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Subject: Re: 4x lower IOPS: Linux MD vs indiv. devices - why?
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From: Tobias Oberstein <tobias.oberstein@gmail.com>
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Date: Mon, 23 Jan 2017 22:49:48 +0100
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To: Andrey Kuzmin <andrey.v.kuzmin@gmail.com>
Cc: fio@vger.kernel.org

Hi Andrey,

> Thanks again for your tips .. the psync thingy in particular. I need to
> verify if that applies to PostgreSQL, because it brings huge gains compared
> to sync!
>
>
> That's easy to explain, it just does one syscall less per IO. It should
> indeed bring home a measurable gain as, with synchronous I/O, I believe
> you're cpu-limited.

Sadly, it seems PostgreSQL currently does lseek/read/write. (I'll double 
check tomorrow running perf against an active PostgreSQL instance).

There was a patch discussed here using pread/pwrite when avail

https://www.postgresql.org/message-id/flat/CABUevEzZ%3DCGdmwSZwW9oNuf4pQZMExk33jcNO7rseqrAgKzj5Q%40mail.gmail.com#CABUevEzZ=CGdmwSZwW9oNuf4pQZMExk33jcNO7rseqrAgKzj5Q@mail.gmail.com

which ends with a comment by Tom Lane (PostgreSQL core developer)

"Well, my point remains that I see little value in messing with
long-established code if you can't demonstrate a benefit that's clearly
above the noise level."

=(

I will post the findings from our discussion here to the PG hackers 
list. Maybe ...

Cheers,
/Tobias