>timex dd if=/dev/zero of=/dev/rmt1 bs=1024 count=1000
>
>The result was real 21.06 seconds.  This falls way short of what I would
>expect when the advertised sustained transfer rate is supposed to be
>15MB/sec native (30MB/sec compressed). ...

1K seems like a very small block size. Did you try other block sizes to see
if that made any difference in speed?  I think I remember some discussion
quite some time ago about what blocksize TSM used - the AIX setting for the
tape device or some other value - I think the answer was that it used a 64K
blocksize.