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>
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>
