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Date: Fri, 23 Nov 2012 16:31:19 +0100
From: Kevin Wolf <kwolf@redhat.com>
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Subject: Re: [Qemu-devel] qcow2: slow internal snapshot creation
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To: Dietmar Maurer <dietmar@proxmox.com>
Cc: "qemu-devel@nongnu.org" <qemu-devel@nongnu.org>

Am 23.11.2012 08:26, schrieb Dietmar Maurer:
> qcow2 snapshot on newly created files are fast:
>=20
> # qemu-img create -f qcow2 test.img 200G
> # time qemu-img snapshot -c snap1 test.img
> real       0m0.014s
>=20
> but if metadata is allocated it gets very slow:
>=20
> # qemu-img create  -f qcow2 -o "preallocation=3Dmetadata" test.img 200G
> # time qemu-img snapshot -c snap1 test.img
> real       1m20.399s
>=20
> but reading the metadata is also fast:
>=20
> # time qemu-img check test.img
> real       0m0.371s
>=20
> So why is creating a new snapshot that slow =96 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