FreeBSD NFS server not responding to TCP SYN packets from Linux/SunOS clients

[Chuck Lever]

rick at snowhite.cis.uoguelph.ca wrote:
>>Tcp always throws away retransmissions.  Doesn't matter whether the data is
>>still in the receive socket queue or not.  The nfs server will never see
>>replayed requests as a result of tcp retransmission.
> 
> 
> 1) Yes, of course. (Brain fart on my part. I mentioned I wasn't a TCP wizzard,
> didn't I?:-)
> There is still the problem that not all
> clients obey the "only retry an RPC after a disconnect/reconnect" rule,

where is that rule stated?  most NFS clients i am aware of retransmit an 
RPC after 60 seconds over TCP.

> but that should be fixed for NFSv4.

agreed.

>>  The problem is "how do you make sure the nfsd threads don't start a
>>  request if the disk I/O subsystem is backlogged".
>>
>>Isn't this simply a matter of choosing the right number of nfsd threads?
> 
> 
> That's the only mechanism available to-day (at least for my server and,
> I think, the current FreeBSD one). Unfortunately load is pretty dynamic
> and it might be nice if "the number of active nfsd threads" changed to
> reflect that, without manual sysadmin intervention.
> 
> However, given 1) and clients that only retry RPCs after a reconnect, I
> don't think it will be all that critical. The clients will see the slow
> response to RPCs and new requests will be limited by the number of threads
> doing I/O on the client.

that assumes the client uses a threaded implementation too, doesn't it?