Today while working in the Datacenter, I accidentally destroyed a fibre cable running to one of our database servers. All the databases went poof and the DBAs freaked out. After I reminded them that this was a non-production database I went about running a new fibre cable.
Being that this box was dual pathed back to the SAN, the server should have had additional paths back to its disks and the databases should have keep running along just fine. However, as luck would have it, the secondary path was down at the time I destroyed the primary.
Anyway after reseating the secondary fibre and running a new primary cable I was able to verify that I had multiple connections back to the disks with the following commands.
# vxdmpadm getdmpnode dmpnodename=3pardata0_4641
NAME STATE ENCLR-TYPE PATHS ENBL DSBL ENCLR-NAME ============================================================================== 3pardata0_4641 ENABLED 3PARDATA 4 4 0 3pardata0
Where 3pardata0_4641 is the disk name and PATHS is obviously the number of paths back to the SAN disk.
Even better yet you can check DMP (dynamic multipath status) for all disks with the command below.
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.
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.
You will also need to make sure that you enable the multipathd daemon. This daemon is in charge of checking for failed paths.
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.