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Re: Understanding how dd works



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 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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