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

[Lars Marowsky-Bree]

On 2005-04-27T12:27:32, "goggin, edward" <egoggin at emc.com> wrote:

> Although I know it sounds a bit radical and counter intuitive,
> but I'm not sure of the utility gained in the current multipathing
> implementation by multipathd periodically testing paths which
> are known to be in an active state in the multipath target driver.
> Possibly someone can convince me otherwise.

Because user-space doesn't know whether any IO has actually gone down a
given path, and that would be the only time the kernel would detect the
error.

> If not, it may be possible to significantly reduce the cpu&io
> resource utilization consumed by multipathd path testing on
> enterprise scale configurations by only testing those paths
> which the kernel thinks are in a failed state -- obviously a
> much smaller set of paths.

I could see not testing paths if we knew IO was hitting them; as an
approximization, the active paths from the active PG might be omitted.
However, the paths in the inactive PG all need to be tested, or else you
are never going to find out that the paths have gone bad on you until
you try to failover.

The best way to minimize path (re-)testing needed is to figure in the
hierarchy of components involved; as long as the FC switch is still bad,
there's no point testing any target which we could reach through it,
etc; testing whether the switch itself is healthy would round-robin
through our various connections to the switch, to make sure we don't
declare the switch down because we got hung up on one failed path.

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.

Not testing paths however isn't a real option.

> multipathd, this will no longer be true.  This seems unlikely
> apparently due to the difficulty in implementing consistently
> accurate path testing in user space.

Uh? How is path testing in user-space difficult?


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

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