Understanding how dd works

Mikkel L. Ellertson mikkel at infinity-ltd.com
Wed Jun 25 16:02:35 UTC 2008


Patrick O'Callaghan wrote:
> On Wed, 2008-06-25 at 13:31 +0100, Dan Track wrote:
>> Thanks for the heads up on this. If the data blocks don't have
>> anything written into them, then what data is written into them when
>> using dd? if I restore the dd image will the blocks then be in the
>> same state i.e unwritten to?
>>
>> Also following on from this if I create a file using dd let's say 2GB,
>> how does the filesystem know that all these blocks belong to the file
>> myfile.img, and where is the information stored to say that a block
>> has data written into it or not?
> 
> It's important to understand that this has nothing to do with 'dd', it's
> simply how the Unix filesystem works, and since Linux is "culturally
> derived" from Unix, it does the same thing. You would see the same
> effect just by using 'cp' or even 'cat'.
> 
cp knows how to handle sparse files. From the cp man page:

By default, sparse SOURCE files are detected by a crude heuristic 
and the corresponding DEST file is made sparse as well.  That is the 
behavior selected by --sparse=auto. Specify --sparse=always to 
create a sparse DEST file whenever the SOURCE file contains a long 
enough sequence of zero bytes. Use --sparse=never to inhibit 
creation of sparse files.

So I would think that cp would give him a good copy...

Mikkel
-- 

   Do not meddle in the affairs of dragons,
for thou art crunchy and taste good with Ketchup!

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