I've concluded testing NILFS and found that even the latest version (2.0.5) is not ready for use. I was hoping to use NILFS to provide snapshots of virtual disks but I ran into nothing but trouble, I think due to the use of O_DIRECT by Xen which is not correctly supported by NILFS. My virtual machines with files on a NILFS filesystem complained of IO errors on their virtual disks. I like the design and tools of NILFS a lot so I will definitely keep an eye on it but it's not ready yet. Next on my list of new filesystems to try: ZFS-FUSE.