[dm-devel] How to debug missing devices in `multipath -l` (and …`-ll`)

[Konstantin Kharlamov]

Hello! Sorry if it's the wrong list, it is referred to by multipath-tools
without saying it's "devel", so I presume user questions are okay…?

How to debug a lack of devices in `multipath -l` and `… -ll` outputs? I'm
studying `multipathd -dv5` logs, but it's not at all clear to me as to what to
look for.

At $WORK I'm trying to replace the older Ubuntu shipped multipath-tools with the
upstream one. Then, given a system with the upstream `multipath-tools`
preinstalled and `multipathd` running, none of the `multipath -l` or `multipath
-ll` show output. Then, if I downgrade `multipath-tools` to the one from Ubuntu
repository and restart `multipathd`, devices do appear.

Then it gets weird: if I upgrade `multipath-tools` back to the latest one
upstream, and reboot, devices are still there! It starts working!

At this point I found the `/etc/multipath/wwids` which might be the reason. I
presume `multipathd` puts devices there under some conditions. But where can I
see how it decides whether to work with devices or to ignore them? I'm looking
through -v5 startup logs when it's working and when it doesn't, there's a lot of
output, but it's hard to `diff` the older (working) and the newer (not working)
output — a lot of style changes. Entries I find suspicious are repeated in both
cases.

I figured, it might ask here for pointers, is there something specific to look
for?