Re: [Qemu-devel] [PATCH 05/21] qcow2: Refcount overflow and qcow2_alloc_bytes()

[Eric Blake <eblake@redhat.com>]

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On 11/10/2014 06:45 AM, Max Reitz wrote:
> qcow2_alloc_bytes() may reuse a cluster multiple times, in which case
> the refcount is increased accordingly. However, if this would lead to an
> overflow the function should instead just not reuse this cluster and
> allocate a new one.

So if recount_order is 1 (2 bits per refcount, max refcount of 4), and
we encounter the same cluster 6 times (say by 5 back-to-back internal
snapshots), does this code optimize to only 2 clusters (both with
refcount 3) or does it result in each of the last 3 clusters spilling to
its own 1-ref cluster for a total of 4 clusters?  Short of Benoit's work
on deduplication, is there even a way to avoid inefficient use of
spilled clusters?  But I guess answering that can be a separate patch;
inefficiency is annoying, but not technically wrong and therefore not a
reason to reject this one.

> 
> Signed-off-by: Max Reitz <mreitz@redhat.com>
> ---
>  block/qcow2-refcount.c | 32 ++++++++++++++++++++++++++++++--
>  1 file changed, 30 insertions(+), 2 deletions(-)
> 

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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