"I've never looked at the Reiser code though the comments I get from friends who use it are on the order of 'extremely reliable but not the fastest filesystem in the world'," Matt Dillon explained when asked to compare his new clustering HAMMER filesystem with ReiserFS, both of which utilize BTrees to organize objects and records. He continued, "I don't expect HAMMER to be slow. A B-Tree typically uses a fairly small radix in the 8-64 range (HAMMER uses 8 for now). A standard indirect block methodology typically uses a much larger radix, such as 512, but is only able to organize information in a very restricted, linear way." He continued to describe numerous plans he has for optimizing performance, "my expectation is that this will lead to a fairly fast filesystem. We will know in about a month :-)"
Among the optimizations planned, Matt explained, "the main thing you want to do is to issue large I/Os which cover multiple B-Tree nodes and then arrange the physical layout of the B-Tree such that a linear I/O will cover the most likely path(s), thus reducing the actual number of physical I/O's needed." He noted, "HAMMER will also be able to issue 100% asynchronous I/Os for all B-Tree operations, because it doesn't need an intact B-Tree for recovery of the filesystem." He went on to describe another potential optimization allowed by the filesystem's design, "HAMMER is designed to allow clusters-by-cluster reoptimization of the storage layout. Anything that isn't optimally layed-out at the time it was created can be re-layed-out at some later time, e.g. with a continuously running background process or a nightly cron job or something of that ilk. This will allow HAMMER to choose to use an expedient layout instead of an optimal one in its critical path and then 'fix' the layout later on to make re-accesses optimal."