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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 15:17:04 +0100 (CET) |
>>Preallocation doesn't matter much these days.
I have done benchmark through nfs on a netapp san, (with directio),
I got 300 iops without preallocation, and 6000 iops with preallocation ....
----- Mail original -----
De: "Kevin Wolf" <address@hidden>
À: "Alexandre DERUMIER" <address@hidden>
Cc: "Dietmar Maurer" <address@hidden>, address@hidden
Envoyé: Vendredi 23 Novembre 2012 15:09:15
Objet: Re: [Qemu-devel] qcow2: slow internal snapshot creation
Am 23.11.2012 15:03, schrieb Alexandre DERUMIER:
> performance is also reduced when snapshot exist. (like if they are no
> preallocated metadatas)
Preallocation doesn't matter much these days.
> 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."
This is a commit message from 2009 that already says that this wasn't
the final solution.
Kevin
Re: [Qemu-devel] qcow2: slow internal snapshot creation, Kevin Wolf, 2012/11/23