Hans Reiser wrote: > > Edward Shishkin wrote: > > >Gregor Zeitlinger wrote: > > > > > >>Hi, > >> > >>I'm new to this list. I'm wondering how the file plugins work. Are they > >>just a means of accessing the file contents or are they also designed to > >>save the file differently. > >> > >>For example, could you create a plugin that automatically zips each > >>text/plain file, which is totally transparent to the user? > >> > >> > > > >Yeah, we could. Although transparentness means a bit worse compression.. > > > ? > > You mean because of choosing to use compression atoms
Yes, but it is not the only reason. > that are node sized? No, compression atoms are limited by common sense ;) Obviously, it doesn't make sense to choose it less then node size, if you want to compress a file presented by tails. On the other hand, I am not sure if you will be delighted from transparent compression supported by 256K clusters even for larger files. > Maybe it might be more precise to say that efficient random > access motivates small compression atoms which are less efficient? > (Assuming that I understood you.) There is one more obscured reason that makes transparent compression worser. I have to remind that its primary assignment is the same: to provide more efficient disk space usage. Let's take a look how transparent compression and the package using the same compression algorithm will provide it: Suppose we want to compress a 8S-byte file, and both transparent compression and package is supported by 4-blocks clusters, S - block size. File system: divides the file into 2 clusters, suppose each cluster was compressed up to (2S+1) bytes, so compressed file occupies 6 blocks. Package: makes compressed file which occupies 2*(2S+1) = 4S+2 bytes and passes it to the file system, which places it (aieee!) into 5 blocks.. So we do have the actual compression ratio produced by the file system is 6/8 = 25% against actual ones (5/8 = 37,5%) produced by the package. The user can observe this effect only by using "du" (when he does "df", he see that file system and package produce the same compression ratio 37,5%). This is another effect caused by the specific of packing policy (for the ext2 files, reiserfs files presented by indirect items, etc..) Obviously we can avoid this by using bigger cluster (8 blocks). Edward.
