I've found my optimal blocksize to be 8 MB (equal to disk cache?) I needed to wipe (some say: wash) the empty space on a disk before creating a compressed image of it.
I used:
cd /media/DiskToWash/
dd if=/dev/zero of=zero bs=8M; rm zero
I experimented with values from 4K to 100M.
After letting dd to run for a while I killed it (Ctlr+C) and read the output:
36+0 records in
36+0 records out
301989888 bytes (302 MB) copied, 15.8341 s, 19.1 MB/s
As dd displays the input/output rate (19.1MB/s in this case) it's easy to see if the value you've picked is performing better than the previous one or worse.
My scores:
bs= I/O rate
---------------
4K 13.5 MB/s
64K 18.3 MB/s
8M 19.1 MB/s <--- winner!
10M 19.0 MB/s
20M 18.6 MB/s
100M 18.6 MB/s
Note: To check what your disk cache/buffer size is, you can use sudo hdparm -i /dev/sda
