Re: [libvirt PATCH 1/2] storage: add support for QCOW2 cluster_size option

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(a)redhat.com&gt; > --- [...] > @@ -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.

> + </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> ?

> + <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>