blocksize when using dd to copy disks? bigger = better?

Jerry McAllister jerrymc at
Tue Sep 23 15:38:12 UTC 2008

On Tue, Sep 23, 2008 at 11:04:13AM -0400, Joachim Rosenfeld wrote:

> When mirroring a disk with dd, I notice that a blocksize of 512 runs
> awfully slow, but with bs=1MB (2^10bytes), it runs fairly quickly.
> Can someone explain the implications of this? Did all the data not
> copy properly with the larger blocksize?

If you are on a beach moving sand and you pick up one grain at a
time and move it, it will take a very long time because the overhead
of moving yourself is much higher than the amount of sand moved.
If you use the largest bucket or scoop that you can handle, then
it goes much faster because the same body motions result in much
more being moved.    Moving data has a similar dynamic.

All the data will get copied properly in either case unless
there is some big error.   A big error is probably more likely
to happen with the smaller block size just because there are
more operations in which an error can occur.


> thanks
> Joe
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