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SubjectRe: [RFC][PATCH 0/3] Skip I/O merges when disabled
Jens Axboe wrote:
> On Thu, Apr 24 2008, Andi Kleen wrote:
>
>>>Not a good idea IMHO, it's much better with an explicit setting. That
>>>way you don't introduce indeterministic behavior.
>>
>>So you would be deterministically slower.
>
>
> Yes, absolutely. Think about the case for a second - the potential gain is in
> fractions of a percent basically, the potential loss however is HUGE.
> There's absolutely no way on earth I'd ever make this dynamic.

If this is intended for databases, it might be backwards (;-))

The commercial unix "forcedirectio" option that Oracle and other
database vendors usually ask for turns out to be a benefit
in large sequential data transfers, because it does two things:

1) transfers directly between user address space and disk, avoiding buffering, and
2) allows enthusiastic coalescence of synchronous writes

Is this intended for DBMSs, or for something esle?

--dave
--
David Collier-Brown | Always do right. This will gratify
Sun Microsystems, Toronto | some people and astonish the rest
davecb@sun.com | -- Mark Twain
(905) 943-1983, cell: (647) 833-9377, (800) 555-9786 x56583
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