[Top][All Lists]
[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
Re: [Qemu-devel] qcow2: slow internal snapshot creation
From: |
Alexandre DERUMIER |
Subject: |
Re: [Qemu-devel] qcow2: slow internal snapshot creation |
Date: |
Fri, 23 Nov 2012 15:03:59 +0100 (CET) |
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" <address@hidden>
À: address@hidden
Cc: "Kevin Wolf (address@hidden)" <address@hidden>
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?
Re: [Qemu-devel] qcow2: slow internal snapshot creation, Kevin Wolf, 2012/11/23