Single Instance Storage

[Fraser Campbell]

Dave Stubbs wrote:

> M$ SIS is a filter and a service that runs on top of the NTFS
> filesystem.  It allows you to copy many redundant copies of the same
> stuff onto a drive, and then during the night the "groveler" service
> walks the filesystem and finds any duplicate files, moving them to a SIS
> repository and replacing them in the user directory with something like
> a hard link.  It is different than the Posix concept of a link though -
> if you go and modify one of these multiple copies of the same file, it
> doesn't change all the others.  Instead it detaches the modified copy
> from the Single Instance Store, and makes it unique.

I believe the Linux vservers project does this:

* each virtual server is basically a chroot
* files in the chroot are hard links to a pristine filesystem
* as a virtual server modifies it's files the hard links are "broken"
   and new file is created
* admin has tools to consolidate differences over time and move common
   files back to hard links

If that's what you're after perhaps you should look at 
http://linux-vserver.org/

Outside of the virtual machine world I think that unionfs and mapfs 
might do what you want.

Regards,
Fraser
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