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Re: [Qemu-devel] [PATCH v2] qcow2: Add bdrv_discard support
From: |
Stefan Hajnoczi |
Subject: |
Re: [Qemu-devel] [PATCH v2] qcow2: Add bdrv_discard support |
Date: |
Wed, 27 Aug 2014 15:12:42 +0100 |
User-agent: |
Mutt/1.5.23 (2014-03-12) |
On Mon, Aug 25, 2014 at 12:38:06AM +0800, Jun Li wrote:
> Why bdrv_discard does not yet pass the discard on to the underlying file
> system driver? And this exist a potential issue: host cluster leak for
> qcow2. As qcow2 image can be created on RAW disk,
> so this is not just "does not yet pass the discard on to the underlying file
> system driver". Could you give some explanation why do not realize this
> function? Thx.
>
> As I am trying to realize qcow2 shrinking, but hit bdrv_discard can not
> release the free cluster on host.
qcow2 is capable of passing discards down to the image file. See
qcow2_process_discards().
It is important that s->discard_passthrough[QCOW2_DISCARD_REQUEST] is
true! Make sure to use the pass-discard-request=on or discard=unmap
options.
Stefan
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