Bug#580972: multipath-tools: unnecessary call to multipath in /etc/udev/rules.d/z60_multipath.rules

[Christian Seiler]

Control: severity -1 important

I discussed this at DebConf15 with Ritesh (and partially with Guido) and
Ritesh and I came to the following conclusions:

 - multipathd will synthesize events for all pre-existing devices on
   startup nowadays - so it will pick up all devices that are already
   there - no need for the udev rule anymore

 - but udev since a couple of versions (including that in Jessie) will
   lock the block devices it is currently processing with LOCK_SH,
   whereas multipath{,d} lock the block devices with LOCK_EX while they
   are trying to set up their mappings (to make sure that the same
   mapping isn't set up at the same time). But if one runs multipath
   as part of a udev rule, this will not work at all, because while any
   udev helper is run for a given block device, udev will have a LOCK_SH
   lock on it, and the multipath binary called from the helper will
   unsuccessfully try and try again to lock them with LOCK_EX, until
   udev runs into its timeout and kills multipath.

Therefore, one should just get rid of the udev rule, because it doesn't
work currently (due to udev locking), previously when it still worked
was potentially slow if there were a lot of mappings, and is unnecessary
with current multipathd, because mappings will just work with it.

(Setting priority to important because it may cause boot to hang until
the udev timeout is reached in certain circumstances.)

Christian

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