> Was there some reason to not use a second process?  I once got fed up
> with the way dd between two disks kept each disk busy only about half
> the time and built a program that had a reader process and a writer
> process, with blocks passed between them via shared memory (mmap
> MAP_ANON|MAP_SHARED, fork, and you have shared memory that doesn't
> stick around on exit the way a SV SHM segment does).  It helped; even
> between two disks on the same SCSI chain, I saw significant overlap
> (based on watching the disk lights).

Wrote a similar beast (tc) for this:

% find . -print | cpio -oc | tc > /dev/rmt0

To get QIC150 tape drives to stream...
provided the writing task (the one with a char special fd) was set
real-time you could detect the tape turn around, even going through
the terminfo database.

    David