From: Mikulas Patocka We can't guarantee that REQ_DISCARD on dm-mirror zeroes the data even if the underlying disks support zero on discard. So this patch sets ti->discard_zeroes_data_unsupported. For example, if the mirror is in the process of resynchronizing, it may happen that kcopyd reads a piece of data, then discard is sent on the same area and then kcopyd writes the piece of data to another leg. Consequently, the data is not zeroed. The flag was made available by commit 983c7db347db8ce2d8453fd1d89b7a4bb6920d56 (dm crypt: always disable discard_zeroes_data). Signed-off-by: Mikulas Patocka Cc: stable@vger.kernel.org Signed-off-by: Alasdair G Kergon --- drivers/md/dm-raid1.c | 1 + 1 file changed, 1 insertion(+) Index: linux-3.5-rc7/drivers/md/dm-raid1.c =================================================================== --- linux-3.5-rc7.orig/drivers/md/dm-raid1.c +++ linux-3.5-rc7/drivers/md/dm-raid1.c @@ -1084,6 +1084,7 @@ static int mirror_ctr(struct dm_target * ti->split_io = dm_rh_get_region_size(ms->rh); ti->num_flush_requests = 1; ti->num_discard_requests = 1; + ti->discard_zeroes_data_unsupported = 1; ms->kmirrord_wq = alloc_workqueue("kmirrord", WQ_NON_REENTRANT | WQ_MEM_RECLAIM, 0);