.img qemu-kvm -drive file=gluster+tcp://1.2.3.4/testvol/a.img qemu-kvm -drive file=gluster+tcp://1.2.3.4:24007/testvol/dir/a.img qemu-kvm -drive file=gluster+tcp://[1:2:3:4:5:6:7:8]/testvol/dir/a.img qemu-kvm -drive file=gluster+tcp://[1:2:3:4:5:6:7:8]:24007/testvol/dir/a.img qemu-kvm -drive file=gluster+tcp://server.domain.com:24007/testvol/dir/a.img qemu-kvm -drive file=gluster+unix:///testvol/dir/a.img?socket=/tmp/glusterd.socket qemu-kvm -drive file=gluster+rdma://1.2.3.4:24007/testvol/a.img qemu-kvm -drive file=gluster://1.2.3.4/testvol/a.img,file.debug=9,file.logfile=/var/log/qemu-gluster.log qemu-kvm 'json:{"driver":"qcow2", "file":{"driver":"gluster", "volume":"testvol","path":"a.img", "debug":9,"logfile":"/var/log/qemu-gluster.log", "server":[{"type":"tcp","host":"1.2.3.4","port":24007}, {"type":"unix","socket":"/var/run/glusterd.socket"}]}}' qemu-kvm -drive driver=qcow2,file.driver=gluster,file.volume=testvol,file.path=/path/a.img, file.debug=9,file.logfile=/var/log/qemu-gluster.log, file.server.0.type=tcp,file.server.0.host=1.2.3.4,file.server.0.port=24007, file.server.1.type=unix,file.server.1.socket=/var/run/glusterd.socket </pre> </div> <div class="section" id="secure-shell-ssh-disk-images"> <h2>Secure Shell (ssh) disk images<a class="headerlink" href="#secure-shell-ssh-disk-images" title="Permalink to this headline">¶</a></h2> <p>You can access disk images located on a remote ssh server by using the ssh protocol:</p> <pre class="literal-block"> qemu-kvm -drive file=ssh://[USER@]SERVER[:PORT]/PATH[?host_key_check=HOST_KEY_CHECK] </pre> <p>Alternative syntax using properties:</p> <pre class="literal-block"> qemu-kvm -drive file.driver=ssh[,file.user=USER],file.host=SERVER[,file.port=PORT],file.path=PATH[,file.host_key_check=HOST_KEY_CHECK] </pre> <p><em>ssh</em> is the protocol.</p> <p><em>USER</em> is the remote user. If not specified, then the local username is tried.</p> <p><em>SERVER</em> specifies the remote ssh server. Any ssh server can be used, but it must implement the sftp-server protocol. Most Unix/Linux systems should work without requiring any extra configuration.</p> <p><em>PORT</em> is the port number on which sshd is listening. By default the standard ssh port (22) is used.</p> <p><em>PATH</em> is the path to the disk image.</p> <p>The optional <em>HOST_KEY_CHECK</em> parameter controls how the remote host’s key is checked. The default is <code class="docutils literal notranslate"><span class="pre">yes</span></code> which means to use the local <code class="docutils literal notranslate"><span class="pre">.ssh/known_hosts</span></code> file. Setting this to <code class="docutils literal notranslate"><span class="pre">no</span></code> turns off known-hosts checking. Or you can check that the host key matches a specific fingerprint: <code class="docutils literal notranslate"><span class="pre">host_key_check=md5:78:45:8e:14:57:4f:d5:45:83:0a:0e:f3:49:82:c9:c8</span></code> (<code class="docutils literal notranslate"><span class="pre">sha1:</span></code> can also be used as a prefix, but note that OpenSSH tools only use MD5 to print fingerprints).</p> <p>Currently authentication must be done using ssh-agent. Other authentication methods may be supported in future.</p> <p>Note: Many ssh servers do not support an <code class="docutils literal notranslate"><span class="pre">fsync</span></code>-style operation. The ssh driver cannot guarantee that disk flush requests are obeyed, and this causes a risk of disk corruption if the remote server or network goes down during writes. The driver will print a warning when <code class="docutils literal notranslate"><span class="pre">fsync</span></code> is not supported:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="n">warning</span><span class="p">:</span> <span class="n">ssh</span> <span class="n">server</span> <span class="n">ssh</span><span class="o">.</span><span class="n">example</span><span class="o">.</span><span class="n">com</span><span class="p">:</span><span class="mi">22</span> <span class="n">does</span> <span class="ow">not</span> <span class="n">support</span> <span class="n">fsync</span> </pre></div> </div> <p>With sufficiently new versions of libssh and OpenSSH, <code class="docutils literal notranslate"><span class="pre">fsync</span></code> is supported.