2.11BSD/src/local/ddd/ddd.1

.TH DDD 1L
.SH NAME
ddd \- double-speed data dumper
.SH SYNOPSIS
.B ddd [option=value] ...
.SH DESCRIPTION
.IR Ddd
works almost the same way as dd(1), but it has a much better
throughput, especially when used with slow i/o-devices, such as
tape drives.  The improvement is achieved mainly by dividing
the copying process into two processes, one of which reads while
the other one writes and vice versa.  Also all code conversion
capabilities are omitted.  There is no additional overhead copying
data between various conversion buffers.

Ddd was inspired by the vast difference in speed between BSD4.2 and
BSD4.3 dumps - in BSD4.3 dump(8) uses alternating processes to write
to raw magnetic tape, thus keeping the tape continuously in motion.
I wanted to get the same improvement to remote dumps, so this
filter was needed.  Directing all physical I/O through ddd usually
increases the throughput of any pipeline of unix commands
(if you have enough MIPS and RAM to handle two extra processes).
.SH OPTIONS
Ddd uses options if, of, ibs and obs exactly as dd(1).  Option bs can
also be used to specify ibs and obs at once.  One option differs slightly
in meaning: cbs can be used to specify the size of the internal buffer.
Input and output processes will swap duties when cbs bytes have been
transferred.  Default values for all sizes are 512 bytes.
As with dd(1), letters k (kilobyte), b (block) or w (word) can be
appended to size values.
Other options are not provided.
.SH HINTS
For best performance, block sizes should be rather large.  For magnetic
tape, I use obs=100b and cbs=500b or so.  Large block sizes (~100b) are
also effective for network connections.  However, cbs should be small
enough for all the data to fit in core, since page faults add
overhead.
.SH AUTHOR
Tapani Lindgren <nispa@cs.hut.fi>
.br
Laboratory of Information Processing Science
.br
Helsinki University of Technology
.br
Finland
.SH SEE ALSO
dd(1), tar(1), dump(8)
.SH BUGS
Should you find one, let me know!
.SH WARNING
(Applies to U.S. residents & citizens only)
.br
Do not use this program!  Get rid of it as soon as you can!
It will probably corrupt all your data, break down your computer
and cause severe injury to the operators.
Even reading the source code may give you a headache.
I warned you!  I will take no responsibility whatsoever!