| Simon Riggs 2005-10-31, 9:23 am |
| On Mon, 2005-10-31 at 14:14 +0100, Martijn van Oosterhout wrote:
> On Mon, Oct 31, 2005 at 12:16:59PM +0000, Simon Riggs wrote:
>
> Is there any particular reason to turn that off?
Well yeh. If things work faster without it, then off it goes - or at
least parameter settings vastly altered.
> You want dirty pages
> written out. Doing them asyncronously beforehand means you don't have
> to wait for it at commit time. It also allows the OS to schedule the
> blocks into a better write order.
Only assuming you have a constant heavy write workload.
>
> Well, it comes down to a thought experiment. Any disk blocks you have in
> the shared buffers will also be in the system cache.
Each have different and independent cache replacement...
> If you give 4GB to
> shared buffers, then there will be 4GB of data in the system cache which
> is not directly useful. So it seems shared buffers should be large
> enough to hold all the info PostgreSQL needs at any particular moment,
> anything else is just wasteful. Getting data out of the system cache is
> not terribly expensive, I timed it at 50 microseconds per page on my
> oldish laptop.
>
> Secondly, you're assuming that PostgreSQLs caching is at least as
> efficient as the OS caching, which is more of an assertion than
> anything else.
Do you doubt that? Why would shared_buffers be variable otherwise?
Best Regards, Simon Riggs
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