Bug#716655: multipath-tools: mention aliases in README.debian

[Vincent McIntyre]

On Thu, Jul 11, 2013 at 12:33:41PM +0530, Ritesh Raj Sarraf wrote:
> On Thursday 11 July 2013 05:52 AM, Vincent McIntyre wrote:
> > Explicit aliases for particular wwwn are a better alternative than
> > user_friendly_names because the mpathN <-> wwwn name mapping can
> > change unexpectedly. The patch below explains this in README.debian.
> > Please consider applying.
> 
> Thanks. I'll apply the patch.

That's great, thank you.

> But user_friendly_names shouldn't break across reboots.
> The problem earlier used to be with locking (which I is
> fixed now) and in general, the idea of the bindings file.
> 
> Do you have any other behavior seen?

I know the mappings file is supposed to prevent this but we have
seen this mechanism go nuts at various times on lenny & squeeze.
I don't have any clear evidence (or you would know about it)
but my suspicion is that on some boots the device enumeration order
changes for some reason and Something Funky (TM) happens at the
coalescence stage.
This may have gone away now. Maybe it was locking.
It may have been us just doing an ill-considered multipath -F
(or _not_ doing one) but it's happened enough that seems unlikely
for that to be the explanation.
It mostly happens when new devices are attached or removed from a host
(FC switch change while host is shut down).

Do you have any advice on how to provide useful information if we
see this again?
We are currently running squeeze but will migrate to wheezy soonish.
We always run multipathd with -v 3 but have never managed to get
anything useful in syslog - it seems to happen before syslog is started.
We always have separate / and /var partitions, /boot is part of /.
We never boot off multipath.

Cheers
Vince