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Date: Fri, 23 Nov 2012 15:03:59 +0100 (CET)
From: Alexandre DERUMIER <aderumier@odiso.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: "Kevin Wolf (kwolf@redhat.com)" <kwolf@redhat.com>, qemu-devel@nongnu.org

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


see initial git commit

http://git.qemu.org/?p=3Dqemu.git;a=3Dcommit;h=3Da35e1c177debb01240243bd656=
caca302410d38c
"qcow2: Metadata preallocation

This introduces a qemu-img create option for qcow2 which allows the metadat=
a 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 dur=
ing
the initial writes. However, as soon as create a snapshot, we're back to th=
e
normal slow speed, obviously. So this isn't the real fix, but kind of a che=
at
while we're still having trouble with qcow2 on virtio."



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

De: "Dietmar Maurer" <dietmar@proxmox.com> 
=C3=80: qemu-devel@nongnu.org 
Cc: "Kevin Wolf (kwolf@redhat.com)" <kwolf@redhat.com> 
Envoy=C3=A9: 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=3Dmetadata" 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 =E2=80=93 any ideas?