Re: Hardware performance for large updates

Josh Berkus wrote:
> Particularly, the difficulty is that this application gets many small
> requests during the day (100 simultaneous uses) and shares a server
> with Apache. So I have to be concerned about how much memory each
> connection soaks up, during the day. At night, the maintainence tasks
> run a few, really massive procedures.
>
> So I should probably restart Postgres with different settings at night,
> hey?

Actually, if you can afford the twice daily changes, it sounds like a
great idea. I think you can get new conf settings to take by sending a
SIGHUP to the postmaster, so you don't even really need any downtime to
do it. Yup, here it is:
http://www.postgresql.org/idocs/index.php?runtime-config.html

>>I do think you probably could increase Shared Buffers, as 256 is
>>pretty small. There's been a lot of debate over the best setting. The
>>usual guidance is start at 25% of physical RAM (16384 == 128MB if you
>>have 512MB RAM), then tweak to optimize performance for your
>>application and hardware.
>
>
> Hmmm... how big is a shared buffer, anyway? I'm having trouble
> finding actual numbers in the docs.

By default it is 8K. It's mentioned here:
http://www.postgresql.org/idocs/index.php?kernel-resources.html
So, as I mentioned above, Shared Buffers of 16384 == 128MB if you have a
default 8K block size.

>>Are you on a Linux server -- if so I found that
>>fdatasync works better than (the default) fsync for wal_sync_method.
>
> Yes, I am. Any particular reason why fdatasync works better?

I can't remember the technical reason (although I've seen one on the
list before), but I have determined it empirically true, at least for my
setup. Ahh, here we go:
http://archives.postgresql.org/pgsql-hackers/1998-04/msg00326.php

Joe