[Techtalk] Detect if Dump used multiple tapes for backup.

[Conor Daly]

On Tue, Jun 22, 2010 at 08:03:22AM -0400 or thereabouts, Little Girl wrote:
> Hey there,
> 
> Conor Daly wrote:
> 
> > I'm backing up a 300Gb filesystem to an SDLT320 tape and,
> > occasionally, the weekly level 0 dump will overflow to a second
> > tape.
> > 
> I'm not sure if this will be of any use to you, but I've attached a
> small Bash script that checks the size of a directory (in KB) and
> displays the results (also ringing the system bell for undesired
> results). Maybe you could add something like this into the mix.

I think the script got stripped by the mailman software...
 
> I don't have any magnetic tapes, so I don't know if regular
> scripts will even work on them. If this is totally off base,
> please ignore. (:

Mag tapes are a different kind of storage.  Basically data gets written /
read sequentially so there's no real way to interact with a tape.  Depending
on the software that wrote the tape, there may be a catalogue stored at the
beginning of the tape but there's no real way to know about size stored.

In particular, tape capacity is usually quoted as uncompressed / compressed
size.  So, our SDLT320 tape is a 160Gb uncompressed / 320Gb compressed tape.
In the normal course, the tape drive will do hardware compression of data
and this will do a good job of squeezing data onto the tape unless the data
consists of already compressed files (eg. images, tarballs etc).  Given that
a lot of our data is stored as images, we will occasionally overrun a single
tape.

My current problem is that I can't come up with an easy way to check if a
particular tape contains a complete or only partial backup.  At the moment
I'm leaning towards the notion of keeping a tally on the main disk of which
tape has what and using that to decide which tape to write to next.

Conor

-- 
Conor Daly,                   
Met Eireann, Glasnevin Hill,  
Dublin 9, Ireland             
Ph +3531 8064276 Fax +3531 8064247

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