multipath, lvm, and you!

Today I went on a magical learning adventure. I learned about multipathing(I had no idea it was a thing). It changed my world in a strange and radical way, left me feeling like the universe was a much bigger place. Insert other cliche statement here.

Anyway, for those of you out there that are just as ignorant as I was, multipathing is a solution to a problem created by SANs/LUNs. Our SAN for example, provides us a few redundant paths to the same storage space, on one of our servers that may look like

# parted -
# print devices
/dev/sde (2147GB)
/dev/sdf (2147GB)
/dev/sdg (2147GB)
/dev/sdh (2147GB)

All of those are 1 drive. How then would you determine which to format, which to work with on a daily basis? MULTIPATH!

by simply issuing “multipath -l” All the information you could hope for is revealed!

mpathg (3600015d00022d500000000000002b858) dm-5 XXX
size=2.0T features='0' hwhandler='0' wp=rw
|-+- policy='round-robin 0' prio=0 status=active
| `- 1:0:0:0 sde 8:64  active undef running
|-+- policy='round-robin 0' prio=0 status=enabled
| `- 1:0:1:0 sdf 8:80  active undef running
|-+- policy='round-robin 0' prio=0 status=enabled
| `- 2:0:0:0 sdg 8:96  active undef running
`-+- policy='round-robin 0' prio=0 status=enabled
  `- 2:0:1:0 sdh 8:112 active undef running

As you can see, the device “mpathg” (located under /dev/mapper/mpathg) is multipathed to “sde”, “sdf”, “sdg”, and “sdh”, it is also set to a multipath device of “dm-5” (located as /dev/dm-5).

Partitions created for this device will end up at “/dev/mapper/mpathgp#”.

Amazingly simple.

Now the LVM part: if your system is multipathing your devices, you MUST use the multipath location when dealing with them for lvm purposes, for example, I had a physical disk “/dev/sdb” that I wanted to add to my primary volume group, I formatted it, did all the necessary steps, issued “pvcreate /dev/sdb1” and got:

Device /dev/sdb1 not found (or ignored by filtering).

Instead I had to issue “pvcreate /dev/mapper/mpathbp1” which worked like a charm.

Such a simple bit of knowledge I know, but I was completely clueless about it for a long time. Remember kids, knowing is half the battle! (the other half of the battle…is…the battle.)