7 Oct
2007
7 Oct
'07
6:21 a.m.
On 10/7/07, Jerome H. Fine <jhfinedp3k at compsys.to> wrote:
Jim Leonard
wrote:
> Ethan Dicks wrote:
>
>> When writing DVDs, I always do a read-verify right after burning, and
>> for archival data, I tend to create recovery data with programs like
>> par2, and burn two to three copies. If it's worth my time to archive,
>> it's worth spending $1 for more than one copy.
>
> This is exactly my procedure as well, but I use QuickPar instead of
> par2. I also burn two copies, since high-quality DVD-Rs can be had
> for less than $0.50 in bulk. My precautions include producing an MD5 value for each
Ghost backup file...
What are Quickpar and Winrar? Please clarify. If
they are similar to
MD5, then I guess I am already doing the same thing....
MD5 generates a simple checksum. Winrar (rar/unrar) splits a large
file into uniform segments with optional data integrity record.
Quickpar (and par2) generate additional recovery records that can be
used to reconstruct a damaged archive. What one might do is to start
with a disk image file, of a, say, 500MB filesystem, then hand that
image file to Winrar (rar), telling it to make as many 20MB parts as
required, then run Quickpar (par2) on the collection of 20MB rar
parts, and to generate, say, 20% excess data, resulting in ~500MB of
rar parts, and ~100MB of par parts, then burn that whole mess to a
CD-R. To reconstitute, one can copy the pars and rars to a DOS or
UNIX machine (or other), run Quickpar (par2) to verify that all the
rar parts are intact, then use rar to rebuild the original 500MB file
from all the 20MB parts. The advantage comes in when
you have one or
more of the 20MB rar parts that can't be read from the CD-R...
you
have previously provided for excess data, so as long as you haven't
lost more than about 100MB of your original data, the par files can be
used with the intact rar parts to reconstitute the missing/damaged rar
parts. One analogy is that this is like RAID 5 for files... as long
as you have a sufficient percentage of the original data and recovery
data intact, you can restore what you are missing.
This allows you to archive your data in a way that allows for less
than perfect retention over time. As long as you catch media
degradation before it passes your self-selected threshold, you can
still recover what you originally wanted to save and migrate it to new
media. For the truely paranoid, you are allowed to save arbitrary
amounts of recovery data, to 100% or more (i.e. - a 500MB filesystem
along with another 500MB or more of recovery data). One could even
save that 100% or 150% of recovery data *without* the original data,
and simply recover the original files when required, but that's an
extreme use of the tools.
I only occasionally go to these lengths for data archiving - for
photographs or software that was never available commercially (i.e.,
my own), since these data aren't likely to be in the hands of anyone
else (unlike commercial software packages and operating system distros
for various platforms).
In short - it's belt-and-suspenders tools for archiving. Not
essential, but the first time you have to restore a missing rar bit
from par files, it makes all the hassle worthwhile.
-ethan