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

From: Eric Blake
Subject: [Qemu-devel] [PATCH v2 2/9] qcow2: Inform block layer about discard boundaries
Date: Thu, 17 Nov 2016 14:13:55 -0600

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.

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)

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