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5. NFS over slow lines

Slow lines include Modems, ISDN and quite possibly other long distance connections.

This section is based on knowledge about the used protocols but no actual experiments. My home computer has been down for 6 months (bad HD, low on cash) and so I have had no modem connection to test this with. Please let me hear from you if try this :-)

The first thing to remember is that NFS is a slow protocol. It has high overhead. Using NFS is almost like using kermit to transfer files. It's slow. Almost anything is faster than NFS. FTP is faster. HTTP is faster. rcp is faster. ssh is faster.

Still determined to try it out? Ok.

NFS' default parameters are for quite fast, low latency, lines. If you use these default parameters over high latency lines it can cause NFS to report errors, abort operations, pretend that files are shorter than they really are, and act mysteriously in other ways.

The first thing to do is not to use the soft mount option. This will cause timeouts to return errors to the software, which will, most likely not handle the situation at all well. This is a good way to get for mysterious failures. Instead use the hard mount option. When hard is active timeouts causes infinite retries instead of aborting whatever it was the software wanted to do. This is what you want. Really.

The next thing to do is to tweak the timeo and retrans mount options. They are described in the nfs(5) man page, but here is a copy:

       timeo=n        The  value  in  tenths  of  a second before
                      sending the first retransmission  after  an
                      RPC timeout.  The default value is 7 tenths
                      of a second.  After the first timeout,  the
                      timeout  is  doubled  after each successive
                      timeout until a maximum timeout of 60  sec-
                      onds  is  reached or the enough retransmis-
                      sions have occured to cause a  major  time-
                      out.   Then,  if  the  filesystem  is  hard
                      mounted, each new timeout cascade  restarts
                      at  twice the initial value of the previous
                      cascade, again doubling at each retransmis-
                      sion.   The  maximum  timeout  is always 60
                      seconds.  Better overall performance may be
                      achieved  by  increasing  the  timeout when
                      mounting on  a  busy  network,  to  a  slow
                      server, or through several routers or gate-

       retrans=n      The number of minor timeouts  and  retrans-
                      missions  that  must  occur  before a major
                      timeout occurs.  The default is 3 timeouts.
                      When a major timeout occurs, the file oper-
                      ation is either aborted or  a  "server  not
                      responding"  message is printed on the con-

In other words: If a reply is not received within the 0.7 second (700ms) timeout the NFS client will repeat the request and double the timeout to 1.4 seconds. If the reply does not appear within the 1.4 seconds the request is repeated again and the timeout doubled again, to 2.8 seconds.

A lines speed can be measured with ping with the same packet size as your rsize/wsize options.

$ ping -s 8192 lugulbanda
PING ( 8192 data bytes
8200 bytes from icmp_seq=0 ttl=64 time=15.2 ms
8200 bytes from icmp_seq=1 ttl=64 time=15.9 ms
8200 bytes from icmp_seq=2 ttl=64 time=14.9 ms
8200 bytes from icmp_seq=3 ttl=64 time=14.9 ms
8200 bytes from icmp_seq=4 ttl=64 time=15.0 ms

--- ping statistics ---
5 packets transmitted, 5 packets received, 0% packet loss
round-trip min/avg/max = 14.9/15.1/15.9 ms

The time here is how long the ping packet took to get back and forth to lugulbanda. 15ms is quite fast. Over a 28.000 bps line you can expect something like 4000-5000ms, and if the line is otherwise loaded this time will be even higher, easily double. When this time is high we say that there is 'high latency'. Generally, for larger packets and for more loaded lines the latency will tend to increase. Increase timeo suitably for your line and load. And since the latency increases when you use the line for other things: If you ever want to use FTP and NFS at the same time you should try measuring ping times while using FTP to transfer files.

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