Re: Asynchronous I/O in Postgres

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. +