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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: jdnewmil@dcn.davis.ca.us <jdnewmil@dcn.davis.ca.us> quote
> > 
> > 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
> here's what i know:
>  cylinders are stacked circles.
>  heads are sides.
>  sectors are arcs.
>  tracks circles.

These are physical coordinates.  Useful to know for bootstrapping.  Often
faked these days, with the numbers passed across the IDE interface having
little to do with the actual structure of the disk.  The BIOS knows how to
access the very first sector to get OS-specific bootstrapping started.

>  cluster are the smallest group of sectors a file can use.

So this is a characteristic of the filesystem type... FAT, ext2, minix...

> maybe i'm not good at interpolating, but i'm still not understanding.
> are you saying that a block is a cluster?  is it a filesystem term or
> a dd term?

dd lets you define what "block" means for the purpose of accessing the
input file or the output file with the read or write calls.  This may
apply if the device driver is picky about the record sizes you read or
write, but may vary from one device/driver to another, or the driver may
be completely flexible on this value.  dd doesn't know what a cluster is,
but by manipulating its idea of what a block is you may be able to extract
better performance out of a device driver.  For example, when copying a
floppy disk to an image file, selecting a relatively large number of bytes
for ibs will let the floppy driver buffering grab as much data as possible
in each system call (ibs or bs = 32768 for example).

> pete
> > <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>
> -- 
> "The following addresses had permanent fatal errors..."      p@dirac.org
>                                -- Mailer Daemon              www.dirac.org/p

Jeff Newmiller                        The     .....       .....  Go Live...
DCN:<jdnewmil@dcn.davis.ca.us>        Basics: ##.#.       ##.#.  Live Go...
                                      Live:   OO#.. Dead: OO#..  Playing
Research Engineer (Solar/Batteries            O.O#.       #.O#.  with
/Software/Embedded Controllers)               .OO#.       .OO#.  rocks...2k

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