First off, (depending on the type of your tape drive) you have a pretty good chance of not needing any compress at all because most of the IBM tape drives have a hardware compression built in. If you send a compressed stream down to the tape drive it will be compressed a second time which will usually make the file a little bigger (because of the second compression overhead adding), not smaller.

Second, even if it is justified to use "compress" in your case, you probably have to add "if=-" to the "dd" command. AFAIK "dd" doesn't use stdin per default for incoming data. It also might be necessary to state the blocksize. Use "bs=<some number>" in this case to state the block size in bytes.

A third reason might be that you have to adress the tape drive with another device number, maybe "/dev/rmt0.1". Have a look in the manual for the meaning of the minor device numbers with tape drives.

I do not recognize your first statement "change tape block_size=512", usually this is accomplished using the "tctl" command, alternatively use the "chdev" command on "/dev/rmtn".

I hope this helps.

bakunin