> > > > Imagine: cp file file2, file and file2 reference the same exact blocks,
> > > > but modified chunks of file2 would be given their own private blocks.
> > >
> > > This is not a microsoft innovation, actually, I believe it was a VMS
> > > innovation. It's called a generational filesystem. the original is
> > > stored, and later generations of the file are stored as diffs.
> >
> > As far as I know, VMS simply stores whole files - no diffs involved. Now
> > if you go back to for instance Univac 1100 and the Exec-8 OS (I suppose
> > it is OS-1100 now), you'll find a system that *did* store the diffs. In
> > the form of punched card images! :-)
>
> Well, not really. That was mostly an application convention rather than
> being done in the OS. And that all the applications wanted to use
> SIR$ SDF to read program file elements was just a coincidence :-)
>
> The cools part of Exec-8 that we still need (we already have sparse
> files) are the virtual filesystem bits. E.g., unloaded files. People
> have been struggling with multi-level storage architectures on UNIX
> for years, while this was pretty much a solved problem on these 1's
> complement 36 bit dinosars 30 years ago.
>
> (The notion was that if you didn't use a file in a while, the system
> would release the data blocks, and mark the file as "unloaded." When
> you "assigned"/opened one of these files, a system process would cause
> the current backup tape to be loaded, and the file restore. When you
> began to get low on disk space, likeway a systen process would start,
> and sort all files based on their priority for being unloaded - based
> on last reference time, do we have a current backup, who created it, etc.
> It would then begin to release the data blocks until you acheived a
> configured threshold.)
>
> louie
Unitree does something like this for UNIX with auto migration to tape.
See http://www.unitree.com
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