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


From: Paolo Bonzini
Subject: Re: [Qemu-block] [Qemu-devel] [PATCH] qcow2: Support BDRV_REQ_MAY_UNMAP
Date: Thu, 29 Sep 2016 14:14:09 +0200
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.3.0


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



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