Re: [opensuse] About Backing Up
- From: David Bolt <bcrafhfr@xxxxxxxxxx>
- Date: Mon, 7 May 2007 13:55:52 +0100
On Mon, 7 May 2007, Carlos E. R. wrote:-
The Monday 2007-05-07 at 03:46 +0200, I wrote:
Yes, I saw then. I ran a few scripts that form a test suite, no errors.
Now I'm preparing a dvd backup, and will add par2 files to it, too. I have
to study the possible command options first.
I'm baffled...
Ok, I have a dozen of big files (roughly the same size each) in a dvd,
with about 250 MB free space that I could use for par data. What options
should I use to create them?
I'd use something like this:
par2 c -s1024000 -c235 -l <basename.for.par2.archives> *
c is to create the recovery files
-s1024000 gives a recovery block size of a little under 1MB
-c235 says to create 235 recovery blocks
-l limits the size of the par2 recovery files to just a bit bigger than
the largest file.
That should create a few recovery files which, with the par2 overheads,
occupy about 235MB and leave around 15MB free. Once it's finished, and
if you're the sort of person that just has to be sure, you can verify
the freshly created files using:
par2 v <basename.for.par2.archives>
And, if there's a failure after the contents has been burnt, copy the
contents off the DVD using either dd or ddrescue, and then use:
par2 r <basename.for.par2.archives>
Now, the bad news is that for a dozen files, totalling a bit over 4GB,
you may not be able to rebuild a broken file with only 235 blocks
without rescuing as much data as possible from the DVD. My guess is that
the files are around 350MB[0], which means you'd need at least 350-360
recovery blocks to rebuild a completely missing file. As long as only
one file is broken, and you manage to recover more than a third of the
data, there _should_ be enough to rebuild it.
There are ways to reduce this problem, and the one I chose was to limit
the size of files to 100MB[1]. That, combined with my using 535 blocks
means I can have 5 completely unreadable files before I am unable to
recover. And when I want to recombine the split files, I just use cat
:-)
[0] After rounding to the nearest MB:
4.35GB - 250MB = 4.1GB
4.1GB / 12 = 350MB
[1] split -b 100M -a 3 -d <filename> <filename>.
^
That is to allow creation of names in the format filename.000,
filename.001, etc.
Regards,
David Bolt
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