Understanding Persistent Reserve

The AIX® server displays the value of reserve_policy and PR_key_value for Persistent Reserve. Use chdev command to set the values for reserve_policy and PR_key_value.

Note: While Persistent Reserve (PR) is supported on both AIX and Linux, reserve_policy is applicable only to AIX.
Persistent Reserve refers to a set of Small Computer Systems Interface-3 (SCSI-3) standard commands and command options. These PR commands and command options give SCSI initiators the ability to establish, preempt, query, and reset a reservation policy with a specified target disk. The functions provided by PR commands are a superset of current reserve and release mechanisms. These functions are not compatible with legacy reserve and release mechanisms. Target disks can only support reservations from either the legacy mechanisms or the current mechanisms.
Note: Attempting to mix Persistent Reserve commands with legacy reserve and release commands will result in the target disk returning a reservation conflict error.
Persistent Reserve establishes an interface through a reserve_policy attribute for SCSI disks. You can optionally use this attribute to specify the type of reservation that the device driver will establish before accessing data on the disk. For devices that do not support the reserve_policy attribute, the drivers will use the value of the reserve_lock attribute to determine the type of reservation to use for the disk. GPFS™ supports four values for the reserve_policy attribute:
no_reserve::
Specifies that no reservations are used on the disk.
single_path::
Specifies that legacy reserve/release commands are used on the disk.
PR_exclusive::
Specifies that Persistent Reserve is used to establish exclusive host access to the disk.
PR_shared::
Specifies that Persistent Reserve is used to establish shared host access to the disk.
Persistent Reserve support affects both the parallel (scdisk) and SCSI-3 (scsidisk) disk device drivers and configuration methods. When a device is opened (for example, when the varyonvg command opens the underlying hdisks), the device driver checks the ODM for reserve_policy and PR_key_value and then opens the device appropriately. For PR, each host attached to the shared disk must use unique registration key values for reserve_policy and PR_key_value. On AIX, you can display the values assigned to reserve_policy and PR_key_value by issuing:
lsattr -El hdiskx -a reserve_policy,PR_key_value
If needed, use the AIX chdev command to set reserve_policy and PR_key_value.
Note: GPFS manages reserve_policy and PR_key_value using reserve_policy=PR_shared when Persistent Reserve support is enabled and reserve_policy=no_reserve when Persistent Reserve is disabled.