Migrating to new SAN switches. Would like to move 1 path at a time to avoid downtime

mistrm2 · 05-07-2009, 09:07 AM

Hello,
We need to replace our SAN switches in our DataCenter. The plan is to move one path at a time, our servers currently have two paths. I would like to know if there is a way to do this that does not require a reboot or downtime.

The storage is EMC Symmetrix and Clariion, the Linux boxes are RedHat 3,4 and 5.

Thanks.

MensaWater · 05-07-2009, 09:34 AM

I haven't done exactly this but did migrate some LUNs from Clariion on RHEL3 a couple of years back to a new Clariion. We were using the Qlogic fibre cards and Qlogic had a downloadable utility on their web site to force reread of the SCSI bus (see /proc/scsi) to get the devices. I think Emulex has a similar utility but haven't used that. More recently we did some work on RHEL5 using a newer version of the Qlogic utility to add new devices. (The RHEL3 one would be different from RHEL4/RHEL5 as the former has a 2.4x kernel and the latter have 2.6x kernels).

Assuming you're using EMC PowerPath rather than the Linux native multipathing stuff you should be able to see the new devices (e.g. /dev/sdb, /dev/sde) with PowerPath and it should recognize them as paths to the same multipath devices (e.g. emcpowera, emcpowerb) and see the original single path you removed as "failed".

Running "powermt display" and "powermt display dev=all" should show you which Linux paths are currently in use and which /dev/sd* devices go to which emcpower* devices. Running again after disconnect of single path should show the failed ones and running "powermt check force" should remove the failed paths. You would then do something like run the Qlogic or Emulex scsi scan utility mentioned earlier to make the OS see the newly attached path(s) after you connect it and running "powermt check force" should make PowerPath see those new paths.

Since I haven't done exactly what you're doing I'd suggest you work out all the details AND do a complete backup prior to doing anything other than the powermt display commands. Don't take what I wrote as a "step by step" because it isn't - its just intended to give you some information for planning.