[dm-devel] Designing a new prio_callout

[Ethan John]

Is it possible to manually set the priority of a path after a connection has
been established?

Since we're doing failover-only (only 1 active path at a time), it would be
nice to tell users that they can manually reset priority after a failure.
For example, in a configuration with two paths, where one is active and the
other is passive for two differeent volumes, a failure of one path will
result in all traffic going through the one remaining path. After the second
path comes back up, all traffic will still be written to the first path
(paths are not rebalanced after a failure).

At this point, we're looking for a decent solution for customers that
doesn't involve ALUA, since we won't have resources to implement that for
this first version of our target. Ideally, we'd like to be able to set the
priority for paths automatically through one of the mpath_prio_* scripts.
Even allowing a user to set these priorities manually would be better than
advising them to use mpath_prio_random as the "easy configuration" solution.

We're not looking to develop our own method of load balancing or failover.
We want to work within the MPIO world, but it's a little difficult given
that we don't support active/active configurations.

Thanks so much for you help so far!

On 7/29/07, Hannes Reinecke <hare at suse.de> wrote:
>
> Ethan John wrote:
> > Thanks for the heads-up about ALUA. We're looking into it.
> >
> > What is the purpose of the custom mpio_prio_* applications that ship
> > with open-iscsi if not to handle multipathing?
> >
> It is. mpath_prio_* are the priority callouts for multipathing.
> They determine the layout of the multipath map.
> Theory is that mpath_prio_* will return the priority for a
> given path in relation to the entire multipath layout.
>
> If used with 'group_by_prio' all paths with the same
> priority will be grouped into one multipath group, and
> the group with the highest priority will become active.
>
> When all paths in a group fail, the group with the next
> highest priority will become active. Additionally some
> failover command (as determined by the hardware handler)
> may be send to the target.
>
> Cheers,
>
> Hannes
> --
> Dr. Hannes Reinecke                   zSeries & Storage
> hare at suse.de                          +49 911 74053 688
> SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
> GF: Markus Rex, HRB 16746 (AG Nürnberg)
>
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-- 
Ethan John
http://www.flickr.com/photos/thaen/
(206) 841.4157
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