dd tricks for holey files
Thursday Mar 13, 2008
Bob Netherton took a look at my last post on corrupted file recovery (?) and asked whether I had considered using the noerror option to dd. Yes, I did experiment with dd and the noerror option.
The noerror option is described in dd(1) as:
noerror
Does not stop processing on an input error.
When
an input error occurs, a diagnostic mes-
sage
is written on standard error, followed
by
the current input and output block counts
in
the same format as used at completion. If
the
sync conversion is specified, the missing
input
is replaced with null bytes and pro-
cessed
normally. Otherwise, the input block
will
be omitted from the output.
This looks like the perfect solution, rather than my dd and iseek script. But I didn't post this because, quite simply, I don't really understand what I get out of it.
Recall that I had a corrupted file which is 2.9 MBytes in size. Somewhere around 1.1 MBytes into the file, the data is corrupted and fails the ZFS checksum test.
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zpool scrub zpl_slim action:
Restore the file in question if possible. Otherwise restore the see:
http://www.sun.com/msg/ZFS-8000-8A
config:
errors:
Permanent errors have been detected in the following files: |
I attempted to use dd with the noerror flag using several different block sizes to see what I could come up with. Here are those results:
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for i in 1k 8k 16k 32k 128k 256k 512k |
hmmm... all of these files are of different sizes, so I'm really unsure what I've ended up with. None of them are the same size as the original file, which is a bit unexpected.
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dd if=libc.so.1
of=/tmp/whaa.1k bs=1k conv=noerror |
hmmm... well, dd did copy some of the file, but seemed to give up after around 5 attempts and I only seemed to get the first 1.1 MBytes of the file. What is going on here? A quick look at the dd source (open source is a good thing) shows that there is a definition of BADLIMIT which is how many times dd will try before giving up. The default compilation sets BADLIMIT to 5. Aha! A quick download of the dd code and I set BADLIMIT to be really huge and tried again.
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bigbaddd if=libc.so.1
of=/tmp/whbb.1k bs=1k conv=noerror |
As dd processes the input file, it doesn't really do a seek, so it can't really get past the corruption. It is getting something, because od shows that the end of the whbb.1k file is not full of nulls. But I really don't believe this is the data in a form which could be useful. And I really can't explain why the new file is much larger than the original. I suspect that dd gets stuck at the corrupted area and does not seek beyond it. In any case, it appears that letting dd do the dirty work by itself will not acheive the desired results. This is, of course, yet another opportunity...
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Don't forget 'conv=sync'; that may help. (otherw...
The ",sync" part is important in the con...
I agree that if dd actually handled the EIO proper...