* Kris Kennaway <[EMAIL PROTECTED]> [080220 13:42] wrote: > Chuck Swiger wrote: > >On Feb 20, 2008, at 1:01 PM, Alfred Perlstein wrote: > >>>Take a look at the level of packet fragmentation you are encountering; > >>>yes, this is expected and things will work but there is extra latency > >>>added when the IP stack has to reassemble packets before the data can > >>>be delivered. Try setting the NFS rsize/wsize to 1024 or perhaps 1400 > >>>and see whether that improves performance. > >>> > >>>Or, if your switch and NICs support it, see whether you can get Gb > >>>Ethernet jumbo frames working so that you don't have to fragment for > >>>2K or 4K data packets.... > >> > >>TCP mounts do not have this problem. You can safely use > >>32k or higher sizes with TCP without fragmentation. > > > >Oh, sure. But there is a bit more overhead with TCP transport than > >UDP-- for local (switched) networks, UDP generally seems to be a > >win...TCP seems to be a better choice over a VPN or some similar kind of > >WAN. > > Actually this is no longer true. At modern LAN speeds (e.g. gige) you > can transmit packets fast enough that two things happen: > > 1) UDP socket buffer overruns are common, leading to packet loss. > > 2) the 16-bit sequence numbers wrap *much* faster than the IP fragment > lifetime (30 seconds). > > These combine to cause data corruption when fragmented packets are > dropped and then reassembled with a later transmission of the same > sequence number. > > TCP mounts should be used whenever possible thesedays (I flipped the > default mode in 8.0 the other day).
Additionally with smaller read/write sizes you must generate more RPCs, for instance using a tcp read size of 32k will allow the client to generate a single NFS_READ request for that 32k, however a UDP mount at 1.2k will need to generate appoximately 26 requests and the server will have to service that many requests as well. -- - Alfred Perlstein _______________________________________________ freebsd-performance@freebsd.org mailing list http://lists.freebsd.org/mailman/listinfo/freebsd-performance To unsubscribe, send any mail to "[EMAIL PROTECTED]"
