Re: Question on disk contention

On Thu, May 31, 2018 at 1:13 AM, Charles Clavadetscher <
clavadetscher(at)swisspug(dot)org> wrote:

> Hi Melvin
>
> As an answer to a previous post you wrote:
>
> "Also, your main problem is that when you have two exact same queries
> executing at the same time, they will cause contention in
> the disk, and neither one will make much progress."
>
> Could you elaborate a little more on the meaning of "contention in the
> disk"?
> What is it that happens?
>
> Thank you and have a good day.
> Regards
> Charles
>
>
>
>Could you elaborate a little more on the meaning of "contention in the
disk"?
>What is it that happens?

To simplify, you have two users/jobs, both wanting the exact same
information. So the system instructs the disk to get
that information from the disk, which causes the disk head to "seek" to the
position of the first eligible row and
continues positioning to other eligible rows. Now the job is not exclusive,
so the system temporarily switches to the
other job, which causes the disk to go back to the first row and work from
there. The switching back and forth continues,
so that instead of one job finishing quickly, they both have to take turns
waiting for needed information. That takes
a lot longer,

Try this, Select a table that has a lot of rows, ideally 1M+. Then start a
query with a WHERE clause and see how long
it takes. Then submit the same query from 5 separate connections
simultaneously and see how long that takes.

--
*Melvin Davidson*
*Maj. Database & Exploration Specialist*
*Universe Exploration Command – UXC*
Employment by invitation only!