Re: [PATCH 1/1] NBD: make nbd default to deadline I/O scheduler

[Paul Clements <paul.clements@steeleye.com>]

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Take 3...this should address all the issues.





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NBD doesn't work well with CFQ (or AS) schedulers, so let's default to something
else.

The two problems I have experienced with nbd and cfq are:
 
1) nbd hangs with cfq on RHEL 5 (2.6.18) -- this may well have been fixed
 
There's a similar debian bug that has been filed as well:
 
http://bugs.debian.org/cgi-bin/bugreport.cgi?bug=447638

There have been posts to nbd-general mailing list about problems with 
cfq and nbd also.
 
2) nbd performs about 10% better (the last time I tested) with deadline 
vs. cfq (the overhead of cfq doesn't provide much advantage to nbd [not 
being a real disk], and you end up going through the I/O scheduler on 
the nbd server anyway, so it makes sense that deadline is better with nbd)
 

Signed-Off-By: Paul Clements <paul.clements@steeleye.com>

--- ./drivers/block/nbd.c.max_nbd_killed	2008-02-07 16:46:24.000000000 -0500
+++ ./drivers/block/nbd.c	2008-02-09 08:14:18.000000000 -0500
@@ -654,6 +654,7 @@ static int __init nbd_init(void)
 
 	for (i = 0; i < nbds_max; i++) {
 		struct gendisk *disk = alloc_disk(1);
+		elevator_t *old_e;
 		if (!disk)
 			goto out;
 		nbd_dev[i].disk = disk;
@@ -667,6 +668,11 @@ static int __init nbd_init(void)
 			put_disk(disk);
 			goto out;
 		}
+		old_e = disk->queue->elevator;
+		if (elevator_init(disk->queue, "deadline") == 0 ||
+			elevator_init(disk->queue, "noop") == 0) {
+				elevator_exit(old_e);
+		}
 	}
 
 	if (register_blkdev(NBD_MAJOR, "nbd")) {