Re: [Qemu-devel] [PATCH] qcow2: Support BDRV_REQ_MAY_UNMAP

[Paolo Bonzini]

On 29/09/2016 14:48, Kevin Wolf wrote:
> Am 29.09.2016 um 14:14 hat Paolo Bonzini geschrieben:
>> On 29/09/2016 12:39, Kevin Wolf wrote:
>>>>> Because there is another check of pass-discard-request value in
>>>>> update_refcount:
>>>>>
>>>>>         if (refcount == 0 && s->discard_passthrough[type]) {
>>>>>             update_refcount_discard(bs, cluster_offset, s->cluster_size);
>>>>>         }
>>> What I mean is that in the second case, you're still uselessly
>>> deallocating the cluster on the qcow2 level while you can't reclaim it
>>> on the filesystem level. So it would be better to leave it allocated in
>>> qcow2, too, so that you don't get an expensive reallocation the next
>>> time you write to it.
>>
>> But if you do a qemu-img convert, the deallocated cluster wouldn't be in
>> the destination.
> 
> Right. I still think that there has to be an option to keep the image
> fully allocated. Perhaps what we really need to check is BDRV_O_UNMAP.

Duh, of course it is.

Paolo