When using 2 Linux servers and PowerPath with a bunch of shared LUNs in order to build a cluster, e.g. Oracle RAC, you have to ensure that on both servers the LUNs are designated as the same /dev/emcpower* devices. Linux is a bit flaky in device enumeration and often, the order of devices gets mixed up. PowerPath can easily fix this.
Use powermt save file=devmap.pp on one host of the cluster and use powermt unmanage dev=emcpower{a,b} and powermt load file=devmap.pp on the other, to make sure they both have the same mapping. Then do a 'powermt save' to make the configuration persistent.
Also, when using LVM in conjunction with PowerPath, you don't want regular SCSI devices (local disks or perhaps iSCSI) to intervene. You can tell LVM to ignore certain devices completely when it comes to LVM. In my case I've included the filters below to make sure Linux VMs and BMs (bare metals) get the proper config. Put this in /etc/lvm/lvm.conf
# add EMCpower devices and sdb, reject all other SCSI disks and any CDs
#BM#filter = [ "a|/dev/emcpower.*|","a|/dev/sdb|","r|/dev/sd.*|","r|/dev/cd.*|" ]
# add any SCSI devices, reject all IDE disks and CDs
#VM#filter = [ "a|/dev/sd.*|","r|/dev/hd.*|","r|/dev/cd.*|" ]
See also the CentOS docs on Controlling LVM Device Scans with Filters
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