ian@bananaPC.SGI.COM (Ian Clements) (06/19/87)
Has anyone had experience with NFS over some sort of Bridge ? I'd like to think that any box that was protocol independant would work but am told that because of the large number and size of packets it may not be true. Anyone got any ideas ? e-mail to ian@sgi please.
hedrick@topaz.rutgers.edu.UUCP (06/19/87)
The gateways we currently use have the old 3Com Multibus Ethernet controllers. (I am reluctant to vendors in this message, because I know that competing vendors have exactly the same problems.) These have essentially no onboard buffering. During a burst of packets, the gateway must be able to copy a packet into memory during the amount of time it takes for the next packet to be received. The processor can't quite do that. The hardware they are about to start shipping has a couple of different improvements that solve this problem, but I would not be surprised to find other vendors with similar problems. Indeed most Multibus-based Suns can't receive NFS at full speed, which I find hilarious, since NFS is Sun's protocol. (The Multibus Suns used the same 3Com controller boards...) Another Unix vendor has a new Ethernet controller which will sometimes exceed the capacity of their older systems, which use Interlan Multibus controllers. From talking to NFS implementors, I get the feeling that there is a sort of a contest to see who can build the fastest NFS implementation. The more implementations you can make fail (by not being able to keep up with you), the happier the implementors feel. It's not clear to me that this contest is in the best interests of the users. For these reasons, NFS has parameters in the mount command that let you control how large the bursts of packets are. The issue is not packet size. That is limited to 1500 bytes by the Ethernet spec. The issue is bursts of packets coming very close together. Few pieces of hardware can keep up with packets arriving continuously at 10Mbits/sec. So the issue is whether there is enough buffer capacity on the Ethernet controller board (or available to it in memory) to hold up to 6 packets (the maximum that NFS will use with its default settings), and whether it can receive packets with the minimum legal spacing. Current versions of NFS let you in effect specify that no more than 2 packets are to be sent in a burst. This should be used in cases where someone you are talking to can't handle your packets. However there are certainly gateways and bridges that can handle long bursts.