[Gllug] How much space left on a tape...

[Mike Brodbelt]

Adrian McMenamin wrote:

> No, as a consequence of earlier advice on this list I am using bz2 and no
> other compression.

There are basically two ways of writing data to a tape drive:-

1/ Write (usually uncompressed) data to a drive with hardware
compression on.
2/ Write (usually compressed) data to a drive with hardware compression off.

In the former case, you have no way of knowing when you'll run out, you
simply have to wait for the drive to return EOT, and handle it
appropriately when that happens. Decent backup software manages this. In
this context, tar does not count....

In the latter case, you can know the size of the compressed data before
you write it, and thus you can be sure - of course if you're writing
through a pipe, you're still stuck though.

Amanda handles this by providing a tapetype program, which will decide
how much data your drive can handle. It does this by writing
compressible and incompressible data to the tape, with few filemarks,
and with lots of filemarks, to determine reliably that you're not using
HW compression, and to gauge the size of filemarks written to the tape.
With that, it knows how much of the tape you've used. To make use of
that though, it needs a holding partition on disk so the compressed
backup images can be written out to disk first, so the size is known
before they get written to tape.

Mike
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