Re: [Qemu-devel] qcow2: slow internal snapshot creation

[Kevin Wolf <kwolf@redhat.com>]

Am 23.11.2012 08:26, schrieb Dietmar Maurer:
> qcow2 snapshot on newly created files are fast:
> 
> # qemu-img create -f qcow2 test.img 200G
> # time qemu-img snapshot -c snap1 test.img
> real       0m0.014s
> 
> but if metadata is allocated it gets very slow:
> 
> # qemu-img create  -f qcow2 -o "preallocation=metadata" test.img 200G
> # time qemu-img snapshot -c snap1 test.img
> real       1m20.399s
> 
> but reading the metadata is also fast:
> 
> # time qemu-img check test.img
> real       0m0.371s
> 
> So why is creating a new snapshot that slow – any ideas?

Had a look at this now. The culprit is the unconditional bdrv_flush() in
update_cluster_refcount(). I suspect it's completely unnecessary by now,
but I need to give it a closer review before I send a patch to remove
it. (And in any case error handling is missing there)

Affected are writing compressed images and creating/deleting internal
snapshots, no other code uses this path.

Kevin