Re: Data Compression on 4mm tapes

In article <1145909660.403472.224060@xxxxxxxxxxxxxxxxxxxxxxxxxxxx>,
"steven_nospam at Yahoo! Canada" <steven_nospam@xxxxxxxx> writes:
I had a questions asked of me today and really could not answer it well
so figured I'd see if anyone else knew.

Is there a way to calculate how much space a file system will take when
it is written to 4mm tape that is using data compression on the tape
drive?

Example:

Filesystem 1024-blocks Used Free %Used Mounted on
/dev/qclv 15728640 6071796 9656844 39% /qc
/dev/oldlv 15728640 11156748 4571892 71% /old
/dev/eomlv 15728640 10150848 5577792 65% /eom

I have a 4mm 4GB tape drive that offers 20gb/40gb when compressed. Is
there any way to calculate how much space will be left on this tape
after data compression in case we want to add another set of 18gb.

Uncompressed, I would need 45gb of space, but compressed it might just
fit.


This certainly is hard to answer, the 20/40 estimate is based on "average"
data mixes. I myself need two DDS-4 cartridges to hold about 60 GB of our data
(mixed text and non-zero binary). Maybe one can use some of the
compression utilities like zip on a subset of the data to estimate their
"compressibility", I don't think the hardware compression of the tape drives
is vastly different (it's probably all Lempel-Ziv anyway).
.

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