Re: [RFC][PATCH 0/3] Skip I/O merges when disabled

["Alan D. Brunelle" <Alan.Brunelle@hp.com>]

David Collier-Brown wrote:
> 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

No, it's intended for devices being used for /random IO loads/ - like
index seeks during OLTP and the like. But in general, the idea is if you
know you have a highly random IO work load, you can set the tunable and
get back a significant chunk of CPU cycles.

Alan