Linux mpath question
 

Hi ,

I am having the following output :

# multipath -ll

mpath0 (3600508b400070aac0000900000080000) dm-1 HP,HSV200
[size=10G][features=1 queue_if_no_path][hwhandler=0]
\_ round-robin 0 [prio=4][active]
\_ 0:0:0:1 sda 8:0 [active][ready]
\_ 0:0:1:1 sdb 8:16 [active][ready]
\_ 1:0:0:1 sdc 8:32 [active][ready]
\_ 1:0:1:1 sdd 8:48 [active][ready]

# fdisk /dev/sda

# kpartx -a /dev/mapper/mpath0

Questions :
-----------------------
1- Does that mean we have 1 LUN with multiple paths to the storage ?
2- Is it mandatory to format the LUN before using it with VG/FS?

part of the answer
 

Two paths each for two channels for four devices that connect you to a single LUN. If one fails, the storage does not fail. IF one channel drops, the storage does not. Normal pattern for critical SAN storage.

I do not believe it is mandatory, but would have to check my documentation for the right way to make use of it. I think I remember that if you pre-format it then dynamic resizing becomes difficult.

You use it by the mpath device or mapper paths, not by the individual device paths. I have only used those when maintaining the LUN: resizing storage.

Only refer to your DM-multipath device via its /dev/mapper/mpath* name (or whatever you've aliased it to in multipath.conf or the bindings file). Do not use the /dev/sd* devices, as those would only give you one path to the LUN, as mentioned. Also, do not use the /dev/dm-* devices, as those can change across reboots.

What do you mean by "format the LUN"? You can partition the LUN prior to adding it to a volume group (vgcreate will automatically mark it as a physical volume if it's not already). You format LVs, not PVs.