[PATCH] Add optional round-robbin read balancing to RAID1

[Roy Keene <rkeene@xxxxxxxxxx>]

All,

	Below is a patch that adds support for an alternate read balancing 
strategy that blindly performs read round-robbin to the MD RAID1 driver. 
This can be configure per array via sysfs.  The default behaviour is to 
preserve existing behaviour (biased read balancing)

I am looking for comments and possible inclusion upstream.

The motivation behind this is that during a sequential read, under some 
circumstances balancing the I/O through the paths produces better 
throughput than reading sequentially from one device.

Thanks,
	Roy Keene



--- linux-2.6.35.4/drivers/md/raid1.c	2010-08-26 18:47:12.000000000 -0500
+++ linux-2.6.35.4-2rrrr/drivers/md/raid1.c	2010-09-06 14:50:15.309000014 -0500
@@ -51,6 +51,52 @@
  */
 #define	NR_RAID1_BIOS 256

+static ssize_t raid1_show_mode_roundrobbin(mddev_t *mddev, char *page)
+{
+	conf_t *conf = mddev->private;
+
+	if (!conf) {
+		return(-ENODEV);
+	}
+
+	return sprintf(page, "%d\n", conf->round_robbin);
+}
+
+static ssize_t raid1_store_mode_roundrobbin(mddev_t *mddev, const char *page, size_t len)
+{
+	conf_t *conf = mddev->private;
+	unsigned long new;
+
+	if (!conf) {
+		return(-ENODEV);
+	}
+
+	if (strict_strtoul(page, 10, &new)) {
+		return -EINVAL;
+	}
+
+	if (new) {
+		conf->round_robbin = 1;
+	} else {
+		conf->round_robbin = 0;
+	}
+
+	return len;
+}
+
+static struct md_sysfs_entry raid1_mode_roundrobbin = __ATTR(round_robbin,
+		S_IRUGO | S_IWUSR, raid1_show_mode_roundrobbin,
+		raid1_store_mode_roundrobbin);
+
+static struct attribute *raid1_attrs[] =  {
+        &raid1_mode_roundrobbin.attr,
+	NULL
+};
+
+static struct attribute_group raid1_attrs_group = {
+	.name = NULL,
+	.attrs = raid1_attrs,
+};

 static void unplug_slaves(mddev_t *mddev);

@@ -456,6 +502,10 @@
 		goto rb_out;
 	}

+	/* If round-robbin mode enabled for this array, select next disk */
+	if (conf->round_robbin) {
+		new_disk = (new_disk + 1) % conf->raid_disks;
+	}

 	/* make sure the disk is operational */
 	for (rdev = rcu_dereference(conf->mirrors[new_disk].rdev);
@@ -480,6 +530,11 @@
 	if (new_disk < 0)
 		goto rb_out;

+	/* Don't bother searching for the best disk if we are in round-robbin mode */
+	if (conf->round_robbin) {
+		goto rb_out;
+	}
+
 	disk = new_disk;
 	/* now disk == new_disk == starting point for search */

@@ -2061,6 +2116,8 @@
 		goto abort;
 	}

+	conf->round_robbin = 0;
+
 	return conf;

  abort:
@@ -2147,6 +2204,15 @@

 	md_set_array_sectors(mddev, raid1_size(mddev, 0, 0));

+	if (mddev->to_remove == &raid1_attrs_group) {
+		mddev->to_remove = NULL;
+	} else {
+		if (sysfs_create_group(&mddev->kobj, &raid1_attrs_group))
+			printk(KERN_WARNING
+				"md/raid:%s: failed to create sysfs attributes.\n",
+				mdname(mddev));
+	}
+
 	mddev->queue->unplug_fn = raid1_unplug;
 	mddev->queue->backing_dev_info.congested_fn = raid1_congested;
 	mddev->queue->backing_dev_info.congested_data = mddev;
@@ -2173,6 +2239,7 @@

 	md_unregister_thread(mddev->thread);
 	mddev->thread = NULL;
+	mddev->to_remove = &raid1_attrs_group;
 	blk_sync_queue(mddev->queue); /* the unplug fn references 'conf'*/
 	if (conf->r1bio_pool)
 		mempool_destroy(conf->r1bio_pool);
--- linux-2.6.35.4/drivers/md/raid1.h	2010-08-26 18:47:12.000000000 -0500
+++ linux-2.6.35.4-2rrrr/drivers/md/raid1.h	2010-09-06 14:26:10.297999975 -0500
@@ -64,6 +64,8 @@
 	 * the new thread here until we fully activate the array.
 	 */
 	struct mdk_thread_s	*thread;
+
+	int round_robbin;
 };

 typedef struct r1_private_data_s conf_t;
--
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