[dm-devel] [RFC] pathchecker

[Heinz Mauelshagen]

On Tue, Feb 24, 2004 at 08:54:10PM +0100, christophe varoqui wrote:
> Kevin Corry wrote:
> >>if test IO succeeds, exec the multipath config tool for this path, which
> >>will reset all paths for the multipath to their active state.
> >
> >Will this reactivate *all* paths, or just the path that's known to have 
> >recovered?
> >
> *all* :)
> like the initial config does ...
> yes, it is suboptimal, but how disturbing ? I don't even know how to 
> activate individual paths. Maybe with path options in tha path list of 
> the PG definition ?
<SNIP>

It will be extremely disturbing because we'll take SCSI timeout handling
adding multi-second delays until we get the errors reported in order to
fail an erroneosly activated path again. During this fs/appls will starve.

Imagine the case, where the first path (P1) in a priority group is long-term
dead and gets reactivated after the test thread found another arbitrary group
member (Px) came back to life. In a larger group with flaky paths this scenario
will render the group effectively useless.

There's 2 options to solve this:

a) load a different table which avoids the long-term dead path;
   IOW: which only contains paths we know to be operational

b) have an interface change to be able to flag a path inactive
   which will prevent io from getting queued


a obviously avoids an interface change at the cost of holding different tables
in the mpath config tool and the driver.


Regards,
Heinz    -- The LVM Guy --

*** Software bugs are stupid.
    Nevertheless it needs not so stupid people to solve them ***

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Heinz Mauelshagen                                 Red Hat GmbH
Consulting Development Engineer                   Am Sonnenhang 11
                                                  56242 Marienrachdorf
                                                  Germany
Mauelshagen at RedHat.com                            +49 2626 141200
                                                       FAX 924446
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