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Date: Wed, 1 May 1996 02:23:59 +0300 (EET DST)
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From: Heikki Suonsivu <hsu@clinet.fi>
To: Peter van Heusden <pvh@leftside.its.uct.ac.za>
Cc: freebsd-fs@freebsd.org
In-reply-to: Peter van Heusden's message of 26 Apr 1996 04:49:56 +0300
Subject: Re: Compressing filesystem: Technical issues
Organization: Clinet Ltd, Espoo, Finland
References: <Pine.BSD.3.91.960426001058.18458A-100000@leftside>
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In article <Pine.BSD.3.91.960426001058.18458A-100000@leftside> Peter van Heusden <pvh@leftside.its.uct.ac.za> writes:
   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'.
...

You might want to have a look at

@inproceedings{Burrows:92,
        AUTHOR="Michael Burrows and Charles Jerian and Butler Lampson and Timoth
y Mann",
        TITLE="On-line Data Compression in a Log-structured File System",
        BOOKTITLE="ASPLOS V Proceedings",
        ORGANIZATION="ACM",
        YEAR=1992,
        MONTH="October",
        PAGES="2--9",
        ABSTRACT = "{We have incorporated on-line data compression into the
        low levels of a log-structured file system (Rosenblum's {\it 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-strucuring 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).

        Hardware compression devices mesh well with this design.  Chips are
        already available that operate at speeds exceeding disk transfer
        rates, which indicates that hardware compression would not only
        remove the performance degradation we observed, but might well
        increase the effective disk transfer rate beyond that obtainable
        from a system without compression.}"
}

It looks like implementing this on BSD LFS would not be too hard, as the
idea is trivial and BSD LFS is based on the same idea.  I do not know how
stable BSD LFS is, anyone tried it lately ?

-- 
Heikki Suonsivu, T{ysikuu 10 C 83/02210 Espoo/FINLAND, hsu@clinet.fi
mobile +358-40-5519679 work +358-0-4375360 fax -4555276 home -8031121