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Re: [Qemu-devel] [PATCH v2 2/9] qcow2: Inform block layer about discard

From: Max Reitz
Subject: Re: [Qemu-devel] [PATCH v2 2/9] qcow2: Inform block layer about discard boundaries
Date: Thu, 17 Nov 2016 22:24:03 +0100
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:45.0) Gecko/20100101 Thunderbird/45.4.0

On 17.11.2016 21:13, Eric Blake wrote:
> At the qcow2 layer, discard is only possible on a per-cluster
> basis; at the moment, qcow2 silently rounds any unaligned
> requests to this granularity.  However, an upcoming patch will
> fix a regression in the block layer ignoring too much of an
> unaligned discard request, by changing the block layer to
> break up a discard request at alignment boundaries; for that
> to work, the block layer must know about our limits.
> However, we can't go one step further by changing
> qcow2_discard_clusters() to assert that requests are always
> aligned, since that helper function is reached on paths
> outside of the block layer.

Also, to do that, we'd have to establish that pdiscard_alignment is a
mandatory alignment. Currently, it's described as the "best alignment",
which doesn't sound too mandatory to me (although the block layer
apparently always does drop unaligned parts).

> CC: address@hidden
> Signed-off-by: Eric Blake <address@hidden>
> ---
>  block/qcow2.c | 1 +
>  1 file changed, 1 insertion(+)
> diff --git a/block/qcow2.c b/block/qcow2.c
> index 6d5689a..e22f6dc 100644
> --- a/block/qcow2.c
> +++ b/block/qcow2.c
> @@ -1206,6 +1206,7 @@ static void qcow2_refresh_limits(BlockDriverState *bs, 
> Error **errp)
>          bs->bl.request_alignment = BDRV_SECTOR_SIZE;
>      }
>      bs->bl.pwrite_zeroes_alignment = s->cluster_size;
> +    bs->bl.pdiscard_alignment = s->cluster_size;
>  }
>  static int qcow2_set_key(BlockDriverState *bs, const char *key)

Reviewed-by: Max Reitz <address@hidden>

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