Matthew Dillon is setting up DragonFly to be able to use a fast disk (like a SSD) for disk cache, reducing the effect swap has on speed.  This means very large amounts of data could be read into memory – greater than the available RAM in the system – without having the normal paging out problems that happen when memory is exhausted.   It’ll work for any filesystem on the machine – HAMMER, UFS, or NFS.  His inital notes have more.  Other notes include details on the NFS benefits, and possibilities with SSDs.  Wear-leveling may make SSDs last much longer.

Work has started, and there’s an update (with examples) that people can try, though it may destroy all your data at this point.  Test results in that update show, if I’m reading it right, a better than doubling of speed on a repeated md5 test on a large file when using the new caching system.  This should be a huge benefit.