NFS reads vs. writes

[Mikhail T.]

I have two similar 10.2-STABLE machines mounting each other's
filesystems via NFS: a:/a and b:/b

When moving some large files around today, I found a huge discrepancy
between writing and reading over NFS. That is, machine a copying files
from its own /a to NFS-mounted b:/b was pushing measly 2-3Mb/s over the
Ethernet. When I cancelled that, logged-in to machine b and proceed to
copy from a:/a to the now-local /b, I got 56Mb/s. I tried changing the
wsize and rsize options, but it did not seem to make a difference...

Any ideas, what is happening here? Why are NFS-writes some 25 times
slower here, than reads? Both filesystems are zfs-backed (though with
different options), both systems are plugged into the same switch, both
use mtu of 9000.

Thanks! Yours,

    -mi