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Date:      Sun, 04 Nov 2012 19:38:02 -0800
From:      "Ronald F. Guilmette" <rfg@tristatelogic.com>
To:        freebsd-questions@freebsd.org
Subject:   Re: Questions about dump/restore to/from DVD media
Message-ID:  <22041.1352086682@tristatelogic.com>
In-Reply-To: <50971B88.40303@herveybayaustralia.com.au>

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In message <50971B88.40303@herveybayaustralia.com.au>, 
Da Rock <freebsd-questions@herveybayaustralia.com.au> wrote:

>Also, you may have considered this already (or not :) ), but you are
>using a direct write to backup your system, and then considering
>compression on top of that. CD/DVD filesystems incorporate some parity
>to allow for defects and scratches, so growisofs might be best to use to
>ensure some integrity to your data.
>
>Minimising your space may be good, but a single bit could render all
>your efforts for nought- especially given the compression leaves no room
>for error ;)

I'm not sure if the error detection/correction on DVDs... either -Rs
or +Rs... is a function of the _filesystem_.  In fact I don't believe
that it is, but I could be wrong.

Google for this:

    DVD+R error correction

and there are plenty of references.  The ones that I read in the past
seemed to suggest that the error detection/correction is a fundamental
aspect of how data gets written to both -R and +R disks, totally independent
of whether the data being written was organized into any type of filesystem
or none at all.

In fact, part of the reason that I only use DVD+Rs these days is because
I read something that said that something like 1/4 of every block of data
on DVD-R disks is not even covered by any error correction code AT ALL.

Ah, yes... here is one such reference:

 http://adterrasperaspera.com/blog/2006/10/30/how-to-choose-cddvd-archival-media

    "The DVD-R specification states that for every 192 bits, 64 of them are
    not protected under any scheme, 24 of them are protected by 24 bits of
    parity, and the last 56 bits are protected by another 24 bits of parity.
    This weird (to put it mildly) scheme allows you to easily scramble or
    lose 25% of the data that is required to read your disk! This information
    is almost more important than the actual data burned on the disc itself.

    The DVD+R specification, however, states that for every 204 bits of
    information, it is split into four blocks of 52 bits containing 1 sync
    bit to prevent misreading because of phase changes, 31 bits of data,
    and a 20 bit parity (that protects all 32 bits of data)..."


Regards,
rfg



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