[dm-devel] Chicken and egg problem with multipath-tools

[Hannes Reinecke]

On 03/02/2013 11:43 AM, Andrei B. wrote:
> Hi,
>
> I've been trying to install multipath-tools on a couple of linux servers with FC storage that I work with.
> They run Linux Slackware.
> I've built from source, created an installable package (Slackware style) and deployed it.
>
> I have discovered the following problem (chicken and egg style):
>
> - multipathd, AFAIK, should start before LVM, because if several volumes need be assembled by LVM,
 > they should exist before LVM starts.
> - multipathd, AFAIK, should start before any service that has it's data stored on the remote storage.
>
> I am not familiar [enough] with the inner workings of the startup sequence on redhat/debian like systems.
> Slackware's startup sequence is much simpler and I placed multipathd in file rc.S (which is the first
 > one called by init) right before LVM.
>
> And here lies the chicken-egg problem. At this point, only the root filesystem is mounted and
 > it is mounted read-only. It is the correct and proper startup 
sequence in Slackware. Filesystems
 > are checked for errors after LVM starts and then root is 
remounted read-write and the rest
 > of the filesystems are mounted.
>
> The problem is that multipathd wants to create a PID file under /var (which is at this time readonly).
 > This fails and multipathd fails to start.
> It took me a lot of testing and reboots until I managed to determine this is the actual problem.
>
Hmm. Why? multipathd can happily live without a PID file; SUSE has 
been doing this for years (for precisely the same reason).
Check the SUSE init scripts.

> The stop-gap solution I have found, so far, is to load dm-multipath module and run multipath -v0
> before LVM, thus creating the multipath devices, then later, after the filesystems are remounted
 > rw, start multipathd.
>
Bah.
Just start multipathd without creating a pid file.
Then you can check if multipathd runs via the multipathd CLI.

Cheers,

Hannes
-- 
Dr. Hannes Reinecke		      zSeries & Storage
hare at suse.de			      +49 911 74053 688
SUSE LINUX Products GmbH, Maxfeldstr. 5, 90409 Nürnberg
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