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Re: [dm-devel] what is the current utility in testing active paths from multipat hd?



On mer, 2005-04-27 at 22:10 +0200, Lars Marowsky-Bree wrote:
> On 2005-04-27T11:17:02, Mike Anderson <andmike us ibm com> wrote:
> 
> > Once support gets completed / utilized the fc_transport class should
> > provide data on the link state and the port state which could be provide
> > indication of path health for deciding if to send a patch check cmd. This
> > would add complication to the tester as each new transport would need some
> > type of handler.
> 
> ACK. Yes, this is part of the additional information to use I was
> referring to. As long as the port is down, why bother...
> 
> > > Another option would be to not mechanically test every N seconds, but to
> > > retest a failed path after 1s - 2s - 4s - ... 32s max as a cascading
> > > back-off, and maybe start at 2 - 64s for paths in inactive PGs.
> > > 
> > A cascading backoff / staggered  timer would require less topology
> > knowledge than the above path health testing method and would provide the
> > reduce IO loading desired (depending on how high a user was willing to go
> > on setting the delta between path tests).
> 
> Yes, it's easier, but it also slows down responsiveness and path
> reactivation of course. One can argue that the combination of the two
> works; we only retest every path every N seconds, but we interleave
> them, so that essentially we test a path every N/M seconds; and as soon
> as one path finds a state change, we shorten the timers for all paths so
> they get all tested faster.
> 
> That's probably a pretty sophisticated heuristic which would work
> reasonably well w/o any additional configuration.
> 
I'm in the process of plugging a uevent listener into a new daemon
thread.

Right now, the goal is to trap sysfs block add/remove events so that I
can get rid of the multipath -> multipathd signaling.

I can see a bright future where the sysfs transport classes will
broadcast over that bus too. And we'll be ready to play with these new
events.

Regards,
-- 
christophe varoqui <christophe varoqui free fr>



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