a35e1c177d
This introduces a qemu-img create option for qcow2 which allows the metadata to be preallocated, i.e. clusters are reserved in the refcount table and L1/L2 tables, but no data is written to them. Metadata is quite small, so this happens in almost no time. Especially with qcow2 on virtio this helps to gain a bit of performance during the initial writes. However, as soon as create a snapshot, we're back to the normal slow speed, obviously. So this isn't the real fix, but kind of a cheat while we're still having trouble with qcow2 on virtio. Note that the option is disabled by default and needs to be specified explicitly using qemu-img create -f qcow2 -o preallocation=metadata. Signed-off-by: Kevin Wolf <kwolf@redhat.com> Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>