[PATCH] fix raid0 readahead size

From: Arjan van de Ven <arjanv@redhat.com>

Readahead of raid0 was suboptimal; it read only 1 stride ahead.  The
problem with this is that while it will keep all spindles busy, it will not
actually manage to make larger IO's, eg each disk would just do the chunk
size IO.  Doing at least 2 chunks is more than appropriate so that each
spindle will get a chance to merge IO's.

(Neil fixed raid6 and raid6 too)

drivers/md/raid0.c

@@ -313,8 +313,8 @@ static int raid0_run (mddev_t *mddev)
 
 	/* calculate the max read-ahead size.
 	 * For read-ahead of large files to be effective, we need to
-	 * readahead at least a whole stripe. i.e. number of devices
-	 * multiplied by chunk size.
+	 * readahead at least twice a whole stripe. i.e. number of devices
+	 * multiplied by chunk size times 2.
 	 * If an individual device has an ra_pages greater than the
 	 * chunk size, then we will not drive that device as hard as it
 	 * wants.  We consider this a configuration error: a larger
@@ -322,8 +322,8 @@ static int raid0_run (mddev_t *mddev)
 	 */
 	{
 		int stripe = mddev->raid_disks * mddev->chunk_size / PAGE_CACHE_SIZE;
-		if (mddev->queue->backing_dev_info.ra_pages < stripe)
-			mddev->queue->backing_dev_info.ra_pages = stripe;
+		if (mddev->queue->backing_dev_info.ra_pages < 2* stripe)
+			mddev->queue->backing_dev_info.ra_pages = 2* stripe;
 	}
 
 

drivers/md/raid5.c

@@ -1602,14 +1602,14 @@ memory = conf->max_nr_stripes * (sizeof(struct stripe_head) +
 
 	print_raid5_conf(conf);
 
-	/* read-ahead size must cover a whole stripe, which is
-	 * (n-1) * chunksize where 'n' is the number of raid devices
+	/* read-ahead size must cover two whole stripes, which is
+	 * 2 * (n-1) * chunksize where 'n' is the number of raid devices
 	 */
 	{
 		int stripe = (mddev->raid_disks-1) * mddev->chunk_size
 			/ PAGE_CACHE_SIZE;
-		if (mddev->queue->backing_dev_info.ra_pages < stripe)
-			mddev->queue->backing_dev_info.ra_pages = stripe;
+		if (mddev->queue->backing_dev_info.ra_pages < 2 * stripe)
+			mddev->queue->backing_dev_info.ra_pages = 2 * stripe;
 	}
 
 	/* Ok, everything is just fine now */

drivers/md/raid6main.c

@@ -1771,14 +1771,14 @@ static int run (mddev_t *mddev)
 
 	print_raid6_conf(conf);
 
-	/* read-ahead size must cover a whole stripe, which is
-	 * (n-2) * chunksize where 'n' is the number of raid devices
+	/* read-ahead size must cover two whole stripes, which is
+	 * 2 * (n-2) * chunksize where 'n' is the number of raid devices
 	 */
 	{
 		int stripe = (mddev->raid_disks-2) * mddev->chunk_size
 			/ PAGE_CACHE_SIZE;
-		if (mddev->queue->backing_dev_info.ra_pages < stripe)
-			mddev->queue->backing_dev_info.ra_pages = stripe;
+		if (mddev->queue->backing_dev_info.ra_pages < 2 * stripe)
+			mddev->queue->backing_dev_info.ra_pages = 2 * stripe;
 	}
 
 	/* Ok, everything is just fine now */