jumbograms (& em) & nfs a no go

I wanted to test gigabit network performance and found out that current
(from 5.0 to up to date -current) doesn't fully work with jumbograms (MTU
set to 6000), Intel adapters and nfs (both UDP and TCP).

I checked that the same thing works with 4.9.

I then left one computer at 4.9 and upgraded the other to 5.0. When I
mount a partition from 5.0 machine I found out, that copying reliably
works only from 5.0 to 4.9. The other way around I see messages 'em0:
discard oversize frame (ether type 800 flags 3 len 67582 > max 6014)' on
5.0 and the copying stalls. On 4.9 machine I later see 'nfs server
10.0.0.2:/usr: not responding'. The interface is stuck for some time - can
be revived by changing mtu back to 1500 and down/up sequence.

-- 
Michal Mertl