On-line Data Compression

We have incorporated on-line data compression into the low levels of a log-structured file system (Rosenblum's Sprite LFS). Each block of data or meta-data is compressed as it is written to the disk and decompressed as it is read. The log-structuring overcomes the problems of allocation and fragmentation for variable-sized blocks. We observe compression factors ranging from 1.6 to 2.2, using algorithms running from 1.7 to 0.4 MBytes per second in software on a DECstation 5000/200. System performance is degraded by a few percent for normal activities (such as compiling or editing), and as much as a factor of 1.6 for file system intensive operations (such as copying multi-megabyte files)