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On 2/4/2014 1:43 PM, Stan Hoeppner wrote:

Everything we've been discussing has been about maximizing write 
throughput.  The fact that you argue this at this point makes it
crystal clear that you don't have no understanding of the
differences in the read/write paths and how buffer cache affects
each differently.  Further discussion is thus pointless.

I am intimately familiar with the two code paths, having written
several applications using them, studied the kernel code extensively,
and been one of the original strong advocates for the kernel to grow
direct aio apis in the first place, since it worked swimmingly well on
WinNT.

So I say again: switching to direct aio, while saving a decent chunk
of cpu time, makes very little difference in streaming write
throughput.  If it did, there would be something terribly broken with
the buffer cache if it couldn't keep the disk queues full.


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