storage: Introduce 'extended_l2' feature for storage volume

QCOW2 images now support 'extended_l2' which splits the default clusters
into 32 subcluster allocation units. This allows the allocation units to
be smaller without increasing the size of L2 table too much and thus also
the cache requirements for holding the full L2 table in memory.

Unfortunately it's incompatible with qemu versions older than 5.2 thus
can't be used as default.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>

docs/formatstorage.rst

@@ -652,6 +652,7 @@ host filesystem. It can contain the following child elements:
      <clusterSize unit='KiB'>64</clusterSize>
      <features>
        <lazy_refcounts/>
+       <extended_l2/>
      </features>
    </target>
 
@@ -714,6 +715,9 @@ host filesystem. It can contain the following child elements:
 
    -  ``<lazy_refcounts/>`` - allow delayed reference counter updates.
       :since:`Since 1.1.0`
+   - ``<extended_l2/>`` - enables subcluster allocation for qcow2 images. QCOW2
+     clusters are split into 32 subclusters decreasing the size of L2 cache
+     needed. It's recommended to increase ``clusterSize``.
 
 Backing store elements
 ~~~~~~~~~~~~~~~~~~~~~~

docs/schemas/storagecommon.rng

@@ -134,6 +134,11 @@
             <empty/>
           </element>
         </optional>
+        <optional>
+          <element name="extended_l2">
+            <empty/>
+          </element>
+        </optional>
       </interleave>
     </element>
   </define>

src/conf/storage_source_conf.c

@@ -66,6 +66,7 @@ VIR_ENUM_IMPL(virStorageFileFormat,
 VIR_ENUM_IMPL(virStorageFileFeature,
               VIR_STORAGE_FILE_FEATURE_LAST,
               "lazy_refcounts",
+              "extended_l2",
 );
 
 

src/conf/storage_source_conf.h

@@ -86,6 +86,7 @@ VIR_ENUM_DECL(virStorageFileFormat);
 
 typedef enum {
     VIR_STORAGE_FILE_FEATURE_LAZY_REFCOUNTS = 0,
+    VIR_STORAGE_FILE_FEATURE_EXTENDED_L2,
 
     VIR_STORAGE_FILE_FEATURE_LAST
 } virStorageFileFeature;

src/storage/storage_util.c

@@ -796,6 +796,17 @@ storageBackendCreateQemuImgOpts(virStorageEncryptionInfoDef *encinfo,
             }
             virBufferAddLit(&buf, "lazy_refcounts,");
         }
+
+        if (virBitmapIsBitSet(info->features,
+                              VIR_STORAGE_FILE_FEATURE_EXTENDED_L2)) {
+            if (STREQ_NULLABLE(info->compat, "0.10")) {
+                virReportError(VIR_ERR_CONFIG_UNSUPPORTED,
+                               _("'extended_l2' not supported with compat level %s"),
+                               info->compat);
+                return -1;
+            }
+            virBufferAddLit(&buf, "extended_l2=on,");
+        }
     }
 
     virBufferTrim(&buf, ",");

tests/storagevolxml2argvdata/qcow2-clusterSize.argv

@@ -1,6 +1,6 @@
 qemu-img \
 create \
 -f qcow2 \
--o compat=0.10,cluster_size=131072 \
+-o compat=1.1,cluster_size=131072,extended_l2=on \
 /var/lib/libvirt/images/OtherDemo.img \
 5242880K

tests/storagevolxml2xmlin/vol-qcow2-clusterSize.xml

@@ -13,5 +13,8 @@
       <label>unconfined_u:object_r:virt_image_t:s0</label>
     </permissions>
     <clusterSize unit='KiB'>128</clusterSize>
+    <features>
+      <extended_l2/>
+    </features>
   </target>
 </volume>

tests/storagevolxml2xmlout/vol-qcow2-clusterSize.xml

@@ -12,6 +12,10 @@
       <group>0</group>
       <label>unconfined_u:object_r:virt_image_t:s0</label>
     </permissions>
+    <compat>1.1</compat>
     <clusterSize unit='B'>131072</clusterSize>
+    <features>
+      <extended_l2/>
+    </features>
   </target>
 </volume>