From: Bruce Momjian <bruce@momjian.us>
Message-Id: <201010210230.o9L2UMX11996@momjian.us>
Subject: Re: Asynchronous I/O in Postgres
In-Reply-To: <AANLkTinygXJv_CFY__PT+hQYWMADC6bDrMQih-kk2wbd@mail.gmail.com>
To: Josh Kupershmidt <schmiddy@gmail.com>
Date: Wed, 20 Oct 2010 22:30:22 -0400 (EDT)
CC: mladen.gogala@vmsinfo.com,
	"pgsql-novice@postgresql.org" <pgsql-novice@postgresql.org>
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Josh Kupershmidt wrote:
> On Fri, Oct 8, 2010 at 8:14 AM, Mladen Gogala <mladen.gogala@vmsinfo.com> wrote:
> > Mladen Gogala wrote:
> >>
> >> So, essentially, the process is reading block by block, in a sequence.
> >> What, exactly, does "effective_io_concurrency" mean?
> >>
> >
> > To rephrase my question, can anybody tell me where in the code is it used?
> 
> The docs are a bit sparse here :-(
> 
> But it looks to me like effective_io_concurrency only affects bitmap
> heap scans. The setting from effective_io_concurrency gets put into
> "target_prefetch_pages" in ./src/backend/utils/misc/guc.c . But the
> only place which uses that variable is
> ./src/backend/executor/nodeBitmapHeapscan.c.
> 
> The EnterpriseDB docs
> <http://www.enterprisedb.com/docs/en/8.3R2/perf/Postgres_Plus_Advanced_Server_Performance_Guide-17.htm>
> mention:
> "effective_io_concurrency is only used for Bitmap Heap Scans. For
> normal sequential scans the operating system should handle read-ahead
> internally (On Linux, see the blockdev command, in particular --setra
> and --setfra)."

So, this this also true for community Postgres?  Can someone suggest
updated docs?


-- 
  Bruce Momjian  <bruce@momjian.us>        http://momjian.us
  EnterpriseDB                             http://enterprisedb.com

  + It's impossible for everything to be true. +