What is the actual tape capacity?

[Sunny K]

Hi friends,
Something has been bugging me for some time. Take a typical DLT8000 with a DLT tape that denotes 40/80GB. To a layman, that would mean 40 GB of data uncompressed and 80 GB if compressed.
Recently I ran a OmniBack backup on this tape and it completed on this 1 tape but with 100GB of data. I am wondering is there further compression that take it beyond the 80GB? If so, how do we know how far we can stretch the tape capacity?
Thanks.

[Kurt Beyers. · Accepted solution]

Sunny,

The data compression of 2:1 is only a rule of fist (a generalization after many observations).

The actual compression ratio is data dependent. Suppose you've got a database space file of 1 GB with only a few records of data in it. You'll achieve a very high compression ratio on this database file as it contains a lot of redundant information. But if the same database space file is completely filled with records, the compression ratio will be around 1.

So you are for sure that you can write at least 40 GB on a DLT800 tape and it is safe to assume that 80 GB of compressed data will fit on the tape. But the actual size that will fit on the tape can only be seen when the tape is full. A tape is always used by backup software till the end of the tape is reached.

Kurt

[Marino Meloni_1]

here is the "compression" explained

http://h20000.www2.hp.com/bizsupport/TechSupport/Document.jsp?objectID=lpg50244

marino