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Danstroem
n00b
Joined: 14 Jun 2004
Posts: 62
Location: Recklinghausen, Germany
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 Posted: Fri Jul 07, 2006 6:49 pm Post subject: broken bytes destroy whole tar archive? |
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I recently made a system-backup with:
| Code: | | tar --wildcards --wildcards-match-slash --exclude 'sys/*' --exclude 'proc/*' --exclude 'mnt/d/Download/*' -cvzO / |
Yes I know, this is not perfect.  My problem is a different:
I burned the archive to a DVD+RW media and some days later, i tried to restore it: Bad blocks on the media! 
I copied the image with dd_rescue. About 120kB of data from the 4.0GB archive were gone. I was still happy, because I read that corrupted data in tar archives is just ommited, and I loose only some sectors (64kB per sector for gzip compressed tars) of data. But I had to realise, that tar did NOT continue to extract the archive. I got
| Code: | tar -xzf archive.tar.gz
gzip: stdin: invalid compressed data--format violated
tar: Unexpected EOF in archive
tar: Unexpected EOF in archive
tar: Error is not recoverable: exiting now |
I read the manpage and tried various options which sounded helping, but I did't get it working. I also tried to copy the image with
| Code: | | dd_rescue -A archive.tar.gz /mnt/d/archive.tar.gz |
which fills the broken sectors with zeros, so that the size of the file stays the same, but tar didn't care.
Do you have an idea how to convince tar from continueing? Any hint greatly appreciated!
Thanks in advance!
-Daniel
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nephros
Advocate
Joined: 07 Feb 2003
Posts: 2139
Location: Graz, Austria (Europe - no kangaroos.)
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| Posted: Fri Jul 07, 2006 6:59 pm Post subject: |
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It's not the tar format that is breaking (tar basically just concatenates all the files together like -- aehm -- lump of tar) but gzip.
I'm not sure if that can be fixed, but the gzip home page has a guide. It is truly non-trivial though.
HTH and Good Luck!
PS. That is why using compression of the archive is not a good idea for a backup. Small corruptions can destroy the whole backup. Much better to compress the individual files before archiving.
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