SCSI-3Persistent Reservations (SCSI-3 PR) are required for I/O fencing and resolve theissues of using SCSI reservations in a clustered SAN environment. SCSI-3 PRenables access for multiple nodes to a device and simultaneously blocks accessfor other nodes.

SCSI-3reservations are persistent across SCSI bus resets and support multiple pathsfrom a host to a disk. In contrast, only one host can use SCSI-2 reservationswith one path. If the need arises to block access to a device because of dataintegrity concerns, only one host and one path remain active. The requirementsfor larger clusters, with multiple nodes reading and writing to storage in acontrolled manner, make SCSI-2 reservations obsolete.

SCSI-3 PR usesa concept of registration and reservation. Each system registers its own "key"with a SCSI-3 device. Multiple systems registering keys form a membership andestablish a reservation, typically set to "Write Exclusive Registrants Only."The WERO setting enables only registered systems to perform write operations.For a given disk, only one reservation can exist amidst numerous registrations.

With SCSI-3 PR technology, blocking write access is assimple as removing a registration from a device. Only registered members can"eject" the registration of another member. A member wishing to eject anothermember issues a "preempt and abort" command. Ejecting a node is final andatomic; an ejected node cannot eject another node. In VCS, a node registers thesame key for all paths to the device. A single preempt and abort command ejectsa node from all paths to the storage device.

When the persistentreservation holder of a Write Exclusive – Registrants Only or ExclusiveAccess – Registrants Only type reservation becomes unregistered thepersistent reservation shall be released. The device server shall establish aunit attention condition for every initiator port associated with a registeredI_T nexus other than the initiator port associated with the I_T nexus thatunregistered. The sense key shall be set to UNIT ATTENTION and the additionalsense code shall be set to RESERVATIONS RELEASED.

The device server shallestablish a unit attention for every initiator port associated with a registeredI_T nexus whose reservation key was removed. The additional sense code shall beset as follows:

a) If the service action was CLEAR, the additional sensecode shall be set to RESERVATIONS PREEMPTED;

or

b) If the serviceaction was PREEMPT or PREEMPT AND ABORT, the additional sense code shall be setto REGISTRATIONS PREEMPTED.