This patch adds RAID1 read balancing to device mapper. A read operation
that is close (in terms of sectors) to a previous read or write goes to
the same mirror.
Signed-off-by: Konstantin Sharlaimov <konstantin.sharlaimov@xxxxxxxxx>
---
Please give it a try, it works for me, yet my results might be system-specific.
Any feedback (bug-reports, suggestions) will be greatly appreciated.
--- linux-2.6.23.1/drivers/md/dm-raid1.c.old 2007-11-03 18:47:10.000000000 +1000
+++ linux-2.6.23.1/drivers/md/dm-raid1.c 2007-11-03 19:54:35.000000000 +1000
@@ -19,6 +19,7 @@
#include <linux/time.h>
#include <linux/vmalloc.h>
#include <linux/workqueue.h>
+#include <linux/random.h>
#define DM_MSG_PREFIX "raid1"
#define DM_IO_PAGES 64
@@ -26,6 +27,9 @@
#define DM_RAID1_HANDLE_ERRORS 0x01
#define errors_handled(p) ((p)->features & DM_RAID1_HANDLE_ERRORS)
+/* Read balancing max hdd head distance */
+#define DM_RAID1_BALANCE_MAX_IO_DISTANCE (256)
+
static DECLARE_WAIT_QUEUE_HEAD(_kmirrord_recovery_stopped);
/*-----------------------------------------------------------------
@@ -116,6 +120,7 @@ struct mirror {
atomic_t error_count;
struct dm_dev *dev;
sector_t offset;
+ sector_t last_io_sector;
};
struct mirror_set {
@@ -741,13 +746,51 @@ static void do_recovery(struct mirror_se
}
}
+static void set_mirror_last_io_sector(struct mirror *m, sector_t sector)
+{
+ /* FIXME: Probably some more work is needed here, however this is unlikely */
+ m->last_io_sector = sector;
+}
+
/*-----------------------------------------------------------------
* Reads
*---------------------------------------------------------------*/
+/*
+ * There is a per-array 'last IO operation' sector number maintained by
+ * read and write handlers for the region. When balancing reads we pick
+ * the disk whose IO operation (HDD head position) is closest.
+ */
static struct mirror *choose_mirror(struct mirror_set *ms, sector_t sector)
{
- /* FIXME: add read balancing */
- return ms->default_mirror;
+ /* If we got here, then the array is in sync and we can pick any mirror */
+
+ unsigned int i;
+ struct mirror *use_mirror;
+ sector_t use_distance, new_distance;
+
+ use_mirror = &ms->mirror[0];
+ use_distance = abs(sector - ms->mirror[0].last_io_sector);
+
+ for (i = 1; i < ms->nr_mirrors; i++) {
+ new_distance = abs(sector - ms->mirror[i].last_io_sector);
+ if (new_distance < use_distance) {
+ use_distance = new_distance;
+ use_mirror = &ms->mirror[i];
+ }
+ }
+
+ /*
+ * If the HDD head is too far from the needed sector then we do stochastic
+ * balancing - chose the mirror randomly. This appers to have a better
+ * chance of chosing an idle disk in case of two or more regions residing
+ * on the same physical disk.
+ *
+ * TODO: Gather more statistical data and verify that the above is correct
+ */
+ if (use_distance > DM_RAID1_BALANCE_MAX_IO_DISTANCE)
+ return &ms->mirror[random32() % ms->nr_mirrors];
+ else
+ return use_mirror;
}
/*
@@ -776,6 +819,9 @@ static void do_reads(struct mirror_set *
else
m = ms->default_mirror;
+ /* Set last IO position for chosen mirror */
+ set_mirror_last_io_sector(m, bio->bi_sector);
+
map_bio(ms, m, bio);
generic_make_request(bio);
}
@@ -800,6 +846,21 @@ static void write_callback(unsigned long
ms = bio_get_ms(bio);
bio_set_ms(bio, NULL);
+
+ /*
+ * Things might be different for various region states:
+ * SYNC: writing is done to all mirrors, reading is balanced
+ * RECOVERING: writing is delayed, reading is done from the default
+ * NOSYNC: writing to default only, reading from the default
+ *
+ * In any case, if we update last IO sector at all mirrors, we will use
+ * the up-to-date data when doing read balancing
+ *
+ * FIXME: update write position only on the region being written
+ */
+
+ for (i = 0; i < ms->nr_mirrors; i++)
+ set_mirror_last_io_sector(&ms->mirror[i], bio->bi_sector);
/*
* NOTE: We don't decrement the pending count here,
-
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