</p> </div> <div class="section" id="nvme-disk-images"> <h2>NVMe disk images<a class="headerlink" href="#nvme-disk-images" title="Permalink to this headline">¶</a></h2> <p>NVM Express (NVMe) storage controllers can be accessed directly by a userspace driver in QEMU. This bypasses the host kernel file system and block layers while retaining QEMU block layer functionalities, such as block jobs, I/O throttling, image formats, etc. Disk I/O performance is typically higher than with <code class="docutils literal notranslate"><span class="pre">-drive</span> <span class="pre">file=/dev/sda</span></code> using either thread pool or linux-aio.</p> <p>The controller will be exclusively used by the QEMU process once started. To be able to share storage between multiple VMs and other applications on the host, please use the file based protocols.</p> <p>Before starting QEMU, bind the host NVMe controller to the host vfio-pci driver. For example:</p> <pre class="literal-block"> # modprobe vfio-pci # lspci -n -s 0000:06:0d.0 06:0d.0 0401: 1102:0002 (rev 08) # echo 0000:06:0d.0 > /sys/bus/pci/devices/0000:06:0d.0/driver/unbind # echo 1102 0002 > /sys/bus/pci/drivers/vfio-pci/new_id # qemu-kvm -drive file=nvme://HOST:BUS:SLOT.FUNC/NAMESPACE </pre> <p>Alternative syntax using properties:</p> <pre class="literal-block"> qemu-kvm -drive file.driver=nvme,file.device=HOST:BUS:SLOT.FUNC,file.namespace=NAMESPACE </pre> <p><em>HOST</em>:<em>BUS</em>:<em>SLOT</em>.<em>FUNC</em> is the NVMe controller’s PCI device address on the host.</p> <p><em>NAMESPACE</em> is the NVMe namespace number, starting from 1.</p> </div> <div class="section" id="disk-image-file-locking"> <h2>Disk image file locking<a class="headerlink" href="#disk-image-file-locking" title="Permalink to this headline">¶</a></h2> <p>By default, QEMU tries to protect image files from unexpected concurrent access, as long as it’s supported by the block protocol driver and host operating system. If multiple QEMU processes (including QEMU emulators and utilities) try to open the same image with conflicting accessing modes, all but the first one will get an error.</p> <p>This feature is currently supported by the file protocol on Linux with the Open File Descriptor (OFD) locking API, and can be configured to fall back to POSIX locking if the POSIX host doesn’t support Linux OFD locking.</p> <p>To explicitly enable image locking, specify “locking=on” in the file protocol driver options. If OFD locking is not possible, a warning will be printed and the POSIX locking API will be used. In this case there is a risk that the lock will get silently lost when doing hot plugging and block jobs, due to the shortcomings of the POSIX locking API.</p> <p>QEMU transparently handles lock handover during shared storage migration. For shared virtual disk images between multiple VMs, the “share-rw” device option should be used.</p> <p>By default, the guest has exclusive write access to its disk image. If the guest can safely share the disk image with other writers the <code class="docutils literal notranslate"><span class="pre">-device</span> <span class="pre">...,share-rw=on</span></code> parameter can be used. This is only safe if the guest is running software, such as a cluster file system, that coordinates disk accesses to avoid corruption.</p> <p>Note that share-rw=on only declares the guest’s ability to share the disk. Some QEMU features, such as image file formats, require exclusive write access to the disk image and this is unaffected by the share-rw=on option.</p> <p>Alternatively, locking can be fully disabled by “locking=off” block device option. In the command line, the option is usually in the form of “file.locking=off” as the protocol driver is normally placed as a “file” child under a format driver. For example:</p> <div class="highlight-default notranslate"><div class="highlight"><pre><span></span><span class="o">-</span><span class="n">blockdev</span> <span class="n">driver</span><span class="o">=</span><span class="n">qcow2</span><span class="p">,</span><span class="n">file</span><span class="o">.