Messages in this thread | | | From | "Stephen C. Tweedie" <> | Date | Thu, 6 May 1999 19:38:55 +0100 (BST) | Subject | Re: Porting Greg Lehey's rawio.c from FreeBSD to Linux... |
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Hi,
On Tue, 4 May 1999 18:58:56 +0200, Brad Knowles <brad@shub-internet.org> said:
> Since sending out that note, I've had this explained to me. Can you > explain to me the semantics of an Oracle or Sybase database wanting to > have direct access to a raw disk device for the database, so that:
> 1) You don't trash your buffer cache
Yes
> 2) You don't waste your time and that of the database when it > could avoid the filesystem redirection entirely?
Yes, and it also avoids the CPU cost of copying data around, so there's a double performance win there.
> Or am I mistaken in believing that the block device interface is also > pumped through the vm/buffer cache system?
The block device interface is layered underneath the buffer cache, but it can be used independently if you fake the buffer_heads. That's what the VM does when swapping or when populating the page cache, for example, and the Linux raw character device patches do the same for block IO.
--Stephen
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