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

[Alexandre DERUMIER <aderumier@odiso.com>]

>>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" <kwolf@redhat.com>
À: "Alexandre DERUMIER" <aderumier@odiso.com>
Cc: "Dietmar Maurer" <dietmar@proxmox.com>, qemu-devel@nongnu.org
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