Commit 48f15b93 authored by Paul Clements's avatar Paul Clements Committed by Linus Torvalds
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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

   There's a similar debian bug that has been filed as well:

   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
Signed-off-by: default avatarPaul Clements <>
Cc: Jens Axboe <>
Signed-off-by: default avatarAndrew Morton <>
Signed-off-by: default avatarLinus Torvalds <>
parent 2f56debd
...@@ -655,6 +655,7 @@ static int __init nbd_init(void) ...@@ -655,6 +655,7 @@ static int __init nbd_init(void)
for (i = 0; i < nbds_max; i++) { for (i = 0; i < nbds_max; i++) {
struct gendisk *disk = alloc_disk(1); struct gendisk *disk = alloc_disk(1);
elevator_t *old_e;
if (!disk) if (!disk)
goto out; goto out;
nbd_dev[i].disk = disk; nbd_dev[i].disk = disk;
...@@ -668,6 +669,11 @@ static int __init nbd_init(void) ...@@ -668,6 +669,11 @@ static int __init nbd_init(void)
put_disk(disk); put_disk(disk);
goto out; goto out;
} }
old_e = disk->queue->elevator;
if (elevator_init(disk->queue, "deadline") == 0 ||
elevator_init(disk->queue, "noop") == 0) {
} }
if (register_blkdev(NBD_MAJOR, "nbd")) { if (register_blkdev(NBD_MAJOR, "nbd")) {
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