Date: Thu, 25 May 2000 13:22:40 -0700 (PDT) From: Matthew Dillon <dillon@apollo.backplane.com> To: Anatoly Vorobey <mellon@pobox.com> Cc: hackers@FreeBSD.ORG Subject: Re: Proper uses for MFS? Message-ID: <200005252022.NAA84015@apollo.backplane.com> References: <200005251705.NAA67491@blackhelicopters.org> <200005251757.KAA83404@apollo.backplane.com> <20000525141623.D6776@sasami.jurai.net>
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:You, Matthew Dillon, were spotted writing this on Thu, May 25, 2000 at 10:57:33AM -0700:
:>
:> I don't particularly like to use MFS for 'large' partitions, mainly
:> because cached data blocks wind up in core memory twice (once in MFS's
:> memory map, and once in the VM page cache).
:
:You've said this several times in threads on MFS during recent months,
:and I've always wanted to ask: is that a necessary 'feature' of MFS's
:architecture, or something which could possibly be fixed without
:too much hard work? For instance, would it be possible to force
:VM not to cache MFS pages, etc.?
:
:--
:Anatoly Vorobey,
The double caching is a consequence of MFS's 'physical media' being
the mmap() rather then real physical media. It would be difficult
to fix, and probably not worth the effort.
MFS's only advantage is that the double-caching tends to keep pages
in-core longer, and you have less 'real' physical I/O because things
like write-behind and buffer cache flushes are doing nothing more
then flusing from the kernel's main VM page cache into MFS's memory
map.
If you have enough memory not to care about the double-caching issue,
then MFS will work fine.
-Matt
Matthew Dillon
<dillon@backplane.com>
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