Re: [Qemu-devel] [PATCH v12 09/10] qcow2: skip writing zero buffers to empty COW areas

[Anton Nefedov]

On 15/1/2019 6:27 PM, Alberto Garcia wrote:
> On Mon 14 Jan 2019 12:18:30 PM CET, Anton Nefedov wrote:
>> If COW areas of the newly allocated clusters are zeroes on the backing image,
>> efficient bdrv_write_zeroes(flags=BDRV_REQ_ALLOCATE) can be used on the whole
>> cluster instead of writing explicit zero buffers later in perform_cow().
>>
>> iotest 060:
>> write to the discarded cluster does not trigger COW anymore.
>> Use a backing image instead.
>>
>> Signed-off-by: Anton Nefedov <anton.nefe...@virtuozzo.com>
>> Reviewed-by: Vladimir Sementsov-Ogievskiy <vsement...@virtuozzo.com>
> 
> Reviewed-by: Alberto Garcia <be...@igalia.com>
> 
>> +        ret = handle_alloc_space(bs, l2meta);
> 
> I insist that it would be nice to have a short comment explaining what
> this does.
> 

Right sorry forgot your comment.
I'd go with:

+        /* Try to efficiently initialize the physical space with zeroes */
          ret = handle_alloc_space(bs, l2meta);
          if (ret < 0) {
              qemu_co_mutex_lock(&s->lock);