[3.13.y.z extended stable] Patch "md/raid5: disable 'DISCARD' by default due to safety concerns." has been added to staging queue

Kamal Mostafa kamal at canonical.com
Thu Oct 9 20:51:50 UTC 2014

This is a note to let you know that I have just added a patch titled

    md/raid5: disable 'DISCARD' by default due to safety concerns.

to the linux-3.13.y-queue branch of the 3.13.y.z extended stable tree 
which can be found at:


This patch is scheduled to be released in version

If you, or anyone else, feels it should not be added to this tree, please 
reply to this email.

For more information about the 3.13.y.z tree, see



>From b61b681ab8fb5a9fcde72061bb221551ab179ba4 Mon Sep 17 00:00:00 2001
From: NeilBrown <neilb at suse.de>
Date: Thu, 2 Oct 2014 13:45:00 +1000
Subject: md/raid5: disable 'DISCARD' by default due to safety concerns.

commit 8e0e99ba64c7ba46133a7c8a3e3f7de01f23bd93 upstream.

It has come to my attention (thanks Martin) that 'discard_zeroes_data'
is only a hint.  Some devices in some cases don't do what it
says on the label.

The use of DISCARD in RAID5 depends on reads from discarded regions
being predictably zero.  If a write to a previously discarded region
performs a read-modify-write cycle it assumes that the parity block
was consistent with the data blocks.  If all were zero, this would
be the case.  If some are and some aren't this would not be the case.
This could lead to data corruption after a device failure when
data needs to be reconstructed from the parity.

As we cannot trust 'discard_zeroes_data', ignore it by default
and so disallow DISCARD on all raid4/5/6 arrays.

As many devices are trustworthy, and as there are benefits to using
DISCARD, add a module parameter to over-ride this caution and cause
DISCARD to work if discard_zeroes_data is set.

If a site want to enable DISCARD on some arrays but not on others they
should select DISCARD support at the filesystem level, and set the
raid456 module parameter.

As this is a data-safety issue, I believe this patch is suitable for
DISCARD support for RAID456 was added in 3.7

Cc: Shaohua Li <shli at kernel.org>
Cc: "Martin K. Petersen" <martin.petersen at oracle.com>
Cc: Mike Snitzer <snitzer at redhat.com>
Cc: Heinz Mauelshagen <heinzm at redhat.com>
Acked-by: Martin K. Petersen <martin.petersen at oracle.com>
Acked-by: Mike Snitzer <snitzer at redhat.com>
Fixes: 620125f2bf8ff0c4969b79653b54d7bcc9d40637
Signed-off-by: NeilBrown <neilb at suse.de>
Signed-off-by: Kamal Mostafa <kamal at canonical.com>
 drivers/md/raid5.c | 18 +++++++++++++++++-
 1 file changed, 17 insertions(+), 1 deletion(-)

diff --git a/drivers/md/raid5.c b/drivers/md/raid5.c
index fd900ef..498ccf3 100644
--- a/drivers/md/raid5.c
+++ b/drivers/md/raid5.c
@@ -64,6 +64,10 @@
 #define cpu_to_group(cpu) cpu_to_node(cpu)

+static bool devices_handle_discard_safely = false;
+module_param(devices_handle_discard_safely, bool, 0644);
+		 "Set to Y if all devices in each array reliably return zeroes on reads from discarded regions");
 static struct workqueue_struct *raid5_wq;
  * Stripe cache
@@ -6114,7 +6118,7 @@ static int run(struct mddev *mddev)
 		mddev->queue->limits.discard_granularity = stripe;
 		 * unaligned part of discard request will be ignored, so can't
-		 * guarantee discard_zerors_data
+		 * guarantee discard_zeroes_data
 		mddev->queue->limits.discard_zeroes_data = 0;

@@ -6139,6 +6143,18 @@ static int run(struct mddev *mddev)
 				discard_supported = false;
+			/* Unfortunately, discard_zeroes_data is not currently
+			 * a guarantee - just a hint.  So we only allow DISCARD
+			 * if the sysadmin has confirmed that only safe devices
+			 * are in use by setting a module parameter.
+			 */
+			if (!devices_handle_discard_safely) {
+				if (discard_supported) {
+					pr_info("md/raid456: discard support disabled due to uncertainty.\n");
+					pr_info("Set raid456.devices_handle_discard_safely=Y to override.\n");
+				}
+				discard_supported = false;
+			}

 		if (discard_supported &&

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