From: Robert Collins <robertc@robertcollins.net>
To: dm-devel@redhat.com
Subject: Load balancing reads on dmraid1 (and 01) arrays
Date: Tue, 5 Jun 2012 12:54:42 +1200 [thread overview]
Message-ID: <CAJ3HoZ0ZsTv1D4vdnQct30gYbejHNvLJOTAaJXHJO8w7d199iA@mail.gmail.com> (raw)
[-- Attachment #1: Type: text/plain, Size: 1973 bytes --]
Hi, I've had this sitting in a local tree for a bit (> 12 months -
blush!), been meaning to send it here...
Its my first dm submission, so please be gentle :) - I'll happily fix
up any issues in the patch (or in other metadata) - though I have
limited bandwidth to spend on this, so I may not get around to it
immediately.
The attached patches change dmraid1 to use all backing devices to
satisfy read requests rather than just one.
The first uses a very simple heuristic - which ever device had the
most recent read dispatched to a given location is preferred for
subsequent reads. There is an obvious theoretical issue with this:
writes move the disk head as well and so effectively should reset
allow the devices to the same position. The problem though is that
this really is a scheduling problem, and one that the multipath stack
would be better at handling. (e.g. multipath shoud be aware of corner
cases like : what if there is a dispatched read on the device we think
is closest, *after* a write. or what if load isn't being balanced
because all the reads are happening in one spot and one device is
totally idle). So, I used a very simple heuristic, which works well
for me - I get significantly better read performance with the first
patch than without.
The second patch tweaks the heuristic to handle the case where one
disk is being monopolised, which I found happens quite frequently with
e.g. kernel builds, where there is a high locality of reference to the
content. Again, its not perfect, and in this case there is a horrible
magic constant :(.
Again, *I* find this a net win, but I have no idea whether other folk
will find it so.
I had a brief look at making this use multipath instead, but that
looks to my naive eye to be both a significant undertaking (which I
don't have time for - sorry!) and also perhaps conceptually
problematic, as this needs to be an asymmetric multipath: reads can
multipath but writes must hit all devices.
Cheers,
Rob
[-- Attachment #2: 0001-Load-balance-dm-raid1-reads-by-most-recently-dispatc.patch --]
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From a7047dcb4f19335089de39bcc5faee27e68f1b74 Mon Sep 17 00:00:00 2001
From: Robert Collins <robertc@robertcollins.net>
Date: Fri, 8 Jul 2011 22:42:02 +1200
Subject: [PATCH 1/2] Load balance dm-raid1 reads by most recently dispatched
sector.
---
drivers/md/dm-raid1.c | 97 ++++++++++++++++++++++++++++++++++++++++++-------
1 file changed, 83 insertions(+), 14 deletions(-)
diff --git a/drivers/md/dm-raid1.c b/drivers/md/dm-raid1.c
index 9bfd057..e16a030 100644
--- a/drivers/md/dm-raid1.c
+++ b/drivers/md/dm-raid1.c
@@ -26,6 +26,14 @@
#define DM_RAID1_HANDLE_ERRORS 0x01
#define errors_handled(p) ((p)->features & DM_RAID1_HANDLE_ERRORS)
+#ifndef PRINTK
+# if DEBUG > 0
+# define PRINTK(x...) printk(KERN_DEBUG x)
+# else
+# define PRINTK(x...)
