[Bug 1549506] [NEW] multipath discovery failed during install due to md arrays locking individual paths

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Similarly to bug 1549504 (for LVM), but this one with LVM on top of md
arrays.

During a Xenial installation, some of the multipath devices were not
discovered, as individual paths were locked by MD arrays.

The fix is to deactivate MD arrays before multipath discovery (in hw-
detect), and activate multipath partitions during partman/init.d (in
partman-multipath), so that MD arrays can be re-activated on top of the
multipath devices (slightly afterward on partman/init.d/25md-devices).

The fix also requires an extra udev rule in the mdadm assembly rules,
because the device-mapper devices have no 'add' event (used by the rule
for assembly), but only a 'change' event.   The rule is required for the
initramfs as well (automatically installed w/ the mdadm install
triggers), otherwise the rootfs won't be found.

Failure:

~ # multipath -v5
<...>
Feb 22 14:46:02 | mpathb: addmap [0 1115734016 multipath 1 queue_if_no_path 1 alua 2 1 round-robin 0 1 1 8:16 1 round-robin 0 1 1 8:128 1]
Feb 22 14:46:02 | libdevmapper: ioctl/libdm-iface.c(1876): device-mapper: reload ioctl on mpathb failed: Device or resource busy
Feb 22 14:46:02 | mpathb: domap (0) failure for create/reload map
<...>
Feb 22 14:46:02 | mpathc: addmap [0 1115734016 multipath 1 queue_if_no_path 1 alua 2 1 round-robin 0 1 1 8:32 1 round-robin 0 1 1 8:144 1]
Feb 22 14:46:02 | libdevmapper: ioctl/libdm-iface.c(1876): device-mapper: reload ioctl on mpathc failed: Device or resource busy
Feb 22 14:46:02 | mpathc: domap (0) failure for create/reload map
<...>
Feb 22 14:46:02 | mpathd: addmap [0 1115734016 multipath 1 queue_if_no_path 1 alua 2 1 round-robin 0 1 1 8:48 1 round-robin 0 1 1 8:160 1]
Feb 22 14:46:02 | libdevmapper: ioctl/libdm-iface.c(1876): device-mapper: reload ioctl on mpathd failed: Device or resource busy
Feb 22 14:46:02 | mpathd: domap (0) failure for create/reload map
<...>
Feb 22 14:46:02 | mpathe: addmap [0 1115734016 multipath 1 queue_if_no_path 1 alua 2 1 round-robin 0 1 1 8:64 1 round-robin 0 1 1 8:176 1]
Feb 22 14:46:02 | libdevmapper: ioctl/libdm-iface.c(1876): device-mapper: reload ioctl on mpathe failed: Device or resource busy
Feb 22 14:46:02 | mpathe: domap (0) failure for create/reload map
<...>
Feb 22 14:46:02 | mpathf: addmap [0 1115734016 multipath 1 queue_if_no_path 1 alua 2 1 round-robin 0 1 1 8:80 1 round-robin 0 1 1 8:192 1]
Feb 22 14:46:02 | libdevmapper: ioctl/libdm-iface.c(1876): device-mapper: reload ioctl on mpathf failed: Device or resource busy
Feb 22 14:46:02 | mpathf: domap (0) failure for create/reload map

~ # dmsetup table | sort
mpatha: 0 1115455488 multipath 0 1 alua 2 1 round-robin 0 1 1 8:0 1 round-robin 0 1 1 8:112 1 
mpathg: 0 1115734016 multipath 0 1 alua 2 1 round-robin 0 1 1 8:96 1 round-robin 0 1 1 8:208 1 
pkvmci845--vg-root: 0 5351333888 linear 259:2 5555
pkvmci845--vg-swap_1: 0 225468416 linear 259:2 5351339443

~ # lvm pvdisplay | grep  Name
  PV Name               /dev/md0p3
  VG Name               pkvmci845-vg

~ # grep ^md /proc/mdstat 
md0 : active raid0 sdm1[4] sdl1[3] sdk1[2] sdj1[1] sdi2[0]


After the patch, the md array is on top of the multipath devices, and show up as a LVM PV correctly.

~ # grep ^md /proc/mdstat 
md0 : active raid0 dm-9[1] dm-10[2] dm-8[0] dm-11[3] dm-12[4]

~ # lvm vgscan
  Reading all physical volumes.  This may take a while...
  Found volume group "pkvmci845-vg" using metadata type lvm2
~ # lvm vgchange -ay
  2 logical volume(s) in volume group "pkvmci845-vg" now active
~ # lvm pvdisplay | grep Name
  PV Name               /dev/md0p3
  VG Name               pkvmci845-vg

** Affects: hw-detect (Ubuntu)
     Importance: Undecided
         Status: New


** Tags: patch
-- 
multipath discovery failed during install due to md arrays locking individual paths
https://bugs.launchpad.net/bugs/1549506
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