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diff -u docs/formatdomain.html.in docs.new/formatdomain.html.in --- docs/formatdomain.html.in 2010-05-27 11:35:33.995122526 +0600 +++ docs.new/formatdomain.html.in 2010-05-27 13:36:27.362623268 +0600 @@ -517,10 +520,30 @@ <dt><code>driver</code></dt> <dd>If the hypervisor supports multiple backend drivers, then the optional <code>driver</code> element allows them to be selected. The <code>name</code> - attribute is the primary backend driver name, while the optional <code>type</code> - attribute provides the sub-type. The optional <code>cache</code> attribute + attribute is the primary backend driver name (for example, "qemu"), + while the optional <code>type</code> attribute provides the sub-type. + For qemu, possible values are: "raw", "qcow", "qcow2", "vmdk" and some other. + + <p>The optional <code>cache</code> attribute controls the cache mechanism, possible values are "default", "none", - "writethrough" and "writeback". <span class="since">Since 0.1.8</span> + "writethrough" and "writeback".</p> + + <p>Writethrough caching means that the host + page cache will be used to read and write data but write notification will + be sent to the guest only when the data has been reported as written by the + storage subsystem.</p> + + <p>Writeback caching will report data writes as completed as soon as + the data is present in the host page cache. This is safe as long as you trust your host. + If your host crashes or loses power, then the guest may experience data corruption.</p> + + <p>The host page cache can be avoided entirely with <code>cache='none'</code>. + This will attempt to do disk IO directly to the guests memory.</p> + + <p>As said in qemu-kvm-0.12.3 manual: Some block drivers perform badly with <code>cache='writethrough'</code>, most notably, + qcow2. If performance is more important than correctness, <code>cache='writeback'</code> + should be used with qcow2.</p> + <span class="since">Since 0.1.8</span>