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

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

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Jens Axboe wrote:
> On Fri, Feb 08 2008, Jens Axboe wrote:
>> On Fri, Feb 08 2008, Paul Clements wrote:
>>> Andrew Morton wrote:
>>>> On Fri, 8 Feb 2008 09:33:41 -0800 Randy Dunlap <randy.dunlap@oracle.com> 
>>>> wrote:
>>>>
>>>>> On Fri, 08 Feb 2008 11:47:42 -0500 Paul Clements wrote:
>>>>>
>>>>>> There have been numerous reports of problems with nbd and cfq. Deadline 
>>>>>> gives better performance for nbd, anyway, so let's use it by default.
>>>> Please define "problems".  If it's just "slowness" then we can live with
>>>> that, but I'd hope that Jens is aware and that it's understood.
>>>>
>>>> It it's "hangs" or "oopses" then we panic.
>>> The two problems I have experienced (which may already be fixed):
>>>
>>> 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
>>>
>>> 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)
>>>
>>> There have been posts to nbd-general mailing list about problems with 
>>> cfq and nbd also.
>> I'm fine with that, it's one of those things we'll do automatically when
>> we have some sort of disk profile system setup. Devices without seek
>> penalties should not use AS or CFQ.
>>
>> Asking for a non-existing elevator is not an issue, but it may trigger
>> both printks and a switch to another elevator. So if you ask for
>> "deadline" and it's modular, you'll get cfq again if it's the default.
>>
>> Your patch looks bad though, you forget to exit the old elevator. And
>> you don't check the return value of elevator_init().
>>
>> All in all, your patch definitely needs more work before it can be
>> included.

Thanks Jens. This one should be better.

--
Paul

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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
 
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)
 
There have been posts to nbd-general mailing list about problems with 
cfq and nbd also.

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	2008-02-08 16:13:01.000000000 -0500
@@ -667,6 +667,12 @@ static int __init nbd_init(void)
 			put_disk(disk);
 			goto out;
 		}
+		if (elevator_init(disk->queue, "deadline") != 0) {
+			if (elevator_init(disk->queue, "noop") != 0) {
+				put_disk(disk);
+				goto out;
+			}
+		}
 	}
 
 	if (register_blkdev(NBD_MAJOR, "nbd")) {