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Date:      Sat, 10 Jan 2009 02:53:43 +0000
From:      RW <rwmaillists@googlemail.com>
To:        freebsd-questions@freebsd.org
Subject:   Re: how to use the MFS ?
Message-ID:  <20090110025343.6f0e95d1@gumby.homeunix.com>
In-Reply-To: <443afs49ix.fsf@be-well.ilk.org>
References:  <2c66535d0901082253k5b8ff098w73234a1944929929@mail.gmail.com> <20090109105458.I8836@wojtek.tensor.gdynia.pl> <49672373.3020704@snaffler.net> <2c66535d0901090340y473ce9dcj74ae2a44e5cc0789@mail.gmail.com> <443afs49ix.fsf@be-well.ilk.org>

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On Fri, 09 Jan 2009 13:30:30 -0500
Lowell Gilbert <freebsd-questions-local@be-well.ilk.org> wrote:

> "PstreeM China" <pstreem@gmail.com> writes:
> 
> > i think the option WRKDIRPREFIX  is a good idea , and i whill test
> > the methon unionfs.
> 
> Definitely benchmark against just using a native local filesystem,
> though.  Taking away all of that memory that FreeBSD would otherwise
> use for *caching* file data could well end up making your builds
> *slower* with the MFS than they would have been without it.

I've a vague recollection that I tried something like this, a few years
ago, and found that the difference was too small to measure. 

I think that however you do it, you basically end-up with something that
looks like: 

   CPU/L1/L2 -> memory -> disk 

and whether the "memory->disk" part is a cached-file or swap-backed
memory is really just a matter of book-keeping - the VM system moves the
physical memory around as it likes. The book-keeping differences may be
significant, but they are not different in the electronic verses
electro-mechanical sense, and such intuitions may not relevant.

Poul-Henning Kamp, has an interesting article in this area for the
varnish project:

"I have spent many years working on the FreeBSD kernel, and only
rarely did I venture into userland programming, but when I had occation
to do so, I invariably found that people programmed like it was still
1975."

http://varnish.projects.linpro.no/wiki/ArchitectNotes


[Not that there's anything wrong with 1975.]



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