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Heuristic Cleaning Algorithms in Log-Structured File Systems



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Heuristic Cleaning Algorithms in Log Structured File Systems Trevor Blackwell Jeffrey Harris Margo Seltzer Harvard University Abstract Research results show that while LogStructured File Systems LFS offer the potential for dramatically improved file system performance the cleaner can seriously degrade performance by as much as 40 in transaction processing workloads 9 Our goal is to examine trace data from live file systems and use those to derive simple heuristics that will permit the cleaner to run without interfering with normal file access Our results show that trivial heuristics perform very well allowing 97 of all cleaning on the most heavily loaded system we studied to be done in the background 1 Introduction The Log Structured File System is a novel disk storage system that performs all disk writes contiguously Since this rids the system of seeks during writing the potential performance of such a system is much greater than in the standard Fast File System 4 which must make writes to several different locations on the disk for common operations such as file creation The mechanism used by LFS to provide sequential writing is to treat the disk as a log composed of a collection of large one half to one megabyte segments each of which is written sequentially New and modified data are appended to the end of this log Since this is an append only system all the segments in the file system eventually become full However as data are updated or deleted blocks that reside in the log become replaced or removed and their space can be reclaimed This reclamation of space gathering the freed blocks into clean segments is called cleaning and is a form of generational garbage collection 3 The critical challenge for LFS in providing high performance is to keep cleaning overhead low and more importantly to ensure that I Os associated with cleaning do not interfere with normal file system activity There are three terms that will be useful in discussing LFS cleaner performance



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