</span><span class="n">filename</span><span class="o">=/</span><span class="n">path</span><span class="o">/</span><span class="n">to</span><span class="o">/</span><span class="n">image</span><span class="p">,</span><span class="n">file</span><span class="o">.</span><span class="n">locking</span><span class="o">=</span><span class="n">off</span><span class="p">,</span><span class="n">file</span><span class="o">.</span><span class="n">driver</span><span class="o">=</span><span class="n">file</span> </pre></div> </div> <p>To check if image locking is active, check the output of the “lslocks” command on host and see if there are locks held by the QEMU process on the image file. More than one byte could be locked by the QEMU instance, each byte of which reflects a particular permission that is acquired or protected by the running block driver.</p> </div> <div class="section" id="filter-drivers"> <h2>Filter drivers<a class="headerlink" href="#filter-drivers" title="Permalink to this headline">¶</a></h2> <p>QEMU supports several filter drivers, which don’t store any data, but perform some additional tasks, hooking io requests.</p> <dl class="option"> <dt id="cmdoption-filter-drivers-arg-preallocate"> <code class="descname">preallocate</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-filter-drivers-arg-preallocate" title="Permalink to this definition">¶</a></dt> <dd><p>The preallocate filter driver is intended to be inserted between format and protocol nodes and preallocates some additional space (expanding the protocol file) when writing past the file’s end. This can be useful for file-systems with slow allocation.</p> <p>Supported options:</p> <dl class="option"> <dt id="cmdoption-preallocate-arg-prealloc-align"> <code class="descname">prealloc-align</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-preallocate-arg-prealloc-align" title="Permalink to this definition">¶</a></dt> <dd><p>On preallocation, align the file length to this value (in bytes), default 1M.</p> </dd></dl> <dl class="option"> <dt id="cmdoption-preallocate-arg-prealloc-size"> <code class="descname">prealloc-size</code><code class="descclassname"></code><a class="headerlink" href="#cmdoption-preallocate-arg-prealloc-size" title="Permalink to this definition">¶</a></dt> <dd><p>How much to preallocate (in bytes), default 128M.</p> </dd></dl> </dd></dl> </div> </div> </div> </div> <footer> <div class="rst-footer-buttons" role="navigation" aria-label="footer navigation"> <a href="virtio-net-failover.html" class="btn btn-neutral float-right" title="QEMU virtio-net standby (net_failover)" accesskey="n" rel="next">Next <span class="fa fa-arrow-circle-right"></span></a> <a href="monitor.html" class="btn btn-neutral" title="QEMU Monitor" accesskey="p" rel="prev"><span class="fa fa-arrow-circle-left"></span> Previous</a> </div> <hr/> <div role="contentinfo"> <p> © Copyright 2021, The QEMU Project Developers. </p> </div> Built with <a href="http://sphinx-doc.org/">Sphinx</a> using a <a href="https://github.com/rtfd/sphinx_rtd_theme">theme</a> provided by <a href="https://readthedocs.org">Read the Docs</a>. <!-- Empty para to force a blank line after "Built with Sphinx ..." --> <p></p> <p>This documentation is for QEMU version 6.2.0.</p> <p><a href="../about/license.html">QEMU and this manual are released under the GNU General Public License, version 2.</a></p> </footer> </div> </div> </section> </div> <script type="text/javascript"> var DOCUMENTATION_OPTIONS = { URL_ROOT:'../', VERSION:'qemu-kvm-6.2.0-53.module+el8.10.0+2055+8eb7870b.4', LANGUAGE:'None', COLLAPSE_INDEX:false, FILE_SUFFIX:'.html', HAS_SOURCE: false, SOURCELINK_SUFFIX: '.txt' }; </script> <script type="text/javascript" src="../_static/jquery.js"></script> <script type="text/javascript" src="../_static/underscore.js"></script> <script type="text/javascript" src="../_static/doctools.js"></script> <script type="text/javascript" src="../_static/js/theme.js"></script> <script type="text/javascript"> jQuery(function () { SphinxRtdTheme.Navigation.enable(true); }); </script> </body> </html>