Re: Random file I/O regressions in 2.6

[Andrew Morton <akpm@osdl.org>]

Ram Pai <linuxram@us.ibm.com> wrote:
>
> I am nervous about this change. You are totally getting rid of
> lazy-readahead and that was the optimization which gave the best
> possible boost in performance. 

Because it disabled the large readahead outside the area which the app is
reading.  But it's still reading too much.

> Let me see how this patch does with a DSS benchmark.

That was not a real patch.  More work is surely needed to get that right.

> In the normal large random workload this extra page would have
> compesated for all the wasted readaheads.

I disagree that 64k is "normal"!

>  However in the case of
> sysbench with Andrew's ra-copy patch the readahead calculation is not
> happening quiet right. Is it worth trying to get a marginal gain 
> with sysbench at the cost of getting a big hit on DSS benchmarks,
> aio-tests,iozone and probably others. Or am I making an unsubstantiated
> claim? I will get back with results.

It shouldn't hurt at all - the app does a seek, we perform the
correctly-sized read.

As I say, my main concern is that we correctly transition from seeky access
to linear access and resume readahead.