[PATCH 0/2] Configurable max discard size

[PATCH 0/2] Configurable max discard size

[Jens Axboe]

Hi,

Most drivers use UINT_MAX (or some variant thereof) for max discard
size, since they don't have a real limit for a non-data transferring
command. This is fine from a throughput point of view, but for a lot
of devices (all?), it truly sucks on latency. We've seen cases of
hundreds of msec in latencies for reads/writes when deleting files
on an fs with discard enabled. For the problematic devices that we
have tested, artificially limiting the size of the discards issued
brings it down to a more manageable 1-2ms max latencies.

[PATCH 1/2] block: have drivers use blk_queue_max_discard_sectors()

[Jens Axboe]

Some drivers use it now, others just set the limits field manually.
But in preparation for splitting this into a hard and soft limit,
ensure that they all call the proper function for setting the hw
limit for discards.

Signed-off-by: Jens Axboe <axboe@fb.com>
---
 drivers/block/brd.c           | 2 +-
 drivers/block/drbd/drbd_nl.c  | 4 ++--
 drivers/block/loop.c          | 4 ++--
 drivers/block/nbd.c           | 2 +-
 drivers/block/nvme-core.c     | 2 +-
 drivers/block/rbd.c           | 2 +-
 drivers/block/skd_main.c      | 2 +-
 drivers/block/zram/zram_drv.c | 2 +-
 drivers/md/bcache/super.c     | 2 +-
 drivers/mmc/card/queue.c      | 2 +-
 drivers/mtd/mtd_blkdevs.c     | 2 +-
 drivers/scsi/sd.c             | 4 ++--
 12 files changed, 15 insertions(+), 15 deletions(-)

diff --git a/drivers/block/brd.c b/drivers/block/brd.c
index 64ab4951e9d6..e573e470bd8a 100644
--- a/drivers/block/brd.c
+++ b/drivers/block/brd.c
@@ -500,7 +500,7 @@ static struct brd_device *brd_alloc(int i)
 	blk_queue_physical_block_size(brd->brd_queue, PAGE_SIZE);
 
 	brd->brd_queue->limits.discard_granularity = PAGE_SIZE;
-	brd->brd_queue->limits.max_discard_sectors = UINT_MAX;
+	blk_queue_max_discard_sectors(brd->brd_queue, UINT_MAX);
 	brd->brd_queue->limits.discard_zeroes_data = 1;
 	queue_flag_set_unlocked(QUEUE_FLAG_DISCARD, brd->brd_queue);
 
diff --git a/drivers/block/drbd/drbd_nl.c b/drivers/block/drbd/drbd_nl.c
index 74df8cfad414..e80cbefbc2b5 100644
--- a/drivers/block/drbd/drbd_nl.c
+++ b/drivers/block/drbd/drbd_nl.c
@@ -1156,14 +1156,14 @@ static void drbd_setup_queue_param(struct drbd_device *device, struct drbd_backi
 			/* For now, don't allow more than one activity log extent worth of data
 			 * to be discarded in one go. We may need to rework drbd_al_begin_io()
 			 * to allow for even larger discard ranges */
-			q->limits.max_discard_sectors = DRBD_MAX_DISCARD_SECTORS;
+			blk_queue_max_discard_sectors(q, DRBD_MAX_DISCARD_SECTORS);
 
 			queue_flag_set_unlocked(QUEUE_FLAG_DISCARD, q);
 			/* REALLY? Is stacking secdiscard "legal"? */
 			if (blk_queue_secdiscard(b))
 				queue_flag_set_unlocked(QUEUE_FLAG_SECDISCARD, q);
 		} else {
-			q->limits.max_discard_sectors = 0;
+			blk_queue_max_discard_sectors(q, 0);
 			queue_flag_clear_unlocked(QUEUE_FLAG_DISCARD, q);
 			queue_flag_clear_unlocked(QUEUE_FLAG_SECDISCARD, q);
 		}
diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index f7a4c9d7f721..f9889b6bc02c 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -675,7 +675,7 @@ static void loop_config_discard(struct loop_device *lo)
 	    lo->lo_encrypt_key_size) {
 		q->limits.discard_granularity = 0;
 		q->limits.discard_alignment = 0;
-		q->limits.max_discard_sectors = 0;
+		blk_queue_max_discard_sectors(q, 0);
 		q->limits.discard_zeroes_data = 0;
 		queue_flag_clear_unlocked(QUEUE_FLAG_DISCARD, q);
 		return;
@@ -683,7 +683,7 @@ static void loop_config_discard(struct loop_device *lo)
 
