From: Jes Sorensen <Jes.Sorensen@xxxxxxxxxx>
Some SSDs handle discard requests badly (very slowly). As an example,
mkfs.xfs on a RAID0 array with 4KB chunk size, constructed from Intel
SSDSC2BF18 180GB SSDs, can jump from 1.2secs to 43mins.
There is no reliable way to easily determine whether a device handles
discard at decent speed, and we cannot just benchmark it since it will
destroy data on the drive.
Signed-off-by: Jes Sorensen <Jes.Sorensen@xxxxxxxxxx>
---
drivers/md/raid0.c | 20 ++++++++++++++++++++
1 file changed, 20 insertions(+)
diff --git a/drivers/md/raid0.c b/drivers/md/raid0.c
index 4f19837..cc8c3c3 100644
--- a/drivers/md/raid0.c
+++ b/drivers/md/raid0.c
@@ -25,6 +25,11 @@
#include "raid0.h"
#include "raid5.h"
+static bool devices_discard_performance = false;
+module_param(devices_discard_performance, bool, 0644);
+MODULE_PARM_DESC(devices_discard_performance,
+ "Set to Y if all devices in each array handles discard requests at proper speed");
+
static int raid0_congested(struct mddev *mddev, int bits)
{
struct r0conf *conf = mddev->private;
@@ -277,6 +282,21 @@ static int create_strip_zones(struct mddev *mddev, struct r0conf **private_conf)
blk_queue_io_opt(mddev->queue,
(mddev->chunk_sectors << 9) * mddev->raid_disks);
+ /* Unfortunately, some devices have awful discard performance,
+ * especially for small sized requests. This is particularly
+ * bad for RAID0 with a small chunk size resulting in a small
+ * DISCARD requests hitting the underlaying drives.
+ * Only allow DISCARD if the sysadmin confirms that all devices
+ * in use can handle small DISCARD requests at reasonable speed,
+ * by setting a module parameter.
+ */
+ if (!devices_discard_performance) {
+ if (discard_supported) {
+ pr_info("md/raid0: discard support disabled due to performance uncertainty.\n");
+ pr_info("Set raid0.devices_discard_performance=Y to override.\n");
+ }
+ discard_supported = false;
+ }
if (!discard_supported)
queue_flag_clear_unlocked(QUEUE_FLAG_DISCARD, mddev->queue);
else
--
2.4.3
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