[dm-devel] Multipath Questions
Heinz Mauelshagen
mauelshagen at redhat.com
Fri Jan 16 09:14:01 UTC 2004
On Wed, Jan 14, 2004 at 01:36:49PM -0600, Kevin Corry wrote:
> Hi,
>
> I just had a couple questions about the overall design and direction of the DM
> multipath module.
>
> 1. Priority Groups.
>
> DM-multipath allows paths to be placed in groups, with each group assigned a
> priority (0 to 1024). What is the intended purpose for these groups? In the
> current code, if a device has multiple groups, only the lowest-priority group
> is used. I'm assuming this will change, but I just wanted to get an idea of
> how the group priority will affect path selection.
Kevin,
low numbers imply high priority (ie, group with prio 1 is used before
group with prio 3).
What this actually comes down to is, that io gets dispatched to the group
with the highest priority (lowest #) until _all_ its paths become faulty.
One point to take into consideration later would be:
do we want to hang on a higher priority group with just 1 path operational,
_while_ we've got lower priority ones with multiple paths ?
>
> 2. Multipath Daemon
>
> The daemon's job is to resubmit failed I/Os to a different path, and to
> periodically test each of the paths to see if they've failed or recovered
> from failure. However, currently the daemon only wakes up on the error path
> for normal I/Os. This means a path could be marked faulty, but never
> reactivated if the path gets fixed.
>
> Obviously the daemon needs a way to wake-up periodically so it can run it's
> test I/Os. The comments in dm-daemon.h say the worker function should return
> a "hint" as to when they should be woken up, but currently this return value
> is unused. I'm trying to look into how to implement this functionality in the
> dm-daemon code, but I figured I'd ask here and see if anyone has already come
> up with a plan for this. Once we get the multipath daemon to do its periodic
> path-testing, I think we'll have a working prototype.
Joe, is that still the case (it is supposed to wake up regularly and test io
on failed paths as Kevin suggests) ?
>
>
> Thanks!
> --
> Kevin Corry
> kevcorry at us.ibm.com
> http://evms.sourceforge.net/
>
>
> _______________________________________________
> dm-devel mailing list
> dm-devel at sistina.com
> http://lists.sistina.com/mailman/listinfo/dm-devel
--
Regards,
Heinz -- The LVM Guy --
*** Software bugs are stupid.
Nevertheless it needs not so stupid people to solve them ***
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
Heinz Mauelshagen Sistina Software Inc.
Senior Consultant/Developer Am Sonnenhang 11
56242 Marienrachdorf
Germany
Mauelshagen at Sistina.com +49 2626 141200
FAX 924446
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
More information about the dm-devel
mailing list