storage: add support for QCOW2 cluster_size option

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@redhat.com>
Reviewed-by: Peter Krempa <pkrempa@redhat.com>

docs/formatstorage.html.in

@@ -791,6 +791,7 @@
   </encryption>
   <compat>1.1</compat>
   <nocow/>
+  <clusterSize unit='KiB'>64</clusterSize>
   <features>
     <lazy_refcounts/>
   </features>
@@ -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 which can affect image file size
+        and performance.
+        <span class="since">Since 7.4.0</span>
+      </dd>
       <dt><code>features</code></dt>
       <dd>Format-specific features. Only used for <code>qcow2</code> now.
         Valid sub-elements are:

docs/schemas/storagecommon.rng

@@ -110,6 +110,13 @@
       </data>
     </element>
   </define>
+
+  <define name="clusterSize">
+    <element name="clusterSize">
+      <ref name="scaledInteger"/>
+    </element>
+  </define>
+
   <define name="fileFormatFeatures">
     <element name="features">
       <interleave>

docs/schemas/storagevol.rng

@@ -124,6 +124,9 @@
             <empty/>
           </element>
         </optional>
+        <optional>
+          <ref name="clusterSize"/>
+        </optional>
         <optional>
           <ref name="fileFormatFeatures"/>
         </optional>

src/conf/storage_conf.c

@@ -1416,6 +1416,13 @@ virStorageVolDefParseXML(virStoragePoolDef *pool,
     if (virXPathNode("./target/nocow", ctxt))
         def->target.nocow = true;
 
+    if (virParseScaledValue("./target/clusterSize",
+                            "./target/clusterSize/@unit",
+                            ctxt, &def->target.clusterSize,
+                            1, ULLONG_MAX, false) < 0) {
+        return NULL;
+    }
+
     if (virXPathNode("./target/features", ctxt)) {
         if ((n = virXPathNodeSet("./target/features/*", ctxt, &nodes)) < 0)
             return NULL;
@@ -1580,6 +1587,11 @@ virStorageVolTargetDefFormat(virStorageVolOptions *options,
 
     virBufferEscapeString(buf, "<compat>%s</compat>\n", def->compat);
 
+    if (def->clusterSize > 0) {
+        virBufferAsprintf(buf, "<clusterSize unit='B'>%llu</clusterSize>\n",
+                          def->clusterSize);
+    }
+
     if (def->features) {
         size_t i;
         bool empty = virBitmapIsAllClear(def->features);

src/storage/storage_util.c

@@ -664,6 +664,7 @@ struct _virStorageBackendQemuImgInfo {
     const char *path;
     unsigned long long size_arg;
     unsigned long long allocation;
+    unsigned long long clusterSize;
     bool encryption;
     bool preallocate;
     const char *compat;
@@ -780,6 +781,9 @@ storageBackendCreateQemuImgOpts(virStorageEncryptionInfoDef *encinfo,
     else if (info->format == VIR_STORAGE_FILE_QCOW2)
         virBufferAddLit(&buf, "compat=0.10,");
 
+    if (info->clusterSize > 0)
+        virBufferAsprintf(&buf, "cluster_size=%llu,", info->clusterSize);
+
     if (info->features && info->format == VIR_STORAGE_FILE_QCOW2) {
         if (virBitmapIsBitSet(info->features,
                               VIR_STORAGE_FILE_FEATURE_LAZY_REFCOUNTS)) {
@@ -1130,6 +1134,7 @@ virStorageBackendCreateQemuImgCmdFromVol(virStoragePoolObj *pool,
         .compat = vol->target.compat,
         .features = vol->target.features,
         .nocow = vol->target.nocow,
+        .clusterSize = vol->target.clusterSize,
         .secretAlias = NULL,
     };
     virStorageEncryption *enc = vol->target.encryption;

tests/storagevolxml2argvdata/qcow2-clusterSize.argv

@@ -0,0 +1,6 @@
+qemu-img \
+create \
+-f qcow2 \
+-o compat=0.10,cluster_size=131072 \
+/var/lib/libvirt/images/OtherDemo.img \
+5242880K

tests/storagevolxml2argvtest.c

@@ -238,6 +238,10 @@ mymain(void)
                  "pool-dir", "vol-qcow2-nocapacity-backing", NULL, NULL,
                  "qcow2-nocapacity", 0);
 
+    DO_TEST("pool-dir", "vol-qcow2-clusterSize",
+            NULL, NULL,
+            "qcow2-clusterSize", 0);
+
     DO_TEST("pool-dir", "vol-file-iso",
             NULL, NULL,
             "iso", 0);

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'/>
+    <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>

tests/storagevolxml2xmlout/vol-qcow2-clusterSize.xml

@@ -0,0 +1,17 @@
+<volume type='file'>
+  <name>OtherDemo.img</name>
+  <key>/var/lib/libvirt/images/OtherDemo.img</key>
+  <capacity unit='bytes'>5368709120</capacity>
+  <allocation unit='bytes'>294912</allocation>
+  <target>
+    <path>/var/lib/libvirt/images/OtherDemo.img</path>
+    <format type='qcow2'/>
+    <permissions>
+      <mode>0644</mode>
+      <owner>0</owner>
+      <group>0</group>
+      <label>unconfined_u:object_r:virt_image_t:s0</label>
+    </permissions>
+    <clusterSize unit='B'>131072</clusterSize>
+  </target>
+</volume>

tests/storagevolxml2xmltest.c

@@ -88,6 +88,7 @@ mymain(void)
     DO_TEST("pool-dir", "vol-qcow2-nobacking");
     DO_TEST("pool-dir", "vol-qcow2-encryption");
     DO_TEST("pool-dir", "vol-qcow2-luks");
+    DO_TEST("pool-dir", "vol-qcow2-clusterSize");
     DO_TEST("pool-dir", "vol-luks");
     DO_TEST("pool-dir", "vol-luks-cipher");
     DO_TEST("pool-disk", "vol-partition");