On 07/30/2014 10:14 AM, Eric Blake wrote:
> On 07/25/2014 12:07 PM, Max Reitz wrote:
>> Actually, we do not need to allocate a new data cluster for every zero
>> cluster to be expanded: It is completely sufficient to rely on qcow2's
>> COW part and instead create a single zero cluster and reuse it as much
>> as possible.

> Also, I have to wonder - since the all-zero cluster is the most likely
> cluster to have a large refcount, even during normal runtime, should we
> special case the normal qcow2 write code to track the current all-zero
> cluster (if any), and merely increase its refcount rather than allocate
> a new cluster any time it is detected that an all-zero cluster is
> needed?  [Of course, the tracking would be runtime only, since
> compat=0.10 header doesn't provide any way to track the location of an
> all-zero cluster across file reloads.  Each new runtime would probably
> settle on a new location for the all-zero cluster used during that run,
> rather than trying to find an existing one.  And there's really no point
> to adding a header to track an all-zero cluster in compat=1.1 images,
> since those images already have the ability to track zero clusters
> without needing one allocated.]


>> +                    ret = bdrv_write_zeroes(bs->file, offset / BDRV_SECTOR_SIZE,
>> +                                            s->cluster_sectors, 0);
> 
> That is, if bdrv_write_zeroes knows how to take advantage of an already
> existing all-zero cluster, it would be less special casing in this code,
> but still get the same benefits of maximizing refcount during the amend
> operation, if all expanded clusters go through bdrv_write_zeroes.

Now that I've looked through both variants, I'm leaning towards the
simplicity of your alternate series, rather than the complexity of this
one, if we can (independently?) optimize bdrv_write_zeroes to reuse a
known-all-zeroes cluster when possible.  Of course, you may want to get
other opinions than just mine before posting your next round of these
patches.

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