MPIO reserve_policy

▶reserve_policy

The MPIO reserve_policy defines whether a reservation methodology is employed when the device is opened. The values are as follows: # lsattr -Rl -a reserve_policy no_reserve single_path PR_exclusive PR_shared   no_reserve Does not apply a reservation methodology for the device. The device might be accessed by other initiators, and these initiators might be on other host systems. single_path Applies a SCSI2 reserve methodology for the device, which means the device can be accessed only by the initiator that issued the reserve. This policy prevents other initiators in the same host or on other hosts from accessing the device. This policy uses the SCSI2 reserve to lock the device to a single initiator (path), and commands routed through any other path result in a reservation conflict. Path selection algorithms that alternate commands among multiple paths can result in thrashing when the single_path reserve value is selected. As an example, assume a device-specific PCM has a required attribute that is set to a value that distributes I/O across multiple paths. When single_path reserve is in effect, the disk driver must issue a bus device reset (BDR) and then issue a reserve using a new path for sending the next command to break the previous reservation. Each time a different path is selected, thrashing occurs and performance degrades because of the overhead of sending a BDR and issuing a reserve to the target device. (The AIX® PCM does not allow you to select an algorithm that could cause thrashing.) PR_exclusive Applies a SCSI3 persistent-reserve, exclusive-host methodology when the device is opened. The PR_key attribute value must be unique for each host system. The PR_key attribute is used to prevent access to the device by initiators from other host systems. PR_shared Applies a SCSI3 persistent-reserve, shared-host methodology when the device is opened. The PR_key value must be a unique value for each host system. Initiators from other host systems are required to register before they can access the device.