+# endif
+#endif
+
static DECLARE_WAIT_QUEUE_HEAD(_kmirrord_recovery_stopped);
/*-----------------------------------------------------------------
@@ -44,6 +52,7 @@ struct mirror {
unsigned long error_type;
struct dm_dev *dev;
sector_t offset;
+ sector_t last_read_position; /* let us map sequential IO to one disk */
};
struct mirror_set {
@@ -403,19 +412,77 @@ static void do_recovery(struct mirror_set *ms)
/*-----------------------------------------------------------------
* Reads
*---------------------------------------------------------------*/
-static struct mirror *choose_mirror(struct mirror_set *ms, sector_t sector)
+static sector_t bio_distance(struct mirror *m, sector_t sector)
{
- struct mirror *m = get_default_mirror(ms);
-
- do {
- if (likely(!atomic_read(&m->error_count)))
- return m;
-
- if (m-- == ms->mirror)
- m += ms->nr_mirrors;
- } while (m != get_default_mirror(ms));
+ return m->last_read_position > sector ? m->last_read_position - sector : sector - m->last_read_position;
+}
- return NULL;
+static struct mirror *choose_mirror(struct mirror_set *ms, sector_t sector, sector_t count, bool update_read_pos)
+{
+ struct mirror *m, *closest = NULL;
+ sector_t distance;
+ sector_t temp_distance;
+ PRINTK("Choosing mirror: %llu %llu %d\n",
+ (unsigned long long) sector,
+ (unsigned long long) count,
+ (int) update_read_pos);
+ /* Find the first usable */
+ for (m = &ms->mirror[ms->nr_mirrors]; m != ms->mirror;) {
+ if (likely(!atomic_read(&(--m)->error_count))) {
+ distance = bio_distance(m, sector);
+ closest = m;
+ PRINTK("Choosing mirror: %llu %llu %d: closest=%p mirror=%p distance=%llu last-read %llu\n",
+ (unsigned long long) sector,
+ (unsigned long long) count,
+ (int) update_read_pos,
+ closest, ms->mirror, (unsigned long long)distance,
+ (unsigned long long)m->last_read_position);
+ break;
+ } else {
+ PRINTK("Choosing mirror: %llu %llu %d: has-errors %d\n",
+ (unsigned long long) sector,
+ (unsigned long long) count,
+ (int) update_read_pos,
+ atomic_read(&m->error_count));
+ }
+ }
+ /* Nothing usable */
+ if (unlikely(closest == NULL))
+ return NULL;
+ if (unlikely(closest == ms->mirror))
+ return closest;
+ /* Now see if there is a closer mirror */
+ for (m = closest; m != ms->mirror;) {
+ if (unlikely(atomic_read(&(--m)->error_count))){
+ PRINTK("Choosing mirror: %llu %llu %d: has-errors %d\n",
+ (unsigned long long) sector,
+ (unsigned long long) count,
+ (int) update_read_pos,
+ atomic_read(&m->error_count));
+ continue;
+ }
+ temp_distance = bio_distance(m, sector);
+ PRINTK("Choosing mirror: %llu %llu %d: closest=%p mirror=%p distance=%llu, temp_distance=%llu last-read %llu\n",
+ (unsigned long long) sector,
+ (unsigned long long) count,
+ (int) update_read_pos,
+ closest, ms->mirror,
+ (unsigned long long) distance,
+ (unsigned long long) temp_distance,
+ (unsigned long long)m->last_read_position);
+ if (temp_distance < distance) {
+ distance = temp_distance;
+ closest = m;
+ }
+ }
+ /* TODO: also track where IO's have completed against: tracking submits
+ * lets us handle sequential requests, but the disks probably have ahci
+ * tagging - if we know where the disk is after either a write or a
+ * read we can dispatch nearer it.
+ */
+ if (likely(update_read_pos))
+ closest->last_read_position = sector + count;
+ return closest;
}
static int default_ok(struct mirror *m)
@@ -431,7 +498,7 @@ static int mirror_available(struct mirror_set *ms, struct bio *bio)
region_t region = dm_rh_bio_to_region(ms->rh, bio);
if (log->type->in_sync(log, region, 0))
- return choose_mirror(ms, bio->bi_sector) ? 1 : 0;
+ return choose_mirror(ms, bio->bi_sector, bio->bi_size >> 9, false) ? 1 : 0;
return 0;
}
@@ -500,6 +567,7 @@ static void read_callback(unsigned long error, void *context)
bio_set_m(bio, NULL);
if (likely(!error)) {
+ /* Should update head position here */
bio_endio(bio, 0);
return;
}
@@ -558,7 +626,7 @@ static void do_reads(struct mirror_set *ms, struct bio_list *reads)
* We can only read balance if the region is in sync.
