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Date:   Sat,  7 Oct 2023 19:21:05 +0800
From:   Yu Kuai <>
Subject: [PATCH -next] md/raid1: don't split discard io for write behind

From: Yu Kuai <>

Currently, discad io is treated the same as normal write io, and for
write behind case, io size is limited to:


For 0.5KB sector size and 4KB PAGE_SIZE, this is just 1MB. For
consequence, if 'WriteMostly' is set to one of the underlying disks,
then diskcard io will be splited into 1MB and it will take a long time
for the diskcard to finish.

Fix this problem by disable write behind for discard io.

Reported-by: Roman Mamedov <>
Reported-and-tested-by: Kirill Kirilenko <>
Signed-off-by: Yu Kuai <>
 drivers/md/raid1.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/drivers/md/raid1.c b/drivers/md/raid1.c
index 3a78f79ee6d5..35d12948e0a9 100644
--- a/drivers/md/raid1.c
+++ b/drivers/md/raid1.c
@@ -1345,6 +1345,7 @@ static void raid1_write_request(struct mddev *mddev, struct bio *bio,
 	int first_clone;
 	int max_sectors;
 	bool write_behind = false;
+	bool is_discard = (bio_op(bio) == REQ_OP_DISCARD);
 	if (mddev_is_clustered(mddev) &&
 	     md_cluster_ops->area_resyncing(mddev, WRITE,
@@ -1405,7 +1406,7 @@ static void raid1_write_request(struct mddev *mddev, struct bio *bio,
 		 * write-mostly, which means we could allocate write behind
 		 * bio later.
-		if (rdev && test_bit(WriteMostly, &rdev->flags))
+		if (!is_discard && rdev && test_bit(WriteMostly, &rdev->flags))
 			write_behind = true;
 		if (rdev && unlikely(test_bit(Blocked, &rdev->flags))) {

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