Determining which blocks to clean is more challenging, and has been the subject of many research papers. In the original LFS paper“Design and Implementation of the Log-structured File System” by Mendel Rosenblum and John Ousterhout. SOSP ’91, Pacific Grove, CA, October 1991. The original SOSP paper about LFS, which has been cited by hundreds of other papers and inspired many real systems., the authors describe an approach that tries to segregate hot and cold segments. A hot segment is one in which the contents are being frequently over-written; thus, for such a segment, the best policy is to wait a long time before cleaning it, as more and more blocks are getting overwritten (in new segments) and thus being freed for use. A cold segment, in contrast, may have a few dead blocks but the rest of its contents are relatively stable. Thus, the authors conclude that one should clean cold segments sooner and hot segments later, and develop a heuristic that does exactly that. However, as with most policies, this policy isn’t perfect; later approaches show how to do better“Improving the Performance of Log-structured File Systems with Adaptive Methods” by Jeanna Neefe Matthews, Drew Roselli, Adam M. Costello, Randolph Y. Wang, Thomas E. Anderson. SOSP 1997, pages 238-251, October, Saint Malo, France. A more recent paper detailing better policies for cleaning in LFS..
