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