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

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Message-ID: <20080219100536.GW23197@kernel.dk>
Date:	Tue, 19 Feb 2008 11:05:36 +0100
From:	Jens Axboe <jens.axboe@...cle.com>
To:	Andrew Morton <akpm@...ux-foundation.org>
Cc:	Paul Clements <paul.clements@...eleye.com>,
	randy.dunlap@...cle.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/1] NBD: make nbd default to deadline I/O scheduler

On Tue, Feb 19 2008, Andrew Morton wrote:
> On Tue, 19 Feb 2008 10:24:28 +0100 Jens Axboe <jens.axboe@...cle.com> wrote:
> 
> > On Tue, Feb 19 2008, Jens Axboe wrote:
> > > On Mon, Feb 18 2008, Andrew Morton wrote:
> > > > > +
> > > > >  	if (e && !try_module_get(e->elevator_owner))
> > > > >  		e = NULL;
> > > > 
> > > > Looks nice and simple.  There might be some of the usual ordering problems
> > > > when this is called during boot, maybe is-initramfs-available-yet problems,
> > > > etc.  But it's unlikely to make things regress from where they are now.
> > > 
> > > Isn't request_module() and below robust enough to handle that?
> > 
> > BTW, I've verified that it works as expected (at least after boot):
> > 
> > carl:/sys/block/sda/queue # cat scheduler 
> > noop [cfq] 
> > carl:/sys/block/sda/queue # echo anticipatory > scheduler 
> > carl:/sys/block/sda/queue # dmesg
> > [...]
> > io scheduler anticipatory registered
> > carl:/sys/block/sda/queue # cat scheduler 
> > noop cfq [anticipatory] 
> > 
> > So it properly loads as-iosched instead of failing, like it would have
> > done before and required the user to do a modprobe as-iosched first.
> > 
> > carl:/sys/block/sda/queue # echo foobar > scheduler 
> > -bash: echo: write error: Invalid argument
> > carl:/sys/block/sda/queue # dmesg
> > [...]
> > elevator: type foobar not found
> 
> Looks promising - let's run with it.  If there _are_ startup ordering
> problems then we won't be any worse off than we are now.

I've merged it for inclusion, since it tests fine here.

> otoh, the system must have _some_ io scheduler installed when talking to
> disks, so perhaps there won't be any such problems at all.

We are guarenteed to always have noop available, since you cannot
de-select that.

-- 
Jens Axboe

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