2.11BSD#431 - /usr/local/man/cat1/ddd.0

DDD(1L)             UNIX Programmer's Manual		  DDD(1L)


NAME
     ddd - double-speed data dumper

SYNOPSIS
     ddd [option=value] ...

DESCRIPTION
     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).

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 pro-
     vided.

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 connec-
     tions.  However, cbs should be small enough for all the data
     to fit in core, since page faults add overhead.

AUTHOR
     Tapani Lindgren <nispa@cs.hut.fi>
     Laboratory of Information Processing Science
     Helsinki University of Technology
     Finland

SEE ALSO
     dd(1), tar(1), dump(8)


Printed 11/24/99						1


DDD(1L)             UNIX Programmer's Manual		  DDD(1L)


BUGS
     Should you find one, let me know!

WARNING
     (Applies to U.S. residents & citizens only)
     Do not use this program!  Get rid of it as soon as you can!
     It will probably corrupt all your data, break down your com-
     puter and cause severe injury to the operators.  Even read-
     ing the source code may give you a headache.  I warned you!
     I will take no responsibility whatsoever!


Printed 11/24/99						2