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

[Paolo Bonzini <pbonzini@redhat.com>]

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.

Paolo