Here is an example of what can happen when you compress files that are already
compressed.  This example is using software compressions on a Unix system using
2 different utilities:  compress and gzip.

Origional file.
-rw-rw-rw-   1 root     sys      50339840 Oct  1 15:36 usr.tar
Unix compress utility.
Unix compress utility.
Compressed, compressed again, and compressed 3rd time.
-rw-rw-rw-   1 root     sys      26751013 Oct  1 15:38 usr.tar.Z
-rw-rw-rw-   1 root     sys      33438204 Oct  1 15:41 usr.tar.Z.Z
-rw-rw-rw-   1 root     sys      33438204 Oct  1 15:41 usr.tar.Z.Z
-rw-rw-rw-   1 root     sys      39067782 Oct  1 16:00 usr.tar.Z.Z.Z
-rw-rw-rw-   1 root     sys      39067782 Oct  1 16:00 usr.tar.Z.Z.Z
gzip utility.
gzip utility.
Compressed, compressed again, and compressed 3rd time.
-rw-rw-rw-   1 root     sys      18601090 Oct  1 15:44 usr.tar.gz
-rw-rw-rw-   1 root     sys      18287838 Oct  1 15:46 usr.tar.gz.gz
-rw-rw-rw-   1 root     sys      18287838 Oct  1 15:46 usr.tar.gz.gz
-rw-rw-rw-   1 root     sys      18290584 Oct  1 15:53 usr.tar.gz.gz.gz
-rw-rw-rw-   1 root     sys      18290584 Oct  1 15:53 usr.tar.gz.gz.gz
Compressed file from compress compressed again by gzip utility.
Compressed file from compress compressed again by gzip utility.
-rw-rw-rw-   1 root     sys      25173084 Oct  1 15:49 usr.tar.Z.gz
What happens  depends on the algorithm used.  It would be
What happens  depends on the algorithm used.  It would be
interesting to here from Sony (AIT), IBM (Magstar) and Qauntum (DLT) how
their compression systems handle data that's already compressed.

rick
----------------------------------------------------------------------
Richard L. Rhodes     e: rhodesr AT firstenergycorp DOT com
Richard L. Rhodes     e: rhodesr AT firstenergycorp DOT com
Ohio Edison Co.       p: 330-384-4904
                      f: 330-384-2514