Re: RFC for multipath queue_if_no_path timeout.

[Hannes Reinecke <hare@suse.de>]

On 09/27/2013 01:49 AM, Mike Snitzer wrote:
> On Thu, Sep 26 2013 at  7:22pm -0400,
> Alasdair G Kergon <agk@redhat.com> wrote:
> 
>> On Thu, Sep 26, 2013 at 10:47:13AM -0700, Frank Mayhar wrote:
>>> Launching it from ramdisk won't help, particularly, since it still goes
>>> through the block layer.  The other stuff won't help if a (potentially
>>> unrelated) bug in the daemon happens to be being tickled at the same
>>> time, or if some dependency happens to be broken and _that's_ what's
>>> preventing the daemon from making progress.
>>  
>> Then put more effort into debugging your daemon so it doesn't have
>> bugs that make it die?  Implement the timeout in a robust independent
>> daemon if it's other code there that's unreliable?
>>
>>> And as far as lvm2 and multipath-tools, yeah, they cope okay in the kind
>>> of environments most people have, but that's not the kind of environment
>>> (or scale) we have to deal with.
>>
>> In what way are your requirements so different that a locked-into-memory
>> monitoring daemon cannot implement this timeout?
> 
> Frank, I had a look at your patch.  It leaves a lot to be desired, I was
> starting to clean it up but ultimately found myself agreeing with
> Alasdair's original point: that this policy should be implemented in the
> userspace daemon.
> 
_Actually_ there is a way how this could be implemented properly:
implement a blk_timeout function.

Thing is, every request_queue might have a timeout function
implemented, whose goal is to abort requests which are beyond that
timeout. EG SCSI uses that for the dev_loss_tmo mechanism.

Multipath what with it being request-based could easily implement
the same mechanism, namely have to blk_timeout function which would
just re-arm the timeout in the default case, but abort any queued
I/O (after a timeout) if all paths are down.

Hmm. I see to draft up a PoC.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		      zSeries & Storage
hare@suse.de			      +49 911 74053 688
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
GF: J. Hawn, J. Guild, F. Imendörffer, HRB 16746 (AG Nürnberg)