[TriLUG] dd limit on disks?

Mike Seda maseda at gmail.com
Sun Sep 3 00:18:14 EDT 2006


Hi Joe,
There is no dd limit that I am aware of. I have used dd to successfully 
clone 160 GB disks in the past. You may be running into bad sectors on 
those 30 GB disks. Try the following command:
dd if=<source_disk> of=<image_file> conv=noerror,sync
The "noerror" and "sync" options tell dd to continue after read errors, 
e.g. bad sectors, and pad the output to match the input block size.
Best,
Mike


Joseph Mack NA3T wrote:
> I do windows backup by dd'ing the whole disk to my linux machine 
> (after defragmenting and filling the disk with zeroes). I check that 
> I've got something sensible by mounting with -o loop, I then gzip the 
> image. This has worked fine so far as the windows disks have been 
> small (6G). If a disk crashes I have to restore to a disk of the same 
> geometry. I find this works to clone a 10G disk to another bigger 
> (40G) disk (maybe I've just been lucky).
>
> I just installed WinXP on a 30G disk and then tried to back it up with 
> dd. I recovered only about 6G (6495068160bytes = 63 sectors * 3 
> surfaces * 16 heads * 5(I don't know what) * 839 (cylinders?) * 512). 
> (total cylinders is 16,383: 16383/839=19 if that helps). Thinking 
> there was something wrong with the disk, I did it again with the same 
> result. Picking another 30G disk (same type) with linux on it, I got 
> the same result. I seem to have got about 20% of the disk.
>
> ulimit is unlimited.
>
> I have some other random sized disks I could try. Since dd doesn't 
> care about the filesystem, I'm only copying bits. Anyone know why I 
> can't dd the whole disk, no matter what's on it? Google didn't find 
> any problems.
>
> Thanks Joe
>




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