> Colin Plumb wrote:
> >
> > One feature they have which is rather cool, is snapshots: they
> > can take a static copy of the file system at any time and keep it
> > on line as a backup. Further writes are done via COW techniques;
> > new data is written to new blocks and the old data is preserved.
>
> Very valuable, yes, but to a rather small audience. I presume (not knowing a
> lot aboutWAFL and such) that there are rollback/commit-style calls you can
> make to "fix" newly written data in ways similar to what relational databases
> usually do. I suppose it could be useful for partitions containing users home
> directories and stuff, given that the appropriate system calls could be
> wrapped in nice tools. Kind of how those trash cans should really work in
> other OSes. Can't think of any other large uses, but yes, it might be useful.
> Possibly for programs handling enormous amounts of data stored on disk,
> wanting undo capabilities; only... what programs?
Another important use, and perhaps a less limited one, is for rolling
backups: by cloning (as it is usually called) the entire FS, you can then
back it up without bringing the system down to single-user mode, and still
be guaranteed that the entire FS has a consistent set of files.
> Problem is, that partition will be problematic for other purposes, most
> notably for LRU-like swapping, where a lot of writes would generate large
> amounts of COW. So you would have to have a mechanism for "flushing" blocks
> at the bottom end of a chain of COWed blocks.
Likewise, these on-line 'backups' would either be deleted immediately
after they are copied off-line, or kept around till more space is needed.
-- Kenneth Albanowski (kjahds@kjahds.com, CIS: 70705,126)
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