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Re: [NOVICE] Asynchronous I/O in Postgres

From: Josh Kupershmidt <schmiddy(at)gmail(dot)com>
To: Bruce Momjian <bruce(at)momjian(dot)us>
Cc: mladen(dot)gogala(at)vmsinfo(dot)com, pgsql-docs(at)postgresql(dot)org
Subject: Re: [NOVICE] Asynchronous I/O in Postgres
Date: 2010-10-27 00:46:25
Message-ID: AANLkTinoFoBdesaPi3C4-VoONcB7gFcRE0UPoLDsbEn9@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-docspgsql-novice
[moving to -docs]

On Wed, Oct 20, 2010 at 10:30 PM, Bruce Momjian <bruce(at)momjian(dot)us> wrote:
> Josh Kupershmidt wrote:
>> 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?

It looks like effective_io_concurrency only has an impact on bitmap
heap scans. I think a brief mention of this fact in the docs for
effective_io_concurrency should suffice, patch attached.

Josh

Attachment: effective_io_concurrency-doc.patch
Description: application/octet-stream (986 bytes)

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