Messages in this thread | | | Date | Fri, 16 Apr 2004 10:03:31 +0100 | From | Jamie Lokier <> | Subject | Re: NFS and kernel 2.6.x |
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Andrew Morton wrote: > > På to , 15/04/2004 klokka 18:14, skreiv Charles Shannon Hendrix: > But Charles was seeing good performance with 2.4-based clients. When he > went to 2.6 everything fell apart.
Perhaps because 2.6 changes the UDP retransmit model for NFS, to estimate the round-trip time and thus retransmit faster than 2.4 would. Sometimes _much_ faster: I observed retransmits within a few hundred microseconds.
On networks with a lot of latency variance, i.e. anything with big queues, that would increase congestion. That'd increase losses, and because NFS over UDP uses large fragmented IP frames (TCP doesn't), fragment loss will greatly increase IP frame loss, as Trond explained.
That's my hypothesis.
There was also a problem with late 2.5 clients and "soft" NFS mounts. Requests would timeout after a fixed number of retransmits, which on a LAN could be after a few milliseconds due to round-trip estimation and fast server response. Then when an I/O on the server took longer, e.g. due to a disk seek or contention, the client would timeout and abort requests. 2.4 doesn't have this problem with "soft" due to the longer, fixed retransmit timeout. I don't know if it is fixed in current 2.6 kernels - but you can avoid it by not using "soft" anyway.
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