Manipulation DAT tapes on linux

[Lennart Sorensen]

On Fri, Nov 19, 2004 at 02:40:03PM +0800, JM wrote:
> thanks for all your replies..
> 
> hmm.. the tape says? 20G for uncompressed and 40G for compressed...
> 
> does this mean doing tar -zcf /dev/st0 is the 40G?
> or
> i have to activate the compression and let the drive its job?

Some drives have hardware compression. which MIGHT get 2:1 compression.
It can in some cases speed up the drive since the drive can write more
data if it gets to compress it in the same time/amount of space.  On the
other hand if you use gzip or bzip2 compression from tar, you use a lot
more cpu, which potentially slows things down, and makes seeking within
the tar file imposible (so to restore you MUST do a read from begining
to end of the entire tar to read anything from it.  I believe some tar
implementations can seek over file contents when the file is not
compressed).  Hardware compression is transparent to software and avoids
those limitations on most drives I believe.  I think there is usually a
seperate device name for talking to the drive with hardware compression
enabled.

Lennart Sorensen
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