Comment 34 for bug 879334

I have been struggling at work with the same problem for the last few months, and accidentally stumbled on a fix that seems to work under Ubuntu 12.04 (3.5.0-32-generic 64-bit kernel) :

1. On clients set rsize=8192,wsize=8192 in /etc/fstab (Smaller values of rsize,wsize will also work, but reduce throughput).

Previously with rsize=wsize=32K, any 2 clients writing large files to our server (with 1 Gbit NICs for clients and 2x1Gbit NICs for the server) would freeze : all clients would appear to hang on accessing the NFS server, but the hang would eventually resolve itself (after several hours of writes at around 100k/sec on the server grinding away continuously with jbd2, or within minutes if one of the 2 clients with the large writes temporarily had its ethernet turned off and then back on after the other client's write had completed).

Now 4 clients can write to the server at 53 MB/sec saturating the server bandwidth at 210 MB/sec and the network and server remain responsive from other interactive clients.