NetBackup will write until it receives an end-of-tape message, then
load up another tape and continue.  A single backup does not need to
fit on a single tape.  

However, there is a bp.conf entry you can add called
DISALLOW_BACKUPS_SPANNING_MEDIA.  I'm not sure what error you'd get
with this, I've never had reason to use it.

A DLT8000 drive with DLT IV tapes (as printed on the cartridge itself,
not to be confused with Media Manager's dlt, dlt2, and dlt3 tape
"types") will hold 40GB of uncompressed data.  There's really no way
to say how much it will hold when using compression.  You can get an
estimate by using the available_media script and averaging the usage
of all the FULL tapes.

The tape selection algorithm is described beginning on page 638 of the
NBU System Administrator's Guide (UNIX 3.4).

If you find this happening on a single tape, I would recommend
freezing the tape so that it is not used again:

bpmedia -freeze -ev <mediaid>

If this happens with most/all tapes, the first thing I would look at
is to make sure the system is configured correctly for that drive
type.  Reference the Media Manager Device Configuration Guide.
 

Konstantin 'Kastus' Shchuka writes:
> Hi NetBackup gurus,
> 
> I have a stupid problem due to the lack of my understanding how NetBackup 
> works.
> 
> I have a Sun StorEdge L9 tape autoloader. It writes to DLT2 tapes. 
> I have a class for full back up of one server which takes appr. 30G.
> One tape can store 40G, if I understand it correctly.
> 
> THe first backup goes to tape without problems. When the second backup is 
> attempted,
> the tape writes to some point, then returns write media error, probably
> because of the end of the tape reached. So the attempt fails with code 84.
> 
> Then another attempt starts, but the same tape is being selected and the 
> story 
> repeats. 
> 
> My question is: should one backup fit on one tape? Or can it start on one tape
> and continue on another? How does NetBackup estimate what can go to some
> particular tape? Is there any way to prevent such behaviour?
> 
> Thank you,
> -- 
> Konstantin 'Kastus' Shchuka
> Unix System Administrator
> ePocrates Inc.
> tel 650.232.4886
> fax 650.592.6995
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-- 
Larry Kingery 
      3 kinds of people: those who can count & those who can't.