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



On Tue, 23 Sep 2008 11:37:00 -0400
Jerry McAllister <jerrymc@xxxxxxx> wrote:

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.

I tried playing around with this once, and I found that the speed rose
rapidly up to a certain blocksize, then levelled-out for a decade or so
and then dropped to half of the peak speed. IIRC in that particular case
the optimum range was something like 20k-200k.

I presume what happens is that you can make the blocksize too big for
the other buffering, and end-up alternating reads and writes rather
than doing them in parallel.
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