While looking at the DHT code, I noticed that it's using a 10-round Davies-Meyer construction to generate the hashes used for file placement. A little surprised, by this, I ran it by a couple of friends who are experts in both cryptography and distributed data storage. The consensus seems to be that the hash used for this purpose needs to be collision resistant but not cryptographically strong. One theorized that the choice made in DHT is probably based on prior examples (e.g. Freenet and Mojo Nation) where cryptographically strong hashes were chosen, but that the requirements driving those decisions probably don't apply to GlusterFS. This is a non-trivial issue because these hashes are used quite frequently and the current one is quite computationally expensive. I note that Hsieh's SuperFastHash is already implemented in GlusterFS and is used for other purposes. It's about 3x as fast as the DM hash, and has better collision resistance as well. MurmurHash (http://murmurhash.googlepages.com/) is even faster and more collision resistant. For future releases, I suggest dropping the DM hash and switching to one of these others.
