linux-kernel - Re: XFS vs Elevators (was Re: [PATCH RFC] nilfs2: continuous snapshotting file system)

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Date:	Mon, 25 Aug 2008 13:01:47 +0100
From:	Jamie Lokier <jamie@...reable.org>
To:	Nick Piggin <nickpiggin@...oo.com.au>,
	gus3 <musicman529@...oo.com>,
	Szabolcs Szakacsits <szaka@...s-3g.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
	xfs@....sgi.com
Subject: Re: XFS vs Elevators (was Re: [PATCH RFC] nilfs2: continuous snapshotting file system)

Dave Chinner wrote:
> To keep on top of this, we keep adding new variations and types and
> expect the filesystems to make best use of them (without
> documentation) to optimise for certain situations. Example - the
> new(ish) BIO_META tag that only CFQ understands. I can change the
> way XFS issues bios to use this tag to make CFQ behave the same way
> it used to w.r.t. metadata I/O from XFS, but then the deadline and
> AS will probably regress because they don't understand that tag and
> still need the old optimisations that just got removed. Ditto for
> prioritised bio dispatch - CFQ supports it but none of the others
> do.


There's nothing wrong with adding BIO_META (for example) and other
hints in _principle_.  You should be able to ignore it with no adverse
effects.  If its not used by a filesystem (and there's nothing else
competing to use the same disk), I would hope to see the same
performance as other kernels which don't have it.

If the elevators are being changed in such a way that old filesystem
code which doesn't use new hint bits is running significantly slower,
surely that's blatant elevator regression, and that's where the bugs
should be reported and fixed?

-- Jamie
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