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Re: [dm-devel] RFC for multipath queue_if_no_path timeout.



On Thu, 2013-09-26 at 13:41 -0400, Mike Snitzer wrote:
> On Thu, Sep 26 2013 at  1:14pm -0400,
> Frank Mayhar <fmayhar google com> wrote:
> > Obviously, if queue_if_no_path is on and multipath runs out of good
> > paths, the I/Os will sit there queued forever barring user intervention.
> > I was doing a lot of failure testing and encountered a daemon bug in
> > which it would abandon its recovery in the middle, leaving the list
> > intact and the I/Os queued, forever.  We fixed the daemon
> Did you share the fix upstream yet?  If not please do ;)

It's a daemon we wrote so the fix only really applies to us, sorry.

> A timeout is always going to be racey.  But obviously with enough
> testing you could arrive at a timeout that is reasonable for your
> needs.. but in general I just don't think a timeout to release the
> queuing is the right way to go.

Having it as an admin-settable option seems reasonable to me, though.  I
agree you don't want one by default.  I expect that the timeout that's
actually used to be on the order of single-digit minutes.

> And I understand Alasdair's point about hardening multipathd and using a
> watchdog to restart it if it fails.  Ultimately that is ideal.  But if
> multipathd does have a bug that makes it incapable of handling a case
> (like the one you just fixed) it doesn't help to restart the daemon.

Believe me, we're hardening out daemon as much as possible, but the
reality is that there's always going to be some situation that wasn't
anticipated.  In our environment, that kind of stuff happens almost
constantly.  No matter how hardened the daemon, _something_ can keep it
from doing its job.

> Therefore I'm not opposed to some solution in kernel.  But I'd think it
> would be the kernel equivalent to multipathd's "queue_without_daemon".
> AFAIK we currently don't have a way for the kernel to _know_ multipathd
> is running; but that doesn't mean such a mechanism couldn't be
> implemented.

If you have a reasonable alternative I'm all ears.  However instantly
failing the I/O if the daemon isn't present and we run out of paths
isn't a good answer for us.  Setting a timeout only if the daemon isn't
present is functionally equivalent to setting a timeout regardless and
having a running daemon nearly instantly reload the table (thereby
turning off the timeout).
-- 
Frank Mayhar
310-460-4042


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