Re: Compressed filesystems: Better compression?

[Matti Aarnio <matti.aarnio@zmailer.org>]

On Tue, Sep 28, 2004 at 11:46:54PM -0400, John Richard Moser wrote:
> I wonder what compression Squashfs, cramfs, and zisofs use.  I think
> cramfs uses zlib compression; I don't know of any other compression in
> the kernel tree, so I assume zisofs uses the same, as does squashfs.  Am
> I mistaken?

Compression algorithms are a bit tough to be used in a random access
smallish blocks environments.  In long streams where you can use megabytes
worth of buffer spaces there is no problem is achieving good performance.
But do try to do that in an environment where your maximum block size
is, say: 4 kB, and you have to start afresh at every block boundary.

Whatever algorithms you use, there will always be data sequences that
are of maximum entropy, and won't compress.  Rather they will be
presented in streams as is with a few bytes long wrappers around
them.

/Matti Aarnio