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

[Ron]

William Park via Talk wrote on 2025-09-02 19:40:

> I was doing one of those occasional backup of my root SSD to HD.  They 
> are identical size, so sector by sector.  I took this occasion to see if 
> blocksize (bs=) matters.
> 
> for i in {4,16,64,256}K {1,4,16,64}M; do
>      echo ==== $i ====
>      dd if=/dev/sda of=/dev/sdi bs=$i
>      sync
> done
> 
> No difference.  All did 117MB/s.

Interesting.

My memory was that I've seen where block size makes a difference, so I 
had to try for myself.


Don't have exactly the same setup so used /dev/random for input and a 
file on a 5400RPM WD Red HD for output, changing the block size:

  $ for i in 1 1000 1000000 10000000 100000000
        echo "*** block size: $i"
        rm ./dd-bs-test
        dd if=/dev/random of=./dd-bs-test bs=$i  count=(math "100000000 
/ $i")
    end

*** block size: 1
100000000 bytes (100 MB, 95 MiB) copied, 29.9448 s, 3.3 MB/s
*** block size: 1000
100000000 bytes (100 MB, 95 MiB) copied, 0.148444 s, 674 MB/s
*** block size: 1000000
100000000 bytes (100 MB, 95 MiB) copied, 0.112547 s, 889 MB/s
*** block size: 10000000
100000000 bytes (100 MB, 95 MiB) copied, 0.115528 s, 866 MB/s
*** block size: 100000000
100000000 bytes (100 MB, 95 MiB) copied, 0.132528 s, 755 MB/s


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:

*** block size: 1
100000000 bytes (100 MB, 95 MiB) copied, 42.0427 s, 2.4 MB/s
*** block size: 1000
100000000 bytes (100 MB, 95 MiB) copied, 0.156072 s, 641 MB/s
*** block size: 1000000
100000000 bytes (100 MB, 95 MiB) copied, 0.10943 s, 914 MB/s
*** block size: 10000000
100000000 bytes (100 MB, 95 MiB) copied, 0.120185 s, 832 MB/s
*** block size: 100000000
100000000 bytes (100 MB, 95 MiB) copied, 0.133916 s, 747 MB/s