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Understanding how NetBackup writes to a tape.

Understanding how NetBackup writes to tapes and what happens when images expire will help you better manage your tapes.

Tapes are linear - meaning that they write from the front to the back.

if we look at a tape.
BOT|day1image|day2image|blankspace|EOT

now the day1image expires on the tape - the assigned date does not change and you have this.

BOT|expiredimage|day2image|blankspace|EOT

if you try to write to this tape again it will ONLY write in the blankspace as tapes are Linear - meaning they can only append (tapes cannot write here and there like a disk can)

Now day2image expires - the assigned date of the tape goes blank - no assigned date

-------

Now look at it with multiplexing.

you use the tape - it gets an assigned date.

BOT|server1part1|server2part1|server1part2|server2part2|blankspace

if the backup for server1 fails or if the backup image expires then you have this

BOT|expiredimage|server2part1|expiredimage|server2part2|blankspace

remember tape is linear it cannot go back and write in those spaces where the failed image went.

So if a tape is full it cannot be written to again until ALL the images on the tape have expired.

 

 


expire the image and expire the tape are two different things.

when NetBackup goes to use a tape and mounts it, it gets assigned -
assigned means that NetBackup took a scratch tape with no assigned date and tried to use the tape today - does not mean that it wrote to it.

Now when NetBackup writes to the tape it gets an images ( this does not change the assigned date).

if the backup is good then the tape has an assigned date, and has a good images.

now say you use the SAME tape tomorrow - the assigned date does NOT change - but the tape does get a new image.  So now you have this (no multiplexing)

BOT|day1image|day2image|blankspace|EOT

now the day1image expires on the tape - the assigned date does not change and you have this.

BOT|expiredimage|day2image|blankspace|EOT

if you try to write to this tape again it will ONLY write in the blankspace as tapes are Linear - meaning they can only append (tapes cannot write here and there like a disk can)

Now day2image expires - the assigned date of the tape goes blank - no assigned date

-------

Now look at it with multiplexing.

you use the tape - it gets an assigned date.

BOT|server1part1|server2part1|server1part2|server2part2|blankspace

if the backup for server1 fails or if the backup image expires then you have this

BOT|expiredimage|server2part1|expiredimage|server2part2|blankspace

remember tape is linear it cannot go back and write in those spaces where the failed image went.

so you do lose space on a tape when a backup fails or expires.

a tape will stay assigned until all images on the tape have expired.
so if you want the tape to be unassigned, you need to expire all the images on it.


Now expiring a tape has to do with the Physical tape.
Not everybody puts an expiration date on a tape.
If you only want to use a tape for 3 years then you can put a date on the tape that sets it for 3 years.  After that date NetBackup will not longer use the tape to do backups on, but the tape can still be used for restores.
To see this you would right click on the media in the console.
that stuff at the top has to do with the expiration of the physical tape, people new to NetBackup quite often get this confused with expiring all the images on the tape.


Understanding how NetBackup writes to tapes and what happens when images expire will help you better manage your tapes.