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I found this great definition of the SCSI3-PR on  “dba-oracle.com”

“SCSI-3 PR, which stands for Persistent Reservation, supports multiple nodes accessing a device while at the same time blocking access to other nodes. SCSI-3 PR reservations are persistent across SCSI bus resets or node reboots and also support multiple paths from host to disk. For SCSI-2 disks, reservations are not persistent which means they do not survive node reboots.

SCSI-3 PR uses a concept of registration and reservation. Systems that participate, register a key with SCSI-3 device. Each system registers its own key. Then registered systems can establish a reservation. With this method, blocking write access is as simple as removing registration from a device. A system wishing to eject another system issues a pre-empt and abort command and that ejects another node. Once a node is ejected, it has no key registered so that it cannot eject others. This method effectively avoids the split-brain condition. ”

To enable the SCSI3_Persistent_Reserve bit on Symmetrix/DMX for Oracle RAC (or any instance where mutliple hosts need to have simultanious access to volumes)

In “configure.txt” on your Solutions Enabler host.

set device 0090 attribute=SCSI3_persist_reserv;

then from the command line:

# symconfigure -sid 1234 -f configure.txt preview

# symconfigure -sid 1234 -f configure.txt commit

Simple.    To verify that it was set, run the following:

# symdev -sid 1234 show 0090

And among the output you’ll find the following:

SCSI-3 Persistent Reserve: Enabled

You can then configure RAC.