 	q->limits.discard_granularity = inode->i_sb->s_blocksize;
 	q->limits.discard_alignment = 0;
-	q->limits.max_discard_sectors = UINT_MAX >> 9;
+	blk_queue_max_discard_sectors(q, UINT_MAX >> 9);
 	q->limits.discard_zeroes_data = 1;
 	queue_flag_set_unlocked(QUEUE_FLAG_DISCARD, q);
 }
diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c
index 0e385d8e9b86..f169faf9838a 100644
--- a/drivers/block/nbd.c
+++ b/drivers/block/nbd.c
@@ -822,7 +822,7 @@ static int __init nbd_init(void)
 		queue_flag_set_unlocked(QUEUE_FLAG_NONROT, disk->queue);
 		queue_flag_clear_unlocked(QUEUE_FLAG_ADD_RANDOM, disk->queue);
 		disk->queue->limits.discard_granularity = 512;
-		disk->queue->limits.max_discard_sectors = UINT_MAX;
+		blk_queue_max_discard_sectors(disk->queue, UINT_MAX);
 		disk->queue->limits.discard_zeroes_data = 0;
 		blk_queue_max_hw_sectors(disk->queue, 65536);
 		disk->queue->limits.max_sectors = 256;
diff --git a/drivers/block/nvme-core.c b/drivers/block/nvme-core.c
index d1d6141920d3..b1eb9d321071 100644
--- a/drivers/block/nvme-core.c
+++ b/drivers/block/nvme-core.c
@@ -1935,7 +1935,7 @@ static void nvme_config_discard(struct nvme_ns *ns)
 	ns->queue->limits.discard_zeroes_data = 0;
 	ns->queue->limits.discard_alignment = logical_block_size;
 	ns->queue->limits.discard_granularity = logical_block_size;
-	ns->queue->limits.max_discard_sectors = 0xffffffff;
+	blk_queue_max_discard_sectors(ns->queue, 0xffffffff);
 	queue_flag_set_unlocked(QUEUE_FLAG_DISCARD, ns->queue);
 }
 
diff --git a/drivers/block/rbd.c b/drivers/block/rbd.c
index d94529d5c8e9..dcc86937f55c 100644
--- a/drivers/block/rbd.c
+++ b/drivers/block/rbd.c
@@ -3803,7 +3803,7 @@ static int rbd_init_disk(struct rbd_device *rbd_dev)
 	queue_flag_set_unlocked(QUEUE_FLAG_DISCARD, q);
 	q->limits.discard_granularity = segment_size;
 	q->limits.discard_alignment = segment_size;
-	q->limits.max_discard_sectors = segment_size / SECTOR_SIZE;
+	blk_queue_max_discard_sectors(q, segment_size / SECTOR_SIZE);
 	q->limits.discard_zeroes_data = 1;
 
