Re: effective_io_concurrency increasing

On Mon, Jun 19, 2017 at 8:49 AM, Merlin Moncure <mmoncure(at)gmail(dot)com> wrote:

> On Mon, Jun 19, 2017 at 10:36 AM, Jeff Janes <jeff(dot)janes(at)gmail(dot)com> wrote:
> > If you have a RAID, set it to the number of spindles in your RAID and
> forget
> > it. It is usually one of the less interesting knobs to play with.
> (Unless
> > your usage pattern of the database is unusual and exact fits the above
> > pattern.)
>
> Isn't that advice obsolete in a SSD world though? I was able to show
> values up to 256 for a single device provided measurable gains for a
> single S3500. It's true though that the class of queries that this
> would help is pretty narrow.

I don't think it is obsolete, you just have to be creative with how you
interpret 'spindle' :)

With a single laptop hard-drive, I could get improvements of about 2 fold
by setting it to very high numbers, like 50 or 80. By giving the hard drive
the option of dozens of different possible sectors to read next, it could
minimize head-seek. But that is with just one query running at a time.
With multiple queries all running simultaneously all trying to take
advantage of this, performance gains quickly fell apart. I would expect
the SSD situation to be similar to that, where the improvements are
measurable but also fragile, but I haven't tested it.

Cheers,

Jeff