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

[Alexandre DERUMIER <aderumier@odiso.com>]

performance is also reduced when snapshot exist. (like if they are no preallocated metadatas)


see initial git commit

http://git.qemu.org/?p=qemu.git;a=commit;h=a35e1c177debb01240243bd656caca302410d38c
"qcow2: Metadata preallocation

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."



----- Mail original -----

De: "Dietmar Maurer" <dietmar@proxmox.com>
À: qemu-devel@nongnu.org
Cc: "Kevin Wolf (kwolf@redhat.com)" <kwolf@redhat.com>
Envoyé: Vendredi 23 Novembre 2012 08:26:13
Objet: [Qemu-devel] qcow2: slow internal snapshot creation



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?