 	blk_queue_merge_bvec(q, rbd_merge_bvec);
diff --git a/drivers/block/skd_main.c b/drivers/block/skd_main.c
index 1e46eb2305c0..586f9168ffa4 100644
--- a/drivers/block/skd_main.c
+++ b/drivers/block/skd_main.c
@@ -4422,7 +4422,7 @@ static int skd_cons_disk(struct skd_device *skdev)
 	/* DISCARD Flag initialization. */
 	q->limits.discard_granularity = 8192;
 	q->limits.discard_alignment = 0;
-	q->limits.max_discard_sectors = UINT_MAX >> 9;
+	blk_queue_max_discard_sectors(q, UINT_MAX >> 9);
 	q->limits.discard_zeroes_data = 1;
 	queue_flag_set_unlocked(QUEUE_FLAG_DISCARD, q);
 	queue_flag_set_unlocked(QUEUE_FLAG_NONROT, q);
diff --git a/drivers/block/zram/zram_drv.c b/drivers/block/zram/zram_drv.c
index fb655e8d1e3b..f439ad2800da 100644
--- a/drivers/block/zram/zram_drv.c
+++ b/drivers/block/zram/zram_drv.c
@@ -1244,7 +1244,7 @@ static int zram_add(void)
 	blk_queue_io_min(zram->disk->queue, PAGE_SIZE);
 	blk_queue_io_opt(zram->disk->queue, PAGE_SIZE);
 	zram->disk->queue->limits.discard_granularity = PAGE_SIZE;
-	zram->disk->queue->limits.max_discard_sectors = UINT_MAX;
+	blk_queue_max_discard_sectors(zram->disk->queue, UINT_MAX);
 	/*
 	 * zram_bio_discard() will clear all logical blocks if logical block
 	 * size is identical with physical block size(PAGE_SIZE). But if it is
diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
index 94980bfca434..fc8e545ced18 100644
--- a/drivers/md/bcache/super.c
+++ b/drivers/md/bcache/super.c
@@ -830,7 +830,7 @@ static int bcache_device_init(struct bcache_device *d, unsigned block_size,
 	q->limits.max_sectors		= UINT_MAX;
 	q->limits.max_segment_size	= UINT_MAX;
 	q->limits.max_segments		= BIO_MAX_PAGES;
-	q->limits.max_discard_sectors	= UINT_MAX;
+	blk_queue_max_discard_sectors(q, UINT_MAX);
 	q->limits.discard_granularity	= 512;
 	q->limits.io_min		= block_size;
 	q->limits.logical_block_size	= block_size;
diff --git a/drivers/mmc/card/queue.c b/drivers/mmc/card/queue.c
index b5a2b145d89f..5daf302835b1 100644
--- a/drivers/mmc/card/queue.c
+++ b/drivers/mmc/card/queue.c
@@ -165,7 +165,7 @@ static void mmc_queue_setup_discard(struct request_queue *q,
 		return;
 
 	queue_flag_set_unlocked(QUEUE_FLAG_DISCARD, q);
-	q->limits.max_discard_sectors = max_discard;
+	blk_queue_max_discard_sectors(q, max_discard);
 	if (card->erased_byte == 0 && !mmc_can_discard(card))
 		q->limits.discard_zeroes_data = 1;
 	q->limits.discard_granularity = card->pref_erase << 9;
diff --git a/drivers/mtd/mtd_blkdevs.c b/drivers/mtd/mtd_blkdevs.c
index 41acc507b22e..1b96cf771d2b 100644
--- a/drivers/mtd/mtd_blkdevs.c
+++ b/drivers/mtd/mtd_blkdevs.c
@@ -423,7 +423,7 @@ int add_mtd_blktrans_dev(struct mtd_blktrans_dev *new)
 
 	if (tr->discard) {
 		queue_flag_set_unlocked(QUEUE_FLAG_DISCARD, new->rq);
-		new->rq->limits.max_discard_sectors = UINT_MAX;
+		blk_queue_max_discard_sectors(new->rq, UINT_MAX);
 	}
 
 	gd->queue = new->rq;
diff --git a/drivers/scsi/sd.c b/drivers/scsi/sd.c
index 3b2fcb4fada0..160e44e7b24a 100644
--- a/drivers/scsi/sd.c
+++ b/drivers/scsi/sd.c
@@ -647,7 +647,7 @@ static void sd_config_discard(struct scsi_disk *sdkp, unsigned int mode)
 	switch (mode) {
 
 	case SD_LBP_DISABLE:
-		q->limits.max_discard_sectors = 0;
+		blk_queue_max_discard_sectors(q, 0);
 		queue_flag_clear_unlocked(QUEUE_FLAG_DISCARD, q);
 		return;
 
