About SCSI persistent reserve
The NetBackup process that reads from or writes to the media in a drive (bptm) issues SCSI persistent reserve commands to do the following:
Register with the tape drive's device server (the server is a logical unit within a drive that processes SCSI tasks)
Request an exclusive access reservation
If the tape drive's device server grants the reservation, the NetBackup process has exclusive use of the device. The reservation prevents other host bus adapters (HBAs) from issuing any commands that can cause data loss.
If the reservation fails, NetBackup fails the job.
When the NetBackup process is finished with the drive, NetBackup unloads the drive and sends a persistent reserve clear command to the drive. The command removes both the reservation and the registration.
SCSI persistent reserve also provides device status detection, which NetBackup uses to resolve reservation conflicts within NetBackup.
The reservation does not prevent other applications on the host that has the reservation from using the same device and from causing data loss. For example, if a user on the same host issues a UNIX mt command, the mt command can take control of the drive.
Also, other HBAs can clear or release a SCSI persistent reservation. Therefore, an application can clear another HBA reservation (although it should not do so).