[kwlug-disc] [GTALUG] block size for dd -- no difference

[William Park]

On 2025-09-03 08:10, Khalid Baheyeldin wrote:
> On Wed, Sep 3, 2025 at 7:17 AM Ron <ron at bclug.ca <mailto:ron at bclug.ca>> 
> wrote:
> 
>     So, in this situation (perhaps a flawed test?), there's a dramatic
>     speedup going above bs=1, a significant speed improvement going above
>     bs=1000, and slight decreases in speed going above bs=1,000,000.
> 
>     Similar results on an SSD, but *slower* on the low end and faster at
>     bs=1000000 (1M).
> 
>     SSD results also slowly decline above bs=1M:
> 
> 
> Looks like bs=1M is the sweet spot, regardless of disk technology.
> Perhaps it is about round trips into the kernel and out (as Chris Frey said
> in the other thread).
> 
> Thanks for doing the tests ...

Someone said I should use 'direct' option.  So, I did

     dd if=/dev/sda of=/dev/sdi bs=$i iflag=direct oflag=direct

And the result is
	
      bs=1K      18.8 MB/s
         4K      51.1
         16K     86.9
         64K     93.3
         256K    103
         1M      107
         4M      107
         16M     105
         64M     106
         256M    99.1

It seems 1M-8M is the optimum, and also the shortest to type.