Why does SCSI-3 Persistent Group Reservation (PGR) need to be enabled when using Storage Foundation (tm) for Windows (SFW) Dynamic Multipathing (DMP) with the Active / Active I/O policy in a cluster environment?

When using SFW DMP with the Active / Active I/O policy in a cluster environment, SCSI-3 PGR must be used due to the fundamental differences between SCSI-2 and SCSI-3 PGR.

SCSI-3 PGR uses a concept of registration and reservation. Hosts accessing a SCSI-3 device register a key with it. Each host registers its own key. Multiple hosts registering keys form a membership. Registered hosts can then establish a reservation with the SCSI-3 device. The reservation type is set to " Exclusive Access - Registrants Only". This means that only some commands to communicate with the device are allowed, and there is only one persistent reservation holder. With SCSI-3 PGR technology, blocking write access can be done by removing a registration from a SCSI-3 device. In the SFW DMP implementation, a host registers the same key for all of its paths to the SCSI-3 device, allowing multiple paths to use it without having to make and release reservations.

In contrast, SCSI-2 reservations can only be used by one host on a single path. SCSI-2 reservations prevent other paths in the same host, or another host from accessing the SCSI-2 device. This limits access to the SCSI-2 device to only one path, and prevents the use of multiple paths to the SCSI-2 device even if they are available. The reserve command in SCSI-2 reservations are used to allow commands to a device from a single initiator, also referred to as path. Only the path that issued the reserve command can access the device. SCSI-2 reservations are not persistent and do not remain active upon a bus or device reset. This is because the SCSI-2 reservations cannot be re-established when a path recovers and the subsequent hard resets that occur.

Because of the nature of SCSI-2, unpredictable results will occur if it is used with SFW DMP in an Active / Active I/O policy environment and is the reason that SCSI-3 PGR must be used in a cluster environment.

For DMP Device Specific Modules (DSMs), the Active / Active I/O policy is implemented by translating SCSI reserve / release commands to SCSI-3 PGR commands. SCSI-3 PGR must be enabled before using the Active / Active I/O policy setting for DMP DSMs (SCSI-3 PGR is disabled and Active / Passive is the I/O policy by default). SFW DMP DSMs and the Active / Active I/O policy are only available in SFW 4.3 and above, and are only supported on Windows 2003 and above. DMP DSMs are not supported with Windows 2000, fibre channel port drivers, fibre channel SCSI Miniport drivers, or with boot and cluster disks on the same path / HBA.

For more information about enabling SCSI-3 PGR for SFW DMP using the Active / Active I/O policy in a cluster environment, please refer to Hardware Compatibility List or the Veritas Storage Foundation Administrator's Guide in the Related Documents section below.