Date: Sun, 19 Sep 1999 12:00:26 -0700 (PDT) From: Matthew Dillon <dillon@apollo.backplane.com> To: Poul-Henning Kamp <phk@critter.freebsd.dk> Cc: Matthew Jacob <mjacob@feral.com>, dg@root.com, Greg Lehey <grog@lemis.com>, cvs-committers@FreeBSD.ORG, cvs-all@FreeBSD.ORG Subject: Re: User block device access Message-ID: <199909191900.MAA73792@apollo.backplane.com> References: <17169.937766010@critter.freebsd.dk>
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:> the superiority of the raw device without taking into account the purpose
:> of using the buffered device in the first place -- i.e. to be able to
:> take advantage of its caching capabilities.
:
:And what I'm having a hard time finding is apps that does that.
:
:All the device using apps I know spend most of chapter one saying
:"ALWAYS USE RAW DEVICES" over and over and over.
:
:--
:Poul-Henning Kamp FreeBSD coreteam member
Your 'All' is a pretty narrow definition of 'all'. Many applications
implement caching of some sort, even if it's only caching of read-data.
The only restriction a database would have, for example, would be to
require write-through rather then write-behind. The data caching would
still be extremely useful.
In fact, a memory-mappable buffered block device with write-through would
be much, much more useful to a database then a character device, and I
think it's only a two line patch to make mmap() work, and probably a
four line patch to implement write-through. It would be virtually
unbeatable... use of mmap() removes the extra copy overhead, read-caching
takes the burden off the application, optional write-through gives you
instant feedback *AND* reblocking. I would even be willing to make the
write-through the default.
That would give us an extremely powerful and useful buffered block device
implementation.
-Matt
Matthew Dillon
<dillon@backplane.com>
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