I've been trying to understand exactly how timeouts work in the NFS client and find that the man page in nfs-utils is not correct.
In particular, the implementation differentiates between TCP and UDP, while the man page does not make that distinction. I have attempted an update to the man page as you can see below. It is entirely possible that I have not got it completely correct (or comprehensible) so I'm asking for people to check that what I have written is correct and clear. This I would particularly like comment on: 1/ I have left 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 gateways. unchanged. Is it still a reasonable thing to say? 2/ I have moved the documentation about major timeouts into the retrans= section. Does that break the description up too much? 3/ the old text seems to say that after the first major-timeout, a slightly different sequence of timeouts are used. I couldn't find evidence of this in the code. Did I miss something, or is my text correct? 4/ Did this change in some ancient kernel version, and should the version number of the change be documented? e.g. is it a 2.4 / 2.6 difference? 5/ As the behaviour is quite different for UDP and TCP, should we introduce a major_timeo= option which calculates an appropriate retrans= based on the actual timeo= and proto= used. and anything else that occurs to anyone. Thanks, NeilBrown diff --git a/utils/mount/nfs.man b/utils/mount/nfs.man index d92da19..0142075 100644 --- a/utils/mount/nfs.man +++ b/utils/mount/nfs.man @@ -83,24 +83,50 @@ Note: Setting this size to a value less than the largest supported block size will adversely affect performance. .TP 1.5i .I 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 seconds is reached or the enough retransmissions -have occured to cause a major timeout. 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 -retransmission. The maximum timeout is always 60 seconds. +The value in tenths of a second for the first RPC timeout. If no +reply has been received in this much time, the message is +retransmitted. +Further timeouts are handled differently depending on the connection +type. + +For UDP (which is unreliable and lacks congestion control), +each successive timeout is twice the previous timeout. As the default +is 11 tenths of a seconds, the timeouts used if +.I timeo= +is not specified are 1.1, 2.2, 4.4, 8.8,... seconds. The timeout for +each retransmission is limited to 60 seconds, so the next few numbers +in the above sequence would be 17.6, 35.2, 60, 60. + +For reliable protocols such as TCP and RDMA, the successive timeouts +grow linearly rather than exponentially to a maximum of 10 minutes. +The default is 1 minute, so the default successive timeout are 1, +2, 3, 4, 5, 6, 7, 8, 9, 10, 10, 10 minutes. + +It is unwise to set +.I timeo= +explicitly without also setting the protocol to use, as it has a +significantly different effect depending on protocol. + 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 gateways. .TP 1.5i .I retrans=n The number of minor timeouts and retransmissions that must occur before -a major timeout occurs. The default is 3 timeouts. When a major timeout -occurs, the file operation is either aborted or a "server not responding" -message is printed on the console. +a major timeout occurs. The default is 2 yielding a total of 3 +attempts (1 transmission and 2 retransmissions). When a major timeout +occurs the behaviour depends on whether the filesystem was mounted +.I hard +or +.IR soft . +In the case of a +.I soft +mount, the operation will abort and typically return an IO error to +the application. In the case of a +.I hard +mount a "server not responding" message will be printed on the +console, and the request will be retried with the original series of +timeouts. .TP 1.5i .I acregmin=n The minimum time in seconds that attributes of a regular file should - To unsubscribe from this list: send the line "unsubscribe linux-nfs" in the body of a message to [EMAIL PROTECTED] More majordomo info at http://vger.kernel.org/majordomo-info.html
