Date: Fri, 26 Apr 1996 00:30:07 +0200 (SAT) From: Peter van Heusden <pvh@leftside.its.uct.ac.za> To: freebsd-fs@freebsd.org Subject: Compressing filesystem: Technical issues Message-ID: <Pine.BSD.3.91.960426001058.18458A-100000@leftside>
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I'm slowly getting started on the issue of writing a compressing filesystem for BSD. The situation thus far: 1) I'm thinking of a model much like the Netware 4.x one, where a file is compressed if it has not been 'touched' (ie. read or written) in a certain time (e.g. a week). It is then decompressed on being 'touched'. 2) I think the correct approach is to base the filesystem on the existing ufs code, and just add a flag which can sit in the i_flag field of the inode which states whether this file is compressed or not. On a successful read or write (i.e. one where data has actually been moved to/from disk successfully) the to_be_compressed flag can be cleared. 3) I am as yet uncertain about some of the design of the mark and sweep process which would do the compressing. My current thinking is that this would be a daemon spawned at mount time which would cycle through the inodes (in numerical order) doing the mark 'n sweep thing using a new filesystem specific ioctl. An unmount would have to gracefully kill the daemon process, of course. I'm currently not certain where to put the temporary data during compression... in memory? In a filesystem? 4) I'll have to think up a good compression strategy which allows recovery from corruption, etc etc. Anyway, in my mind, issue 3, the process to do the compressing, is the one I am having the most problems with. Any suggestions on the design of something like this would be appreciated. Thanks, Peter
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