Additional information on Virtual Tape Libraries (VTL) , tape capacity and virtual media management

Article:TECH172896  |  Created: 2011-10-26  |  Updated: 2013-07-26  |  Article URL http://www.symantec.com/docs/TECH172896
Article Type
Technical Solution


Issue



NetBackup image cleanup process treats VTLs just like real tape libraries.
i.e. NetBackup cleanup lesson 101…
When images expire, the image db, media db & vol db are updated. In summary:
-          Image db – Image is removed.
-          mediaDB – number of valid images decremented.
-          volDB – When number of valid images reach zero, media is unassigned, most likely removed from existing pool back into ‘Scratch’ pool.
Nothing in the above process ‘touches’ the tape. With "real" tape libraries, the tape does not have to be in the library for the above process to occur.

As VTLs use disk space to store the data, VTLs have a finite capacity. Often VTLs are configured where the number and size of virtual media exceed the physical VTL disk space – Perhaps because the admins assume a better compression ratio / dedupe ratio than achieved in practice.

As NetBackup expires images, ‘pre-used’ media are unassigned and placed back into the scratch pool, the disk space allocated to the media is still occupied within the VTL. This can result in VTLs running out of disk space and hence calls to NetBackup support for help with failing backups.

Solution



By writing a small amount of data to the beginning of the media, the amount of space allocated to the virtual media will be reduced and the VTL will be free to reuse this reclaimed space.

One method to ‘write a few kb of data’ is to use the NetBackup ‘bplabel’ command to re-label the media AFTER the images on the media has expired. Care needs to be used with bplabel as it does NOT check if media has any valid data on it (assigned/deassigned status).

Note: NetBackup 7.5 contains an improvement to bplabel where it will check the assigned status before the label.
 
The recommended NetBackup hook is the ‘media_deassign_notify’ script, which must be promoted from the “goodies” directory (i.e. copy from netbackup/bin/goodies to netbackup/bin).
-          The script is called as each media is deassigned (by EMM)
-          In the script, place a ‘bplabel’ call and specify the tape using the ‘Scratch’ pool.
The usage of the ‘Scratch’ pool protects against bplabel overwriting media which has been assigned. i.e. Once assigned, the media will no longer be in the Scratch pool and bplabel will fail.
 
The parameters passed to media_deassign_notify are:
"-----------------------------------------"
"     MEDIA ID:  $1"
"     LEGACY MEDIA TYPE:  $2"
"     BARCODE:  $3"
"     ROBOT NUMBER:  $4"
"     ROBOT TYPE:  $5"
"----------------------------------------"
 
It is also recommended to check the ‘ROBOT NUMBER’ so the script only ever attempts to bplabel VTL media.

Additional information with regard to the bplabel command can be found in the Symantec NetBackup Commands Reference Guide

 

The creation and support of any scripts is left as an exercise for the reader.




Article URL http://www.symantec.com/docs/TECH172896


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