@@ -675,7 +675,7 @@ static void sd_config_discard(struct scsi_disk *sdkp, unsigned int mode)
 		break;
 	}
 
-	q->limits.max_discard_sectors = max_blocks * (logical_block_size >> 9);
+	blk_queue_max_discard_sectors(q, max_blocks * (logical_block_size >> 9));
 	queue_flag_set_unlocked(QUEUE_FLAG_DISCARD, q);
 }
 
-- 
2.4.1.168.g1ea28e1

[PATCH 2/2] block: make /sys/block/<dev>/queue/discard_max_bytes writeable

[Jens Axboe]

Lots of devices support huge discard sizes these days. Depending
on how the device handles them internally, huge discards can
introduce massive latencies (hundreds of msec) on the device side.

We have a sysfs file, discard_max_bytes, that advertises the max
hardware supported discard size. Make this writeable, and split
the settings into a soft and hard limit. This can be set from
'discard_granularity' and up to the hardware limit.

Signed-off-by: Jens Axboe <axboe@fb.com>
---
 Documentation/block/queue-sysfs.txt |  4 +++-
 block/blk-settings.c                |  4 ++++
 block/blk-sysfs.c                   | 26 +++++++++++++++++++++++++-
 include/linux/blkdev.h              |  1 +
 4 files changed, 33 insertions(+), 2 deletions(-)

diff --git a/Documentation/block/queue-sysfs.txt b/Documentation/block/queue-sysfs.txt
index 3a29f8914df9..3748cf827131 100644
--- a/Documentation/block/queue-sysfs.txt
+++ b/Documentation/block/queue-sysfs.txt
@@ -20,7 +20,7 @@ This shows the size of internal allocation of the device in bytes, if
 reported by the device. A value of '0' means device does not support
 the discard functionality.
 
-discard_max_bytes (RO)
+discard_max_bytes (RW)
 ----------------------
 Devices that support discard functionality may have internal limits on
 the number of bytes that can be trimmed or unmapped in a single operation.
@@ -28,6 +28,8 @@ The discard_max_bytes parameter is set by the device driver to the maximum
 number of bytes that can be discarded in a single operation. Discard
 requests issued to the device must not exceed this limit. A discard_max_bytes
 value of 0 means that the device does not support discard functionality.
+Writing a lower value to this file can limit the maximum discard size issued
+to the device, which can help latencies.
 
 discard_zeroes_data (RO)
 ------------------------
diff --git a/block/blk-settings.c b/block/blk-settings.c
index 12600bfffca9..b38d8d723276 100644
--- a/block/blk-settings.c
+++ b/block/blk-settings.c
@@ -116,6 +116,7 @@ void blk_set_default_limits(struct queue_limits *lim)
 	lim->chunk_sectors = 0;
 	lim->max_write_same_sectors = 0;
 	lim->max_discard_sectors = 0;
+	lim->max_hw_discard_sectors = 0;
 	lim->discard_granularity = 0;
 	lim->discard_alignment = 0;
 	lim->discard_misaligned = 0;
@@ -303,6 +304,7 @@ EXPORT_SYMBOL(blk_queue_chunk_sectors);
 void blk_queue_max_discard_sectors(struct request_queue *q,
 		unsigned int max_discard_sectors)
 {
+	q->limits.max_hw_discard_sectors = max_discard_sectors;
 	q->limits.max_discard_sectors = max_discard_sectors;
 }
 EXPORT_SYMBOL(blk_queue_max_discard_sectors);
@@ -641,6 +643,8 @@ int blk_stack_limits(struct queue_limits *t, struct queue_limits *b,
 
 		t->max_discard_sectors = min_not_zero(t->max_discard_sectors,
 						      b->max_discard_sectors);
+		t->max_hw_discard_sectors = min_not_zero(t->max_hw_discard_sectors,
+							 b->max_hw_discard_sectors);
 		t->discard_granularity = max(t->discard_granularity,
 					     b->discard_granularity);
 		t->discard_alignment = lcm_not_zero(t->discard_alignment, alignment) %
diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
index 6264b382d4d1..3d1dba600228 100644
--- a/block/blk-sysfs.c
+++ b/block/blk-sysfs.c
@@ -151,6 +151,29 @@ static ssize_t queue_discard_max_show(struct request_queue *q, char *page)
 		       (unsigned long long)q->limits.max_discard_sectors << 9);
 }
 
