The previously rule of thumb which I had used was that after doing a
random seek, the time needed to write 4k and 32k was pretty much in
the noise.  But that was a number from several years ago.  I suppose I
should do some quick experiments to see what is a good number these
days....

Indeed.  fio to the rescue.

I remember Christoph saying something about 64k somewhat recently?  But,
helpfully, I can't recall the details :).

- z