I/O Fencing via SCSI-3 PGR

AIX supports I/O fencing with SCSI-3 Persistent Group Reservation (PGR) enabled.

There are two ways that AIX supports I/O Fencing via SCSI-3 PGR:

The SCSI-3 PGR API
The AIX DKPRES API

When the disk reserve_policy is set to:

no_reserve – Adaptive Server uses raw SCSI-3 PGR API, where # is the hdisk number:
```
chdev -l hdisk# -a reserve_policy=no_reserve
```
In this configuration, remote applications cannot accidentally create and remove registrations on disk.
PR_shared – Adaptive Server uses AIX DKPRES API, where # is the hdisk number.:
```
chdev -l hdisk# -a reserve_policy=PR_shared
```
This PR_shared setting may allow any remote application (such as database devices) to open the disk that automatically creates and removes registrations on the device. This can make the database devices susceptible to data corruption.

Note: syb_cluster/UAF are known to open database disks and cause this problem, and therefore should not be left running when using AIX DKPRES.

The preferred method is raw SCSI-3 PGR API, however, some IBM storage devices like DS5800 do not support raw SCSI-3 PGR. For this reason, set the reserve_policy to no_reserve first. If the Adaptive Server instance fails to start due to I/O fencing issues, then the device does not support raw SCSI-3 PGR API. In such cases, the devices must be configured to use DKPRES API, in which case, set the reserve_policy to PR_shared.