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

[Jamie Lokier <jamie@shareable.org>]

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