Comment 8 for bug 688068

Mr. Gardner was kind enough to explain to me the issue here:

When a KVM guest mounts an NFS filesystem from the Host system (e.g. the nfs server is the same physical hardware on which the guest is running), there is no throttling to the amount of memory used. A file copy (or a big conversion, like I was doing with kvm-img convert) can completely exhaust memory -- and it does so in kernel space. In this case, the hard hang I experienced was likely due to memory exhaustion (that is supposition on my part). A more preferred alternative is to utilize rsync over ssh, since that should provide some throttling to the connection. In addition, ssh runs in userspace, rather than in kernel space, so memory exhaustion can be dealt with far more gracefully.

Alternate solutions would be to move the NFS server off to another machine, or to move the kvm guest to another piece of hardware. Mr. Gardner wasn't sure if Samba would also be affected by this, so I have devirtualized the single kvm guest which needed read-write NFS access.

The natty narwhal kernel has been stable so far, but I will be returning to the 2.6.32-26-server kernel in short order, to continue testing it.