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

[Benoît Canet]

The Thursday 22 May 2014 à 11:42:50 (+0200), 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.

I think we should fix this to allocate a whole extra cluster instead
of 512B.
These days most SSD pages (smalled write unit) are 4KB or 16KB.
Having most of the file shifted by 512B mean that the SSD controller
will have to to Read/Modify/Write cycles for most write hence impacting
performance and SSD life.

Best regards

Benoît

> 
> Reported-by: Maria Kustova <address@hidden>
> Signed-off-by: Stefan Hajnoczi <address@hidden>
> ---
>  docs/specs/qcow2.txt | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/docs/specs/qcow2.txt b/docs/specs/qcow2.txt
> index f19536a..a46ee57 100644
> --- a/docs/specs/qcow2.txt
> +++ b/docs/specs/qcow2.txt
> @@ -4,6 +4,10 @@ A qcow2 image file is organized in units of constant size, 
> which are called
>  (host) clusters. A cluster is the unit in which all allocations are done,
>  both for actual guest data and for image metadata.
>  
> +If the end of the image file is not on a cluster boundary, the missing data 
> is
> +treated as zeroes.  This layout can occur when an L1 table or refcount table 
> is
> +the last thing in the file and not all entries in the table are used.
> +
>  Likewise, the virtual disk as seen by the guest is divided into (guest)
>  clusters of the same size.
>  
> -- 
> 1.9.0
>