*/
if (likely(region_in_sync(ms, region, 1)))
- m = choose_mirror(ms, bio->bi_sector);
+ m = choose_mirror(ms, bio->bi_sector, bio->bi_size >> 9, 1);
else if (m && atomic_read(&m->error_count))
m = NULL;
@@ -940,6 +1008,7 @@ static int get_mirror(struct mirror_set *ms, struct dm_target *ti,
atomic_set(&(ms->mirror[mirror].error_count), 0);
ms->mirror[mirror].error_type = 0;
ms->mirror[mirror].offset = offset;
+ ms->mirror[mirror].last_read_position = 0;
return 0;
}
@@ -1181,7 +1250,7 @@ static int mirror_map(struct dm_target *ti, struct bio *bio,
* The region is in-sync and we can perform reads directly.
* Store enough information so we can retry if it fails.
*/
- m = choose_mirror(ms, bio->bi_sector);
+ m = choose_mirror(ms, bio->bi_sector, bio->bi_size >> 9, 1);
if (unlikely(!m))
return -EIO;
--
1.7.9.5
[-- Attachment #3: 0002-Force-switching-drives-after-enough-linear-IO-allows.patch --]
[-- Type: application/octet-stream, Size: 7553 bytes --]
From 693806f872ff119aa526216ee45628e929c386f6 Mon Sep 17 00:00:00 2001
From: Robert Collins <robertc@robertcollins.net>
Date: Mon, 4 Jun 2012 15:01:14 +1200
Subject: [PATCH 2/2] Force switching drives after enough linear IO - allows
readahead to exercise parallelism as well.
---
drivers/md/dm-raid1.c | 59 ++++++++++++++++++++++++++++++++++++++++++-------
1 file changed, 51 insertions(+), 8 deletions(-)
diff --git a/drivers/md/dm-raid1.c b/drivers/md/dm-raid1.c
index e16a030..b9504d8 100644
--- a/drivers/md/dm-raid1.c
+++ b/drivers/md/dm-raid1.c
@@ -22,11 +22,19 @@
#define DM_MSG_PREFIX "raid1"
#define MAX_RECOVERY 1 /* Maximum number of regions recovered in parallel. */
+/* Maximum sector count of sequential IO sent to one mirror. This needs to be
+ * large enough not to saturate drive command queues but small enough that we
+ * can keep both drives active.
+ */
+#define DM_SEQUENTIAL_IO_MIRROR_LIMIT 131072
#define DM_RAID1_HANDLE_ERRORS 0x01
#define errors_handled(p) ((p)->features & DM_RAID1_HANDLE_ERRORS)
#ifndef PRINTK
+# ifndef DEBUG
+# define DEBUG 0
+# endif
# if DEBUG > 0
# define PRINTK(x...) printk(KERN_DEBUG x)
# else
@@ -52,6 +60,7 @@ struct mirror {
unsigned long error_type;
struct dm_dev *dev;
sector_t offset;
+ sector_t sequential_io_start_position; /* lets us detect long runs of sequential IO for load balancing */
sector_t last_read_position; /* let us map sequential IO to one disk */
};
@@ -417,11 +426,18 @@ static sector_t bio_distance(struct mirror *m, sector_t sector)
return m->last_read_position > sector ? m->last_read_position - sector : sector - m->last_read_position;
}
+/* Return the number of sequential read IO in sectors undertaken so far by m */
+static sector_t sequential_ios(struct mirror *m)
+{
+ return m->last_read_position - m->sequential_io_start_position;
+}
+
static struct mirror *choose_mirror(struct mirror_set *ms, sector_t sector, sector_t count, bool update_read_pos)
{
struct mirror *m, *closest = NULL;
sector_t distance;
sector_t temp_distance;
+ bool prefer_other_mirror;
PRINTK("Choosing mirror: %llu %llu %d\n",
(unsigned long long) sector,
(unsigned long long) count,
@@ -431,12 +447,15 @@ static struct mirror *choose_mirror(struct mirror_set *ms, sector_t sector, sect
if (likely(!atomic_read(&(--m)->error_count))) {
distance = bio_distance(m, sector);
closest = m;
- PRINTK("Choosing mirror: %llu %llu %d: closest=%p mirror=%p distance=%llu last-read %llu\n",
+ PRINTK("Choosing mirror: %llu %llu %d: closest=%p mirror=%p distance=%llu last-read %llu sequential %llu\n",
(unsigned long long) sector,
(unsigned long long) count,
(int) update_read_pos,
closest, ms->mirror, (unsigned long long)distance,
- (unsigned long long)m->last_read_position);
+ (unsigned long long)m->last_read_position,
+ (unsigned long long)sequential_ios(m));
+ /* Actively prefer a different mirror on sequential IO if we have exceeded the threshold */
+ prefer_other_mirror = distance==0 && (sequential_ios(m) >= DM_SEQUENTIAL_IO_MIRROR_LIMIT);
break;
} else {
PRINTK("Choosing mirror: %llu %llu %d: has-errors %d\n",
@@ -449,9 +468,15 @@ static struct mirror *choose_mirror(struct mirror_set *ms, sector_t sector, sect
/* Nothing usable */
if (unlikely(closest == NULL))
return NULL;
+ /* Only one drive availale */
if (unlikely(closest == ms->mirror))
return closest;
- /* Now see if there is a closer mirror */
+ /* Now see if there is another usable mirror that is closer and has not
+ * had too much sequential read IO dispatched to it yet.
