Message-ID: <7bc57e3ad40f0baa364d1c3c142f9dccb8f5cb01.camel@cybertec.at>
Subject: Re: Would PostgreSQL adapt to a HYPOTHETICAL hardware that grows
 and shrinks as load increases?
From: Laurenz Albe <laurenz.albe@cybertec.at>
To: Jean Baro <jfbaro@gmail.com>, pgsql-novice@lists.postgresql.org
Date: Tue, 21 Dec 2021 15:16:42 +0100
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On Mon, 2021-12-20 at 15:33 -0300, Jean Baro wrote:
> Let's imagine there is a type of Server(Physical, Unikernel, or Hypervisor,
> doesn't matter here) that could allocate more RAM, IO bus, Network bandwidth,
> or CPU power through API calls. The increase or decrease would take a few
> milliseconds to effectively take place.
> 
> This hypothetical VM would receive commands from external monitoring tools
> that detect load and complex queries (through statistics and ML) and would
> decide to adjust the VM size accordingly,
> 
> I have a rough idea how complex (or impossible) it would be for ANY modern
> database to rely on an always-changing "hardware". As there are many CORE
> datasets in memory and statistics around the hardware, so changing this
> every now and then seems to require a completely different strategy.
> 
> I am not an expert but as I see more and more Serverless solutions tackling
> RDBMS, and PostgreSQL being the leader in the cloud world, I was wondering
> if one day PostgreSQL could take advantage of these scalable VMs.
>  * It seems to me that scaling up could be less complex, as PG wouldn't
>    have to select what to invalidate from its cache layers and statistics.
>  * Statistics would be affected every time the size of the instance changes
>  * The instance should probably have a minimum size to avoid cold starts.
>    So scaling to zero wouldn't be possible.
>  * If the server where the VM/Unikernel is running has no more available
>    resources to grow, what should be done (maybe this machine should share
>    resources with SPOT only, so it could forcibly deallocate the spot
>    instances before increasing the size of the DB VM?)

PostgreSQL does not examine the operating system resources.  Doing so would be
difficult, because that would have to support all (or at least most of the)
operating systems supported by PostgreSQL, and the differences are substantial.

On the other hand, there are a couple of parameters, like "effective_cache_size",
that tell PostgreSQL something about the operating system resources, and other
parameters like "work_mem" that might change with the available resources
are configurable at runtime.

So while I don't see PostgreSQL "self-tuning", it should be possible to write
a piece of software that monitors the resources on a given system and
adjusts PostgreSQL accordingly.  This will of course never be a "one size fits all",
because the correct configuration depends a lot on the workload.

Yours,
Laurenz Albe
-- 
Cybertec | https://www.cybertec-postgresql.com