SCSI-3 Persistent Reservation

SCSI-3 Persistent Reservation

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.

Another benefit of the SCSI-3 PR method is that since a node registers the same key down each path, ejecting a single key blocks all I/O paths from that node. For example, SCSI-3 PR is implemented by EMC Symmetrix, Sun T3, and Hitachi Storage systems. In case of SCSI-2 reservation, it works only with one path with one host.