There’s a nasty bug in 2.6.32.x: when UFO (UDP Fragmentation Offload) is enabled on an interface and datagrams follow the software fallback path, NFS sessions get corrupted. It’s easy to reproduce – just copy a large (few hundred MB) file to a UDP NFS mount. The transfer will stall within 1-5 minutes and your copy process on the client will go into D state – stuck on
Sadly, it’s not fixed in 18.104.22.168, though I reported it back in early October.
The workaround is obvious: disable UFO.
BTW: this will also hit you if you use qdev on KVM, as it enables several offload features in the guests, including UFO.
UPDATE: still not fixed in 22.214.171.124. See bug #33972.