Re: Question on disk contention

On Thu, May 31, 2018 at 10:04 AM, Ron <ronljohnsonjr(at)gmail(dot)com> wrote:

> On 05/31/2018 08:52 AM, Melvin Davidson wrote:
>
>
>
> 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.
>
>
> Why isn't the OS caching the disk blocks, and why isn't Postgres using the
> cached data?
>
>
> --
> Angular momentum makes the world go 'round.
>

> Why isn't the OS caching the disk blocks, and why isn't Postgres using
the cached data?
It does, but the cache is for each connection/job. They are not shared.
--
*Melvin Davidson*
*Maj. Database & Exploration Specialist*
*Universe Exploration Command – UXC*
Employment by invitation only!