Re: Background Writer and performances

On Mon, Jul 10, 2006 at 02:56:48PM +0200, DANTE Alexandra wrote:
> **********************************
> I would like to send charts to show you exactly what happens on the
> server but, with the pictures, this e-mail is not posted on the mailing
> list.
> I can send charts to a personal e-mail adress if needed.
> **********************************

The best idea is to upload them to a website.

> By comparing the charts, I can see that the checkpoints are less
> expensive in term of Disk activity, IO/s and disk write throughput when
> the parameters are set to the maximum values but I don?t not reach to
> have constant disk IO/s, disk activity, disk write throughput before and
> after a checkpoint. I was expecting to see more activity on the disks
> during the bench (and not only a peak during the checkpoint) when the
> parameters are set to the maximum values. Is it possible ?

I have very little experience with the bgwriter, but on the whole, I
don't think the bgwriter will change the total number of I/Os. Rather,
it changes the timing to make them more consistant and the load more
even.

However, reading the descriptions, there's always going to be an
unavoidable spike at checkpoint time because the checkpoint actually
needs to sync() the data to disk, whereas the bgwriter merely queues it
to the kernel.

> Have you already experimented the Background Writer ? What results could
> I obtain by setting properly the parameters ?bgwriter_xxx? ? Could
> somenone explain me how I can used it ?

Looking arond I mostly see that people are still testing. I'm not
really in a position to know however...

Have a nice day,
--
Martijn van Oosterhout <kleptog(at)svana(dot)org> http://svana.org/kleptog/
> From each according to his ability. To each according to his ability to litigate.