To understand NetBackup's behavior with Drive Cleaning TapeAlerts, it is important to understand the TapeAlert interface to a drive.

The TapeAlert interface to the tape drive is via the SCSI bus, based on a new LOG SENSE page, which contains 64 one-byte alert flags.

The specific conditions for any one flag to be set and cleared are device-specific, and thus must be defined by the device vendor (DRIVE VENDOR) implementing them.

The configuration of the Log Sense page is via a Mode Select page. The Mode Sense/Select configuration of the TapeAlert interface is compatible with the SMART diagnostic standard for disc drives.

The TapeAlert LOG SENSE page is read by NetBackup at the following times:

           * At the beginning of a write/read job

For this Technote Alert flags 20 to 25: are used for cleaning management. Some Drive vendors implementations may vary, however "Flag 20:  Clean Now" is used by NetBackup as an indication of when a drive needs cleaning.

When a drive is selected by NetBackup for a backup the LOG SENSE PAGE is looked at by avrd for status. If the "Clean Now" flag is set. The backup will continue with the current tape and clean the drive when complete.  

If a backup is in progress and "Flag 20" is set, the flag would not be read until the a tape is mounted in the drive.

If a job spans media and during the first tape the Clean Now flag is set,  the cleaning light comes on,  the second piece of media is mounted in the drive,  the LOG SENSE PAGE "Clean Now" flag is consumed, the second tape is written too and after the second tape the drive is cleaned. If a third piece of media is needed it will be mounted next.

The implication is that the present job will conclude its write despite a TapeAlert Clean Now message.  (i.e. The TapeAlert will not require the loss of what has been written to tape so far.)  This is true regardless of the number NetBackup jobs involved in writing out the rest of the media.  

If there are a large number of media becoming FROZEN as a result of having implemented TapeAlert, there is a strong likelihood of underlying media and/or tape drive issues.

Future Patches may change the operation listed above. At that point this TechNote will be withdrawn and updated to reflect any changes.

Details of note:  

Given the options of TapeAlert, robotic cleaning, or frequency based cleaning, VERITAS Software recommends TapeAlert.  For greater detail of each option see the linked technote:  Public Link: http://seer.support.veritas.com/tnotes/NETBACKUP/231451.htm