Questions about the dd command

Heinrich Rebehn rebehn at
Fri Feb 29 16:40:27 UTC 2008

elmo wrote:
> I've spent literally hours trying to find direct answers on the dozens 
> of websites that discuss 'dd'
> but the couldn't find what I wanted.
> A few questions about the ' dd ' command:
>   For example:     sudo dd if=/dev/sda2 of=/dev/sdb2 bs=1440
>   1. Which is preferrable;    execute the command from an installed
>        ubuntu (/dev/sda2) or from a ubuntu CD?
>   2. How do you  modify the example  to show the data as it is being 
> copied? 
>      Would this have a drastic effect on how long it takes to complete 
> the copy
>       compared to just watching a blinking cursor?
>   3.  What effect on the does the ' bs = ' number have on
>       the time to complete the copy or the accuracy of the copy?  Is 
> there a
>       default number if the the bs=    is omitted from the command?
1. You should not use dd to copy a filesystem that is currently mounted, 
because parts of it may be cached in RAM. The copy would be 
inconsistent. So you better boot from a ubuntu CD.
2. Standard dd does not have a "progress bar" option. However, it will 
report its progress when sent a SIGHUP.
3. I don't know, make measurements yourself. 1440 is a somewhat strange 
value, use a multiple of 512 (The standard blocksize)


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