[dm-devel] 2.6.10-rc1-udm1: multipath work in progress

[Lars Marowsky-Bree]

On 2004-11-07T19:40:45, Alasdair G Kergon <agk at redhat.com> wrote:

> > I've had to think about this one a bit. But there's also a flaw in the
> > bypassed logic here (at least as implemented now, unless you got some
> > patch I've not yet seen). Namely, we don't ever reset it. 
> We rely on userspace to reset it.

Yes, the problem I was hinting at is that this does not in fact protect
against 'automatic' PG switches, because as soon as a path from the
higher priority non-bypassed PG becomes available again it would
immediately switch.

Of course this can also be solved by ensuring that user-space first
takes care of the bypassed flags before reinstating a path, but under
very much constructed timings and events, that might still fail. (ie, PG
is bypassed by the kernel exactly in between user-space checking the
flags and the path being reinstated).

> > So yes, we probably ought to remember that we switched PGs, but probably
> > also should clear that flag after some IO has succeeded once w/o causing
> > a PG switch over.
> At the moment, I still prefer to leave that to userspace.
> (On the grounds that there's policy involved here - how much I/O or
> elapsed time?  Depends on the failure mode.)

Hm. Ok. Probably these corner-cases don't matter much.

> > Well there might be the reason to set them on the initial table load to
> > get us off started correctly.
> To set the number of the PG to be used first, like you suggested.

Yes, that's what I meant.

> That all seems to make for a coherent approach again, cleaner to use 
> than before, though slightly more complex internally currently.

Ok, great.

Christophe, any particular wishes on how to implement this in the
multipath-tools? Would you want an additional helper to call to answer
the question of which PG should be active or bypassed first?


Sincerely,
    Lars Marowsky-Brée <lmb at suse.de>

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