Re: Block size: 8K or 16K?

Curt Sampson wrote:
> On Thu, 25 Apr 2002, mlw wrote:
>
> > ...but my gut tells me that using 16K blocks will increase performance
> > over 8K. Aleady I have seen a sequential scan of a large table go from 20
> > seconds using 8K to 17.3 seconds using 16K.
>
> You should be able to get the same performance increase with 8K
> blocks by reading two blocks at a time while doing sequential scans.
> That's why I've been promoting this idea of changing postgres to
> do its own read-ahead.
>
> Of course, Bruce might be right that the OS read-ahead may take
> care of this anyway, but then why would switching to 16K blocks
> improve sequential scans? Possibly because I'm missing something here.

I am almost sure that increasing the block size or doing read-ahead in
the db will only improve performance if someone is performing seeks in
the file at the same time, and hence OS readahead is being turned off.

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