Re: [Qemu-devel] [PATCH] docs: clarify that qcow2 file size is not always a cluster multiple

[Eric Blake <eblake@redhat.com>]

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On 05/22/2014 03:42 AM, Stefan Hajnoczi wrote:
> Normally one would expect that qcow2 image file lengths are multiples of
> the cluster size.  This is not true in all cases and the spec should
> document this so implementers remember to accept such files.
> 
> $ qemu-img create -f qcow2 foo.qcow2 2G
> Formatting 'foo.qcow2', fmt=qcow2 size=2147483648 encryption=off cluster_size=65536 lazy_refcounts=off
> $ ls -l foo.qcow2
> -rw-r--r-- 1 stefanha stefanha 197120 May 22 11:40 foo.qcow2
> $ bc -q
> 3 * (64 * 1024) + 512
> 197120
> 
> The extra sector are the 4 L1 table entries that a 2 GB disk with 64 KB
> cluster size needs.  The rest of the L1 table is omitted from the file
> but allocation will continue at the next cluster boundary.
> 
> Reported-by: Maria Kustova <maxa@catit.be>
> Signed-off-by: Stefan Hajnoczi <stefanha@redhat.com>
> ---
>  docs/specs/qcow2.txt | 4 ++++
>  1 file changed, 4 insertions(+)

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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