Horrible zpool resilver perormance on local hast

[Pete French]

Partially answering my own question here, as it occured to
me that the zpool is scattering writes across the disc in 4k blocks,
but hast has a minimum extent size of 2meg by default. Thats eems like
a likely culprit for the 'dirty blocks' multiplcation I am seeing.

I shrunk down the extent size to 16k to see if that helped, and I
now see a far smaller dirty block to written block ration, but
also the write speed has now shrunk down even more to around 50k/s :-(

Any advice on tuning this properly ?

-pete.