Re: [Qemu-devel] [PATCH v2] qcow2: Metadata preallocation

[Kevin Wolf <kwolf@redhat.com>]

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 <kwolf@redhat.com>
> ---
> 
> 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