Re: Question on Opteron performance

On Tue, 2004-03-09 at 11:06, Steve Wolfe wrote:
> Right now, our production DB server is getting a bit more heavily loaded
> than we'd like, and we expect its usage to double in the next few months,
> so we're looking at where to put our money for a better machine.
>
> Right now, we're using a dual 2.8GHz Xeon with 3 gigs of memory, and run
> without fsync() enabled. Between disk cache and shared buffers, the disk
> system isn't an issue - vmstat shows that the disk I/O is nearly always at
> zero, with the occasional blips of activity rarely being more than a few
> hundred kilobytes.

You do know that turning off fsync() means your data will all get
trashed if you get an OS crash or power problem or H/W crash or ...

> The main question in my mind is whether a 4-way Opteron is going to
> give me enough of a performance benefit over a 2-way Opteron to make the
> extra $10k worth it. My first guess was that it would, as going from 2
> Opterons to 4 will give you twice the potential memory bandwidth.
> However, as PostgreSQL pulls heavily from the global buffers, I may not be
> able to utilize all of that potential bandwidth.

Is this true? Did they really double the size of the memory bus, or is
it a case of 4 CPUs fighting for the same memory bandwidth that 2 had
before?

> If anyone has done tests with PostgreSQL on 2- vs. 4-way machines under
> heavy load (many simultaneous connections), I would greatly appreciate
> hearing about the results.

What sort of load is "heavy load" to you?

Stephen