How do I tell how full the backup is?

How do I tell how full the backup is?

Post by Andy Lawso » Fri, 28 Jul 2000 04:00:00

Hi all,

Recently we had a problem where a backup tape became full before the
stated capacity was reached. I found a document on the Cheyenne KB
( which pointed
me in the direction of the compressed files being backed up from the
NW volume.

A closer inspection showed we were attempting to fit 1.5Gb of
compressed files and 6Gb of uncompressed files onto a 4-8Gb DAT cart.
It didn't fit, and the backup ran over onto two tapes. This is in a
remote office where we need to fit each backup onto a single cart. I
had realised we reaching capacity but though we might have a while
yet. I was working to the following (rough) equation;

Space remaining = native capacity - (total data / 2)

The stated compression ration of the tape drive is 2:1.

But the correct one (as far as I can figure) should be;

Space remaining = (native cap - comp data) - (uncomp data / 2)

Using this equation, I figure that I'm currently fitting 16.4Gb of
data (after NW, and then tape unit compression) onto a 20-40Gb DLT
cart (on a different server). ie.

3.6 = (20 - 6.0) - (20.8 / 2)

Not a problem, then. Except.........

What I want to find out is the remaining _native_ capacity of the
cart, after a full backup. The amount of data written to it is only
really useful as a rule of thumb. Arcserve is unable to determine how
much of the data was compressed before it was passed to the tape unit,
or what compression ratio was actually achieved by the tape drive as
it was writing to the tape.

So rather than just waiting for the tape to become full, and the
nightly backups to overrun, has anybody any ideas on how to check the
remaining native cart capacity?


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