ZFS Storage Overhead (via)
is not quite about what you might think. It's not about, say, the
overhead added by ZFS's RAIDZ storage (where there are surprises); instead it's about some interesting
low level issues of where space disappears to even in very simple
pools. The bit about metaslabs was especially interesting to me.
It goes well with Matthew Ahrens' classic ZFS RAIDZ stripe width,
or: How I Learned to Stop Worrying and Love RAIDZ,
which is endlessly linked and cited for very good reasons.