+static ssize_t queue_discard_max_store(struct request_queue *q,
+				       const char *page, size_t count)
+{
+	unsigned long max_discard;
+	ssize_t ret = queue_var_store(&max_discard, page, count);
+
+	if (ret < 0)
+		return ret;
+
+	if (max_discard & (q->limits.discard_granularity - 1))
+		return -EINVAL;
+
+	max_discard >>= 9;
+	if (max_discard > UINT_MAX)
+		return -EINVAL;
+
+	if (max_discard > q->limits.max_hw_discard_sectors)
+		max_discard = q->limits.max_hw_discard_sectors;
+
+	q->limits.max_discard_sectors = max_discard;
+	return ret;
+}
+
 static ssize_t queue_discard_zeroes_data_show(struct request_queue *q, char *page)
 {
 	return queue_var_show(queue_discard_zeroes_data(q), page);
@@ -361,8 +384,9 @@ static struct queue_sysfs_entry queue_discard_granularity_entry = {
 };
 
 static struct queue_sysfs_entry queue_discard_max_entry = {
-	.attr = {.name = "discard_max_bytes", .mode = S_IRUGO },
+	.attr = {.name = "discard_max_bytes", .mode = S_IRUGO | S_IWUSR },
 	.show = queue_discard_max_show,
+	.store = queue_discard_max_store,
 };
 
 static struct queue_sysfs_entry queue_discard_zeroes_data_entry = {
diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
index d4068c17d0df..243f29e779ec 100644
--- a/include/linux/blkdev.h
+++ b/include/linux/blkdev.h
@@ -268,6 +268,7 @@ struct queue_limits {
 	unsigned int		io_min;
 	unsigned int		io_opt;
 	unsigned int		max_discard_sectors;
+	unsigned int		max_hw_discard_sectors;
 	unsigned int		max_write_same_sectors;
 	unsigned int		discard_granularity;
 	unsigned int		discard_alignment;
-- 
2.4.1.168.g1ea28e1

Re: [PATCH 2/2] block: make /sys/block/<dev>/queue/discard_max_bytes writeable

[Mike Snitzer]

On Tue, Jul 14 2015 at 11:02am -0400,
Jens Axboe <axboe@fb.com> wrote:

> Lots of devices support huge discard sizes these days. Depending
> on how the device handles them internally, huge discards can
> introduce massive latencies (hundreds of msec) on the device side.
> 
> We have a sysfs file, discard_max_bytes, that advertises the max
> hardware supported discard size. Make this writeable, and split
> the settings into a soft and hard limit. This can be set from
> 'discard_granularity' and up to the hardware limit.

Looks pretty good, but we'll lose the original discard_max_bytes once it
is changed.  That information loss will prevent users from knowing what
adjustments are possible over time.

This may be OK, but figured i'd raise it.

Re: [PATCH 2/2] block: make /sys/block/<dev>/queue/discard_max_bytes writeable

[Jens Axboe]

On 07/14/2015 09:23 AM, Mike Snitzer wrote:
> On Tue, Jul 14 2015 at 11:02am -0400,
> Jens Axboe <axboe@fb.com> wrote:
>
>> Lots of devices support huge discard sizes these days. Depending
>> on how the device handles them internally, huge discards can
>> introduce massive latencies (hundreds of msec) on the device side.
>>
>> We have a sysfs file, discard_max_bytes, that advertises the max
>> hardware supported discard size. Make this writeable, and split
>> the settings into a soft and hard limit. This can be set from
>> 'discard_granularity' and up to the hardware limit.
>
> Looks pretty good, but we'll lose the original discard_max_bytes once it
> is changed.  That information loss will prevent users from knowing what
> adjustments are possible over time.
>
> This may be OK, but figured i'd raise it.

