ASHIFT=13 by default.

[Zaphod Beeblebrox]

So... I think I answered at least a small part of my confusion with the
test ZFS datasets.  I created them with 'zpool create vrx raidz
/dev/zvol/*' ... and it would appear that the default behavior for this is
to assign ASHIFT to be 13 ... which makes these test pools rater
unrepresentative of ordinary zfs pools.

That means that many people using test ZFS pools are testing with 8k blocks
(not even just 4k blocks).  While I can immediately understand what I need
to do to get a representative test, I'm posting because other people may
have been similarly confused.