Re: Huge number of disk writes after migration to 8.1 - Mailing list pgsql-bugs

From Bruce Momjian
Subject Re: Huge number of disk writes after migration to 8.1
Date
Msg-id 200602011732.k11HWdQ00507@candle.pha.pa.us
Whole thread Raw
In response to Re: Huge number of disk writes after migration to 8.1  (Tom Lane <tgl@sss.pgh.pa.us>)
List pgsql-bugs
Added to TODO:

    * Allow statistics collector information to be pulled from the collector
      process directly, rather than requiring the collector to write a
      filesystem file twice a second?


---------------------------------------------------------------------------

Tom Lane wrote:
> "Magnus Hagander" <mha@sollentuna.net> writes:
> > In most cases you're going to see extremely few reads compared to writes
> > on pg_stats, right? So why not have the backends connect to the stats
> > process (or perhaps use UDP, or use the pipe, or whatever) and fetch the
> > data when needed. So when nobody fetches any data, there is no overhead
> > (except for the stats process adding up values, of course).
>
> That's a thought.  You'd still want the stats file to preserve the data
> across shutdowns, but the update rate could be far slower, maybe once
> every few minutes.  The other nice thing is that when you do want the
> stats, you could get current values, not half-a-second-behind values.
>
> > Then you could also push down some filtering to the stats process - for
> > example, when you are reading from pg_stat_activity there is no need to
> > send over the row level stats. IIRC, today you have to read (and write)
> > the whole stats file anyways.
>
> No; the current behavior of grabbing a snapshot of the whole stats
> dataset is a feature, not a bug.  It lets you sit there and correlate
> the data using multiple queries, without worrying that the numbers are
> changing under you.  We'd lose this ability if the data had to be
> re-fetched for each query because we didn't grab it all.
>
>             regards, tom lane
>
> ---------------------------(end of broadcast)---------------------------
> TIP 6: explain analyze is your friend
>

--
  Bruce Momjian                        |  http://candle.pha.pa.us
  pgman@candle.pha.pa.us               |  (610) 359-1001
  +  If your life is a hard drive,     |  13 Roberts Road
  +  Christ can be your backup.        |  Newtown Square, Pennsylvania 19073

pgsql-bugs by date:

Previous
From: Tom Lane
Date:
Subject: Re: Bug in query planer ?
Next
From: Clifford Wolf
Date:
Subject: Re: Bug in query planer ?