That's true, I should have mentioned that. But if you write a higher 
value than the device supports, then it will be truncated to the max 
value that the device supports. So it's not really lost, and it was the 
best alternative I could think of.

-- 
Jens Axboe

Re: [PATCH 0/2] Configurable max discard size

[Christoph Hellwig]

On Tue, Jul 14, 2015 at 09:02:17AM -0600, Jens Axboe wrote:
> Hi,
> 
> Most drivers use UINT_MAX (or some variant thereof) for max discard
> size, since they don't have a real limit for a non-data transferring
> command. This is fine from a throughput point of view, but for a lot
> of devices (all?), it truly sucks on latency. We've seen cases of
> hundreds of msec in latencies for reads/writes when deleting files
> on an fs with discard enabled. For the problematic devices that we
> have tested, artificially limiting the size of the discards issued
> brings it down to a more manageable 1-2ms max latencies.

This looks reasonable to me.  Any chance you could also come up
with reasonable start values for the hardware you've done this for
so that we can get a good out of the box experience?

Re: [PATCH 0/2] Configurable max discard size

[Jens Axboe]

On 07/14/2015 10:01 AM, Christoph Hellwig wrote:
> On Tue, Jul 14, 2015 at 09:02:17AM -0600, Jens Axboe wrote:
>> Hi,
>>
>> Most drivers use UINT_MAX (or some variant thereof) for max discard
>> size, since they don't have a real limit for a non-data transferring
>> command. This is fine from a throughput point of view, but for a lot
>> of devices (all?), it truly sucks on latency. We've seen cases of
>> hundreds of msec in latencies for reads/writes when deleting files
>> on an fs with discard enabled. For the problematic devices that we
>> have tested, artificially limiting the size of the discards issued
>> brings it down to a more manageable 1-2ms max latencies.
>
> This looks reasonable to me.  Any chance you could also come up
> with reasonable start values for the hardware you've done this for
> so that we can get a good out of the box experience?

Based on experimentation with two different vendors, 64MB would be a 
good default. Question is how best to set that. At least with the 
current patch, untouched, 'discard_max_bytes' will still show the hw max 
value. If I default it to 64MB, we'd lose that information until people 
started bumping it up in size. Maybe that's not such a big deal, however.

-- 
Jens Axboe

Re: [PATCH 0/2] Configurable max discard size

[Christoph Hellwig]

On Tue, Jul 14, 2015 at 10:04:12AM -0600, Jens Axboe wrote:
> Based on experimentation with two different vendors, 64MB would be a good 
> default. Question is how best to set that. At least with the current patch, 
> untouched, 'discard_max_bytes' will still show the hw max value. If I 
> default it to 64MB, we'd lose that information until people started bumping 
> it up in size. Maybe that's not such a big deal, however.

Just expose the max value as another read-only sysfs value?

Re: [PATCH 0/2] Configurable max discard size

[Jens Axboe]

On 07/14/2015 11:53 AM, Christoph Hellwig wrote:
> On Tue, Jul 14, 2015 at 10:04:12AM -0600, Jens Axboe wrote:
>> Based on experimentation with two different vendors, 64MB would be a good
>> default. Question is how best to set that. At least with the current patch,
>> untouched, 'discard_max_bytes' will still show the hw max value. If I
>> default it to 64MB, we'd lose that information until people started bumping
>> it up in size. Maybe that's not such a big deal, however.
>
> Just expose the max value as another read-only sysfs value?

Sure, just didn't want to clutter it with Yet Another sysfs file. But we 
could add 'discard_max_hw_bytes', that would be the obvious choice.

-- 
Jens Axboe