+ * TODO: When N = or > nr_mirrors sets of sequential read IO are
+ * happening, allow each mirror to specialise, and avoid extraneous
+ * seeking.
+ */
for (m = closest; m != ms->mirror;) {
if (unlikely(atomic_read(&(--m)->error_count))){
PRINTK("Choosing mirror: %llu %llu %d: has-errors %d\n",
@@ -462,17 +487,31 @@ static struct mirror *choose_mirror(struct mirror_set *ms, sector_t sector, sect
continue;
}
temp_distance = bio_distance(m, sector);
- PRINTK("Choosing mirror: %llu %llu %d: closest=%p mirror=%p distance=%llu, temp_distance=%llu last-read %llu\n",
+ PRINTK("Choosing mirror: %llu %llu %d: closest=%p m=%p distance=%llu, temp_distance=%llu last-read %llu sequential %llu prefer-other %d\n",
(unsigned long long) sector,
(unsigned long long) count,
(int) update_read_pos,
- closest, ms->mirror,
+ closest, m,
(unsigned long long) distance,
(unsigned long long) temp_distance,
- (unsigned long long)m->last_read_position);
- if (temp_distance < distance) {
+ (unsigned long long)m->last_read_position,
+ (unsigned long long)sequential_ios(m),
+ (int)prefer_other_mirror);
+ /* Use this valid mirror if:
+ * - the first usable mirror has hit its sequential limit
+ * - or this mirror is closer and (on sequential IO) has not hit its limit
+ */
+ if (prefer_other_mirror ||
+ (temp_distance < distance &&
+ (temp_distance != 0 || sequential_ios(m) < DM_SEQUENTIAL_IO_MIRROR_LIMIT))) {
distance = temp_distance;
closest = m;
+ prefer_other_mirror = false;
+ PRINTK("Choosing mirror: %llu %llu %d: selected %p\n",
+ (unsigned long long) sector,
+ (unsigned long long) count,
+ (int) update_read_pos,
+ m);
}
}
/* TODO: also track where IO's have completed against: tracking submits
@@ -480,8 +519,11 @@ static struct mirror *choose_mirror(struct mirror_set *ms, sector_t sector, sect
* tagging - if we know where the disk is after either a write or a
* read we can dispatch nearer it.
*/
- if (likely(update_read_pos))
+ if (likely(update_read_pos)) {
+ if (closest->last_read_position != sector)
+ closest->sequential_io_start_position = sector;
closest->last_read_position = sector + count;
+ }
return closest;
}
@@ -1009,6 +1051,7 @@ static int get_mirror(struct mirror_set *ms, struct dm_target *ti,
ms->mirror[mirror].error_type = 0;
ms->mirror[mirror].offset = offset;
ms->mirror[mirror].last_read_position = 0;
+ ms->mirror[mirror].sequential_io_start_position = 0;
return 0;
}
--
1.7.9.5
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next reply other threads:[~2012-06-05 0:54 UTC|newest]
Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top
2012-06-05 0:54 Robert Collins [this message]
2012-07-17 19:20 ` Load balancing reads on dmraid1 (and 01) arrays Alasdair G Kergon
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