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Re: [Qemu-devel] [PATCH v2] qcow2: Metadata preallocation
From: |
Kevin Wolf |
Subject: |
Re: [Qemu-devel] [PATCH v2] qcow2: Metadata preallocation |
Date: |
Fri, 21 Aug 2009 13:17:01 +0200 |
User-agent: |
Thunderbird 2.0.0.21 (X11/20090320) |
Kevin Wolf schrieb:
> 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.
>
> Note that the option is disabled by default and needs to be specified
> explicitly using qemu-img create -f qcow2 -o preallocation=metadata.
>
> Signed-off-by: Kevin Wolf <address@hidden>
> ---
>
> v2: Use bdrv_truncate() to set the right file size
This one seems to be wrong actually, possibly some metadata being stored
after the last allocated cluster. Please apply version 1 instead.
Kevin