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The following is an archive of a post made to our 'vox-tech mailing list' by one of its subscribers.

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Re: [vox-tech] backing up the master boot record
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Re: [vox-tech] backing up the master boot record



On Tue, 19 Jun 2001, Peter Jay Salzman wrote:

> begin: Mister Resistor <xxxxxx@uiuc.edu> quote
> > 
> > > also, a question about dd.  according to the man page, bs=BYTE tells dd to
> > > read and write BYTE bytes.  count=BLOCK tells dd to copy only BLOCK input
> > > blocks.
> > >
> > > what is a block?
> > 
> > Probably the equivalent of a sector.  Read the man pages. :-)
> 
> i'd be glad to if you could tell me which man page to read...  :)

That would be nice, but it is in

 man dd

if you interpolate:

---Quote---
       count=BLOCKS
              copy only BLOCKS input blocks

       ibs=BYTES
              read BYTES bytes at a time
[...]
       obs=BYTES
              write BYTES bytes at a time
[...]
       seek=BLOCKS
              skip BLOCKS obs-sized blocks at start of output

       skip=BLOCKS
              skip BLOCKS ibs-sized blocks at start of input
-----------

<hypothesizing>There is something historical about dd that I never quite
understood.  It was described to me as a tool for converting data from
storage on one type of device to storage on another, with 9-track tapes
from various sources being common examples at the time it was described to
me.  Of course, I have only used a 9-track tape once, and that was on a
VMS system, so I don't know much about them.  I imagine that if a device
driver could only support certain block sizes, a tool like dd would be
useful for extracting data from tapes with an odd block size and putting
it on tapes with a size more to your liking.  I don't know if such
restrictions remain in modern device drivers...</hypothesizing>

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