Fibre Channel Multipath attach race condition

[Walt Boring]

When the system is under load and a Fibre Channel attach happens with multipath installed, sometimes only 1 devices shows up under multipath -l command. This causes the detach to fail to detach all of the devices since the libvirt volume code runs multipath -l at attach time, and not all devices show up at that time due to system load and/or FC fabric traffic.

We need to rediscover all of the devices at detach time and ensure we are removing all of those devices. The devices should exist at detach time.

[Walt Boring (walter-boring)]

Here are example output for multipath -l /dev/sdl at different times.

ATTACH time
350002ac110e2383d dm-5 3PARdata,VV
size=4.8G features='0' hwhandler='0' wp=rw
`-+- policy='round-robin 0' prio=-1 status=active
  `- 2:0:0:5 sdl 8:176 active undef running

DETACH TIME
350002ac110e2383d dm-5 3PARdata,VV
size=4.8G features='0' hwhandler='0' wp=rw
`-+- policy='round-robin 0' prio=-1 status=active
  |- 2:0:0:5 sdl 8:176 active undef running
  `- 1:0:0:5 sdm 8:192 active undef running

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/29320
Committed: http://github.com/openstack/nova/commit/080476b2d383b148f6fc8d202c3b0509f9bb1d66
Submitter: Jenkins
Branch: master

commit 080476b2d383b148f6fc8d202c3b0509f9bb1d66
Author: Walter A. Boring IV <email address hidden>
Date: Wed May 15 16:00:43 2013 -0700

    Fix dangling LUN issue under load with multipath

    This fixes an issue where not all of the LUNs are seen
    by the kernel at attach time, but later become available.
    We now rescan the list of devices seen by multipath at
    detach time.

    Also added another unit test case.

    Fixes Bug: #1175366

    Change-Id: Id5b313b17454ec32672373b7b564b9450466b7a2