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DragonFly kernel List (threaded) for 2003-09
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Re: Ridiculous idea: Cache as ramdisk?

From: Chris Pressey <cpressey@xxxxxxxxxxxxxxx>
Date: Tue, 23 Sep 2003 09:45:10 -0700

On Tue, 23 Sep 2003 18:11:43 +0200
ibotty <bsd@xxxxxxxxxx> wrote:

> > b) he wants a possibility to retain a specific file in the cache.
> >> this can be generalized to the idea, that each file has different
> >> buffer-cache priority.
> > 
> > Would softupdates' snapshot ability approximate this?  i.e. Put a
> > snapshot into memory, and then only initial accesses would cause a
> > disk read.  I'm relatively sure someone beat me to this thought.
> in my little understanding about vfs and vm, this sounds like a good
> idea(but i thought snapshots cannot be demand-loaded and -saved)
> but as according to matt we are retiring softupdates and snashots
> anyway, so few thoughts need to be spend with this idea (be it good or
> bad is left to general opinion).

Hm, my only suggestion is to abstract it just a little further - say
every file has a different "importance" - from (say) 0 for "not
important at all, I don't care if it suddenly disappears" to (say) 9 for
"very important, make sure it's in NV storage ASAP"

This way the behaviour isn't tied to any particular mechanism, and that
should theoretically make it easier for both implementers (who can use a
different mechanism, should it come down to that) and users (who don't
have to think in terms of "buffer cache" and other concepts they don't
necessarily understand well.)

Random brainfarts to go with this:

Temporary files have long been a source of security, correctness, and
performance issues.  This may (or may not) ameliorate that.  If it
doesn't... something else probably should :)

Filesystem metadata can never be too rich.  Even AmigaDOS had a comment
field on files. :)

Journalling file systems seem to be rather optimized for recovery, vs
the impression I get when I hear the word "journalling" - I think of
something more like CVS, where you can get any older version of a file
merely by requesting a different tag.  Such a "write once" filesystem
would be very nice to use, I think.

Anyway... back to your regularly scheduled BSD forking...

-Chris (in Vancouver, if you care)

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