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Re: XFS vs Elevators (was Re: [PATCH RFC] nilfs2: continuous snapshottin

To: Dave Chinner <david@xxxxxxxxxxxxx>
Subject: Re: XFS vs Elevators (was Re: [PATCH RFC] nilfs2: continuous snapshotting file system)
From: david@xxxxxxx
Date: Mon, 25 Aug 2008 20:50:14 -0700 (PDT)
Cc: Jamie Lokier <jamie@xxxxxxxxxxxxx>, Nick Piggin <nickpiggin@xxxxxxxxxxxx>, gus3 <musicman529@xxxxxxxxx>, Szabolcs Szakacsits <szaka@xxxxxxxxxxx>, Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>, linux-fsdevel@xxxxxxxxxxxxxxx, linux-kernel@xxxxxxxxxxxxxxx, xfs@xxxxxxxxxxx
In-reply-to: <20080826030759.GY5706@disturbed>
References: <20080821051508.GB5706@disturbed> <200808211933.34565.nickpiggin@xxxxxxxxxxxx> <20080821170854.GJ5706@disturbed> <200808221229.11069.nickpiggin@xxxxxxxxxxxx> <20080825015922.GP5706@disturbed> <20080825120146.GC20960@xxxxxxxxxxxxx> <20080826030759.GY5706@disturbed>
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On Tue, 26 Aug 2008, Dave Chinner wrote:

On Mon, Aug 25, 2008 at 01:01:47PM +0100, Jamie Lokier wrote:
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

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.

Right, but it's what we need to do to make use of that optimisation
that is the problem. For XFS, it needs to replace the current
BIO_SYNC hints we use (even for async I/O) to get metadata
dispatched quickly. i.e. CFQ looks at the sync flag first then the
meta flag.  Hence to take advantage of it, we need to remove the
BIO_SYNC hints we currently use which will change the behaviour on
all other elevators as a side effect.

This is the optimisation problem I'm refering to - the BIO_SYNC
usage was done years ago to get metadata dispatched quickly because
that is what all the elevators did with sync I/O. Now to optimise
for CFQ we need to remove that BIO_SYNC optimisation which is still
valid for the other elevators....

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?

Sure, but in reality getting ppl to go through the pain of triage is
extremely rare because it only takes 10s to change elevators and
make the problem go away...

it sounds as if the various flag definitions have been evolving, would it be worthwhile to sep back and try to get the various filesystem folks to brainstorm together on what types of hints they would _like_ to see supported?

it sounds like you are using 'sync' for things where you really should be saying 'metadata' (or 'journal contents'), it's happened to work well enough in the past, but it's forcing you to keep tweaking the filesystems. it may be better to try and define things from the filesystem point of view and let the elevators do the tweaking.

basicly I'm proposing a complete rethink of the filesyste <-> elevator interface.

David Lang

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