*BSD News Article 19221


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From: jan@encap.Hanse.DE (Jan-Oliver Neumann)
Newsgroups: comp.os.386bsd.development
Subject: Compressing file system ?
Date: 5 Aug 1993 19:48:07 +0200
Organization: Hanse Networking e.V. Hamburg, Germany.
Lines: 38
Message-ID: <23rh55$ct@encap.Hanse.DE>

Hello folks.

I've installed DOS 6.0 (yuck!) for a friend yesterday and I was 
quite impressed how little the overhead  for the compressed 
disks was.

I mean: CPU performace has increased dramatically in the past 
years, but disk-speed didn't grow in that manner. 

Let's imagine a file system that compresses disk blocks that
have not been used recently with a fast algorithm and blocks
that haven't been used for a long time with a slow but effective
algorithm. I think the overhead for reading disk blocks should
be very little as heavily used blocks are already in the buffer
cache. The compressing of disk-blocks could be done asynchrously
when the system is idle or the load is very low.

Those fstype would be ideal for partitions with news of other
text-files.

But I also see the technical problems: disk blocks must be large
for the compressing to be effective. But large blocks also force
the fragement size to be larger. And this isn't very good for
news-partitions. Also, what about seeks ? I think bmap() can't find
out which block to read in, 'cause it doesn't know where byte 'n'
can be found. Then the file must be read in (and decompressed)
linearly to find the block. And what about the inodes ? I think
the disk-inode must contain the compressed file size and the inode
that are in-core must contain both. This implies a lot of decompressing.

Has somebody already thought about such a file-system ? This are only
some thoughts I made while waiting for the compressing of a 240 MB 
drive. :)

Jan-Oliver
-- 
Jan-Oliver Neumann	                                  <jan@encap.Hanse.DE>
Hanse Networking e.V. Hamburg, FRG. Mail to vorstand@Hanse.DE for information.