Re: Index Scans become Seq Scans after VACUUM ANALYSE

J. R. Nield wrote:
> So since we have all this buffering designed especially to meet our
> needs, and since the OS buffering is in the way, can someone explain to
> me why postgresql would ever open a file without the O_DSYNC flag if the
> platform supports it? 

We sync only WAL, not the other pages, except for the sync() call we do
during checkpoint when we discard old WAL files.

> > I concur with Bruce: the reason we keep page images in WAL is to
> > minimize the number of places we have to fsync, and thus the amount of
> > head movement required for a commit.  Putting the page images elsewhere
> > cannot be a win AFAICS.
> 
> 
> Why not put all the page images in a single pre-allocated file and treat
> it as a ring? How could this be any worse than flushing them in the WAL
> log? 
> 
> Maybe fsync would be slower with two files, but I don't see how
> fdatasync would be, and most platforms support that. 

We have fdatasync option for WAL in postgresql.conf.

--  Bruce Momjian                        |  http://candle.pha.pa.us pgman@candle.pha.pa.us               |  (610)
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