A little background…
Most of the time, I have used the RDAC driver in Linux to manage SAN disks in Linux. The RDAC driver is used to hide the complexity of multiple paths and to
present redundant paths as a single path which can be used as you would
a standard SCSI / IDE / SAS / SATA drive. Seeing only one device makes managing your disks much easier.
However where I work we only use RDAC with our IBM FastT, Sun 6140 and STK Flexline storage arrays. RDAC is not for LSI based storage such as Hitachi, Clarion, and EMC. For these servers we manage SAN disk with DM-Multipath.
Setup…
Setting up DM-Multipath is not hard, first you need to make sure that you install the package, device-mapper-mulitpath, and you will need to configure your multipath.conf and drop it into /etc. Below is some info on how to do so.
http://kbase.redhat.com/faq/docs/DOC-3691
You will also need to make sure that you enable the multipathd daemon. This daemon is in charge of checking for failed paths.
Multipath Command…
For those use to using RDAC, DM-Multipath takes some getting used to, especially when you see the output from fdisk -ll.
In one particular instance I was given the disk name of /dev/sdm as the name of the new disk on this box. The output from the fdisk -l command is not exactly helpful, as there are a ton of psuedo devices showing up in my output. This is where the multipath command comes in handy.
In the example below I see a multipathed device of mpath8. This psuedo device is make up of /dev/sdm, which is the disk that I was given, and its mirror pathed bretheren, /dev/sdo
multipath -ll
mpath8 (360060e801004e780052fab2800000024) dm-9 HITACHI,DF600F
[size=35G][features=1 queue_if_no_path][hwhandler=0]
\_ round-robin 0 [prio=0][active]
\_ 3:0:0:6 sdm 8:192 [active][undef]
\_ 4:0:0:6 sdo 8:224 [active][undef]
At this point I need to make LVM aware of the partition on mpath8.
kpartx -a /dev/mapper/mpath0
Now I should have the following in /dev/mapper, which is the partition that I created. This is the device that I will use in LVM
/dev/mapper/mpath0p1