FC multipath LUN configuration

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FC multipath LUN configuration


I need to setup a multipath disk I receive through Fibre Channel. The
LUN has been already set up and assigned to the server by my
colleague, so all I need to do is to configure the server to use the
multipath storage. I have no experience with using multipath FC LUNs

Since I installed multipath-tools, I see the paths:

root@mgmt-bckp01:~# multipath -ll
mpatha (360050763008087ee5000000000000004) dm-3 IBM,2145
size=5.0T features='1 queue_if_no_path' hwhandler='0' wp=rw
|-+- policy='round-robin 0' prio=50 status=active
| |- 0:0:0:0 sdb 8:16 active ready running
| `- 7:0:0:0 sde 8:64 active ready running
`-+- policy='round-robin 0' prio=10 status=enabled
  |- 0:0:1:0 sdd 8:48 active ready running
  `- 7:0:1:0 sdf 8:80 active ready running

I understand that I should use /dev/mapper/mpatha, but I have some concerns:

1. Why do I see the paths with regular block device names 4 times
(/dev/sd[b,d,e,f])? I was told that I have 2 paths to the LUN. I'd
understand if I would see 1 disk per path, so 2 disks altogether, but
seeing 4 surprises me. Is something wrong? Do I need to do some
further configuration to hide the excess disks? I tested that all four
disks are the same: when I write something to one, I see it on all the
others (after an "echo 3 > /proc/sys/vm/drop_caches").

2. I intend to create an LVM PV on the LUN. But then, LVM identifies
PVs by PV UUIDs, and if it finds the UUID on /dev/sdb earlier than on
/dev/mapper/mpatha, then it will read/write /dev/sdb directly. That is
one path. That would defeat the whole purpose of multipath, because
then if I lose the path to /dev/sdb, LVM loses access to the PV, as it
will not fail over to the other path, am I right? So how can I force
LVM to only look for the PV on /dev/mapper/mpatha? Somewhere I read
that I can blacklist regular disks and force LVM to ignore disks those
match "/dev/sd*", but then it wouldn't be feasible either, because I
have PVs on local disks too (/dev/sda and /dev/sdc). Also, disks may
not come up in the same order; as I noticed it's totally random which
disk gets which letter – as you see, my local disks didn't even get
consecutive letters (a, c). So only blacklisting "b, d, e, f" would
not help, as I can't predict if the LUN will get the same letters
after a reboot.

3. Is there any way to simply hide the regular disk nodes? So I would
see my local disks as /dev/sda and /dev/sdb, and the multipath LUN as
/dev/mapper/mpatha only, without any "/dev/sd?" node associated to it.

Thanks for any help in advance.


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