Commit 48f15b93 authored Feb 23, 2008 by Paul Clements Committed by Linus Torvalds Feb 23, 2008

NBD: make nbd default to deadline I/O scheduler

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>
Cc: Jens Axboe <jens.axboe@oracle.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

parent 2f56debd

drivers/block/nbd.c

+6 −0

Original line number	Diff line number	Diff line
		@@ -655,6 +655,7 @@ 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;
		@@ -668,6 +669,11 @@ 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")) {