updated geom_sched code

Ivan Voras ivoras at freebsd.org
Sun Mar 8 11:39:04 PDT 2009

Luigi Rizzo wrote:
> Hi all,
> this is an update on the work that Fabio Checconi and I are doing
> on disk scheduling, which was first announced here a couple of
> months ago:
>     http://lists.freebsd.org/pipermail/freebsd-stable/2009-January/047597.html
> Since the previous version, we have done some massive cleanup and
> consolidation of the code, fixed a bug in the system's disksort,
> and added amd64 support.

> NOTE: due to the way we store classification info, the schedulers
> are probably incompatible with g_journal.
> Apart from that (which needs to be fixed by adding a field to the
> struct bio), we believe the code to be quite stable now, so future
> work will be mainly focused on adding more scheduling algorithms
> and doing a thorough performance evaluation under various workloads.
> In the meantime we'd really appreciate any comment from you on the
> approach we took, and anyone testing the code.


Do you have some documentation about the long-term plans? You will have
to add an additional field to bio, of course, but there is one more
thing: the schedulers will have to be integrated into the GEOM in an
"invisible" way. I.e. instead of /dev/ad0-sched-s1f the users should see
only /dev/ad0 like they're used to, and get the schedulers by default.
Otherwise, your work will not get much use.

This is not the only case where "invisible" classes are needed. Back
when I was working on GEOM logging (what has since turned into
gjournal), there was also a need to be able to insert a class
transparently in between two classes in a tree without disturbing either
(so for example you could turn IO request logging if needed on a "hot"
server). IIRC Pawel had some similar needs and maybe some ideas but I
don't know much there.

I've CC-ed freebsd-geom@ so the discussion can get more technical there :)

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