Re: Does larger i/o size make sense?

Merlin Moncure <mmoncure(at)gmail(dot)com> writes:
> On Thu, Aug 22, 2013 at 2:53 PM, Kohei KaiGai <kaigai(at)kaigai(dot)gr(dot)jp> wrote:
>> An idea that I'd like to investigate is, PostgreSQL allocates a set of
>> continuous buffers to fit larger i/o size when block is referenced due to
>> sequential scan, then invokes consolidated i/o request on the buffer.

> Isn't this dealt with at least in part by effective i/o concurrency
> and o/s readahead?

I should think so. It's very difficult to predict future block-access
requirements for anything except a seqscan, and for that, we expect the
OS will detect the access pattern and start reading ahead on its own.

Another point here is that you could get some of the hoped-for benefit
just by increasing BLCKSZ ... but nobody's ever demonstrated any
compelling benefit from larger BLCKSZ (except on specialized workloads,
if memory serves).

The big-picture problem with work in this area is that no matter how you
do it, any benefit is likely to be both platform- and workload-specific.
So the prospects for getting a patch accepted aren't all that bright.

regards, tom lane