On Wed, May 19, 2021 at 02:49:14PM +0200, Peter Krempa wrote: > On Thu, May 13, 2021 at 13:23:04 +0200, Pavel Hrdina wrote: > > The default value hard-coded in QEMU (64KiB) is not always the ideal. > > Having a possibility to set the cluster_size by user may in specific > > use-cases improve performance for QCOW2 images. > > > > QEMU internally has some limits, the value has to be between 512B and > > 2048KiB and must by power of two, except when the image has Extended L2 > > Entries the minimal value has to be 16KiB. > > > > Since qemu-img ensures the value is correct and the limit is not always > > the same libvirt will not duplicate any of these checks as the error > > message from qemu-img is good enough: > > > > Cluster size must be a power of two between 512 and 2048k > > > > Resolves: https://gitlab.com/libvirt/libvirt/-/issues/154 > > > > Signed-off-by: Pavel Hrdina <phrdina@xxxxxxxxxx> > > --- > > [...] > > > @@ -867,6 +868,11 @@ > > the file image is used in VM. To create non-raw file images, it > > requires QEMU version since 2.1. <span class="since">Since 1.2.7</span> > > </dd> > > + <dt><code>clusterSize</code></dt> > > + <dd> Changes the qcow2 cluster size where smaller size can improve image > > + file size whereas larger size can improve performance. > > + <span class="since">Since 7.4.0</span> > > I think we should refrain from advice on how the cluster size can be > configured because it has way more nuance than this short sentence. Reasonable, I can drop it as I was just rephrasing what qemu has in their documentation. > > + </dd> > > <dt><code>features</code></dt> > > <dd>Format-specific features. Only used for <code>qcow2</code> now. > > Valid sub-elements are: > > > > > diff --git a/tests/storagevolxml2xmlin/vol-qcow2-clusterSize.xml b/tests/storagevolxml2xmlin/vol-qcow2-clusterSize.xml > > new file mode 100644 > > index 0000000000..22534982a1 > > --- /dev/null > > +++ b/tests/storagevolxml2xmlin/vol-qcow2-clusterSize.xml > > @@ -0,0 +1,17 @@ > > +<volume> > > + <name>OtherDemo.img</name> > > + <key>/var/lib/libvirt/images/OtherDemo.img</key> > > + <capacity unit="G">5</capacity> > > + <allocation>294912</allocation> > > + <target> > > + <path>/var/lib/libvirt/images/OtherDemo.img</path> > > + <format type='qcow2'/> > > I wonder whether it wouldn't make more sense to stash cluster size under > the 'format' tag. > > Does the LVM volume XML have <format> ? No it doesn't have it. Only for pool XML. In addition currently it has only path attribute. For qcow2 we have most of the options represented outside the <format> element. I'm not saying it's correct as I cannot decide which place would be better, but following what we already do with other qcow2 options I figured we should do the same here. It is definitely possible to have it in <format>. Pavel > > + <permissions> > > + <mode>0644</mode> > > + <owner>0</owner> > > + <group>0</group> > > + <label>unconfined_u:object_r:virt_image_t:s0</label> > > + </permissions> > > + <clusterSize unit='KiB'>128</clusterSize> > > + </target> > > +</volume>
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