On Tue, 2006-01-31 at 21:53 -0800, Luke Lonergan wrote:
> Jeffrey,
>
> On 1/31/06 8:09 PM, "Jeffrey W. Baker" <jwbaker@acm.org> wrote:
> >> ... Prove it.
> > I think I've proved my point. Software RAID1 read balancing provides
> > 0%, 300%, 100%, and 100% speedup on 1, 2, 4, and 8 threads,
> > respectively. In the presence of random I/O, the results are even
> > better.
> > Anyone who thinks they have a single-threaded workload has not yet
> > encountered the autovacuum daemon.
>
> Good data - interesting case. I presume from your results that you had to
> make the I/Os non-overlapping (the "skip" option to dd) in order to get the
> concurrent access to work. Why the particular choice of offset - 3.2GB in
> this case?
No particular reason. 8k x 100000 is what the last guy used upthread.
>
> So - the bandwidth doubles in specific circumstances under concurrent
> workloads - not relevant to "Huge Data sets, simple queries", but possibly
> helpful for certain kinds of OLTP applications.
Ah, but someday Pg will be able to concurrently read from two
datastreams to complete a single query. And that day will be glorious
and fine, and you'll want as much disk concurrency as you can get your
hands on.
-jwb