Path: utzoo!utgpu!jarvis.csri.toronto.edu!rutgers!apple!voder!zok!mark
From: mark@zok.UUCP (Mark W. Snitily)
Newsgroups: comp.unix.i386
Subject: dd curiosity
Message-ID: <396@zok.UUCP>
Date: 30 Dec 89 19:42:14 GMT
Organization: The distant planet Zok
Lines: 31

This is really just a trivia question about the implementation of dd.
Have recently been using dd to read/write sectors to/from a raw disk
partition.  Noticed a large speed difference based upon the direction
of the I/O.

For example, if I write 4 sectors into the partition starting at sector
20000, the write will complete very fast, (i.e. a few seconds):
      dd if=myfile of=/dev/rdsk/0p1 seek=20000 count=4

But, if I read 4 sectors from the partition starting at sector 20000,
the read will take forever to complete, (i.e. we're talking minutes to
hours depending upon the skip value):
      dd if=/dev/rdsk/0p1 of=myfile skip=20000 count=4

Anyone have any ideas why reading with dd takes so long?

Any ideas of how to speed it up?  (e.g. Don't use dd, use program xyz
instead.  Or something like, if you specify undocumented option "inseek",
then dd will seek to the specified sector of the input file rather than
reading every sector.  etc...)

BTW, Running 386/ix 2.0.2, raw disk partition is on an ESDI 14ms drive.

Thanks...

-- Mark

Mark W. Snitily                 Consulting Services:
894 Brookgrove Lane             Graphics, Operating Systems, Compilers
Cupertino, CA 95014             (408) 252-0456
mark@zok.uucp