Re: ANNOUNCEMENT: Squashfs released (a highly compressed filesystem)

[Phillip Lougher <phillip@lougher.demon.co.uk>]

Jeff Garzik wrote:
 > Phillip Lougher wrote:

 >
 >> 2. Squashfs compresses inode and directory information in addition
 >> to file data.  Inodes/directories generally compress down to 50%, or
 >> say on average 8 bytes or less per inode.
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 > squashfs or mksquashfs?

mksquashfs...

 >
 > A r/w compressed filesystem would be darned useful too :)
 >
 > Jeff
 >

A r/w compressed filesystem may be my next project...  As a couple of
people have mentioned there are compressed r/w filesystems already
out there.

As you'll know, there are always tradeoffs with filesystem design,
it is very difficult to get as good compression with a r/w fs
than with a read only filesystem.  I wanted to get maximum
compression, and quite a few of the techniques I use rely on
its read-only nature.

An append only (i.e. files can be added, but not modified), fs might
be a useful compromise.  With compressed metadata, any modification
of files will inevitably achive different compression ratios, and so
modification of metadata/files in place is not an option.  Appending
modified metadata/data brings you to log-structured (journalling)
filesystems and compaction (log cleaning) requirements with consequent
loss of compression.

Phillip


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