HASt and write amplification on the seconday

[Pete French]

> Extents (2 meg blocks) are used for synchronization after the
> secondary is reconnected. Normally (the secondary is connected) writes
> to the primary and the secondary are the same.

Ah, Ok, that was not clear to me from the documentation. That would
also expain what I was seeing when I did a resyc a while ago. OK, thats
arekief as I was worrying that I had a wirte apliification factor
of 4096 on top of an SSD!

Thanks for taking the time to explain...

-pete.