Understanding how dd works

Dan Track dan.track at gmail.com
Wed Jun 25 12:31:55 UTC 2008


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?

Thanks
Dan



On Wed, Jun 25, 2008 at 12:48 PM, Chris G <cl at isbd.net> wrote:
> On Wed, Jun 25, 2008 at 12:27:04PM +0100, Dan Track wrote:
>> Hi
>>
>> I've got a xen vm file called test, if I copy it with dd I get the following
>> dd if=/opt/xen/test of=/opt/test-vm.img bs=4096
>> du -s /opt/xen/test = 1934112
>> du -s /opt/test-vm.img = 26240040
>>
>> My question is why is the test-vm.img larger in size than the original?
>>
> Perhaps because the original file is 'sparse', i.e. it has large
> unused chunks in it, when originally created these will be unallocated
> and use no space, only when written to will the space be allocated.
> However when you dd the file it writes everything (including 'nul'
> data) to the destination file.
>
> --
> Chris Green
>
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