Followup to DVD Burning question

[Brooks Davis]

On Wed, Apr 13, 2005 at 02:11:39PM +0200, Søren Schmidt wrote:
> Arne Wörner wrote:
> >--- Søren Schmidt <sos at DeepCore.dk> wrote:
> >
> >>Arne Wörner wrote:
> >>
> >>>That's how I do backups with a DVD-RW:
> >>> tar cf - ... | dd of=/dev/acd0 obs=32k conv=osync
> >>>
> >>>Unluckily my DVD-RW's cannot be used for UFS with my
> >>>burner... I do not know why.
> >>
> >>DVD-RW doesn't quite cut it for UFS, you need DVD+RW media for
> >>that.
> >>
> >
> >Why? I can read and write the same block of a DVD-RW round robin.
> >I mean:
> >
> >>dd if=a of=/dev/acd0 bs=32k count=1 oseek=10
> >>dd if=/dev/acd0 of= /dev/null bs=32k count=1 iseek=10
> >>dd if=b of=/dev/acd0 bs=32k count=1 oseek=10
> >>dd if=/dev/acd0 of= /dev/null bs=32k count=1 iseek=10
> >>dd if=c of=/dev/acd0 bs=32k count=1 oseek=10
> >>dd if=/dev/acd0 of= /dev/null bs=32k count=1 iseek=10
> >
> >...
> >This looks just like a hard disc to me...
> 
> I forgot the exact details, but the DVD-RW media (or rather the DVD-RW 
> method of accessing RW media) doesn't allow for non 32K transfers, 
> whereas the DVD+RW method does read-modify-write behind the scene to get 
> it to work seamlessly. I guess this could be worked around in the burner 
> firmware but that would probably offend the DVD consortium ;)

One could write a GEOM that did read-modify-write to do blocksize
translation. :-)

-- Brooks

-- 
Any statement of the form "X is the one, true Y" is FALSE.
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