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

[Peter Krempa]

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 at redhat.com>
> ---

[...]

> @@ -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>