This patch adds read balancing.  The round-robin method is used, with
MIN_READS (128) requests going to each device.

From: Jonathan E Brassow <jbrassow@redhat.com>

[The 128 needs changing into a runtime parameter.]

Index: linux-2.6.19-rc4/drivers/md/dm-raid1.c
===================================================================
--- linux-2.6.19-rc4.orig/drivers/md/dm-raid1.c	2006-11-01 21:40:51.000000000 +0000
+++ linux-2.6.19-rc4/drivers/md/dm-raid1.c	2006-11-01 21:40:53.000000000 +0000
@@ -129,6 +129,9 @@ struct mirror_set {
 	struct workqueue_struct *kmirrord_wq;
 	struct work_struct kmirrord_work;
 
+	atomic_t read_count;      /* Read counter for read balancing */
+	struct mirror *read_mirror; /* Last mirror read. */
+
 	unsigned int nr_mirrors;
 	struct mirror mirror[0];
 };
@@ -684,10 +687,46 @@ static void do_recovery(struct mirror_se
 /*-----------------------------------------------------------------
  * Reads
  *---------------------------------------------------------------*/
-static struct mirror *choose_mirror(struct mirror_set *ms, sector_t sector)
+/* Switch to next dev, via round-robin, after MIN_READS reads */
+#define MIN_READS 128
+
+/* choose_mirror
+ * @ms: the mirror set
+ *
+ * This function is used for read balancing.
+ *
+ * Returns: chosen mirror, or NULL on failure
+ */
+static struct mirror *choose_mirror(struct mirror_set *ms)
 {
-	/* FIXME: add read balancing */
-	return ms->default_mirror;
+	struct mirror *start_mirror = ms->read_mirror;
+
+	/*
+	 * Perform MIN_READS on each working mirror then
+	 * advance to the next one.  start_mirror stores
+	 * the first we tried, so we know when we're done.
+	 */
+	do {
+		if (likely(!atomic_read(&ms->read_mirror->error_count) &&
+			   !atomic_dec_and_test(&ms->read_count)))
+			goto use_mirror;
+
+		atomic_set(&ms->read_count, MIN_READS);
+
+		if (ms->read_mirror-- == ms->mirror)
+			ms->read_mirror += ms->nr_mirrors;
+
+	} while (ms->read_mirror != start_mirror);
+
+	/*
+	 * We've rejected every mirror.
+	 * Confirm that start_mirror can still be used.
+	 */
+	if (unlikely(atomic_read(&ms->read_mirror->error_count)))
+		return NULL;
+
+use_mirror:
+	return ms->read_mirror;
 }
 
 /*
@@ -712,7 +751,7 @@ static void do_reads(struct mirror_set *
 		 * We can only read balance if the region is in sync.
 		 */
 		if (rh_in_sync(&ms->rh, region, 0))
-			m = choose_mirror(ms, bio->bi_sector);
+			m = choose_mirror(ms);
 		else
 			m = ms->default_mirror;
 
@@ -898,6 +937,7 @@ static struct mirror_set *alloc_context(
 	ms->nr_regions = dm_sector_div_up(ti->len, region_size);
 	ms->in_sync = 0;
 	ms->default_mirror = &ms->mirror[DEFAULT_MIRROR];
+	ms->read_mirror = &ms->mirror[DEFAULT_MIRROR];
 
 	if (rh_init(&ms->rh, ms, dl, region_size, ms->nr_regions)) {
 		ti->error = "Error creating dirty region hash";
@@ -1147,7 +1187,7 @@ static int mirror_map(struct dm_target *
 		return DM_MAPIO_SUBMITTED;
 	}
 
-	m = choose_mirror(ms, bio->bi_sector);
+	m = choose_mirror(ms);
 	if (!m)
 		return -EIO;