Re: undelete?

Rogan Dawes (
Tue, 29 Jul 1997 08:54:39 +0200

On Mon, Jul 28, 1997 at 01:25:46PM -0600, Tim Bird wrote:
> > > Now, solve this one, you open the file read/write and you start changing
> > > the file in random places. Since even changing one byte will change the
> > > compression dynamics you end up re-compressing/re-writing everything
> > > following the position at the write(), block compression or no... I
> > > challenge you to code that efficiently.
> >
> > My experience with computers indicates, that most activity comes in
> > bursts, we therefor should not try to compress on the fly when the CPU
> > has better things to do [unless it reduces IO and that's the current
> > bottleneck]. We should rather store the uncompressed data and
> > compress it later when we're in need of disk-space or have nothing
> > better to do. [...]
> This is how NetWare does it and it works quite well. It is very nice
> to have the compression and decompression performance requirements
> decoupled. NetWare implements an asymetric algorithm that sacrifices
> compression speed (which is done at off-peak periods) in exchange
> for blazing decompression speeds. In this regard, NetWare actually
> gets a double win. Compressed files not only take less space on
> disk, but are served faster from the file system than uncompressed
> files (because the decompression is so fast, and less disk blocks
> are read.)
> Tim Bird

Folks interested in this might want to check out the LZO
compression/decompression routines posted recently to c.o.l.a :
real-time decompression, pretty fast compression, GPL, etc.

>From the announcement :

LZO implements a number of algorithms with the following features:
- Decompression is simple and *very* fast.
- Requires no memory for decompression.
- Compression is pretty fast.
- Requires 64 kB of memory for compression.
- Allows you to dial up extra compression at a speed cost in the
compressor. The speed of the decompressor is not reduced.
- Includes compression levels for generating pre-compressed
data which achieve a quite competitive compression ratio.
- There is also a compression level which needs only 8 kB for compression.
- Algorithm is thread safe.
- Algorithm is lossless.


Rogan Dawes   Tel: (011) 490-7379  Fax: (011) 490-7327
- The software said it requires Windows 3.1 or better, so I installed Linux -