Thread: Parameter value in RDS

Hi,

I am new to RDS postgres, I have version 14 running on it with m7g.large

I found that lots of parameters has DBInstanceClassMemory written, so what exactly is the value of this variable ?

How should I calculate it?

On Wed, Jan 17, 2024 at 02:42:16AM +0530, Atul Kumar wrote:
> Hi,
> 
> I am new to RDS postgres, I have version 14 running on it with m7g.large
> 
> I found that lots of parameters has DBInstanceClassMemory written, so what
> exactly is the value of this variable ?
> 
> How should I calculate it?

We can't answer RDS-specific questions here.  I suggest you ask the
vendor.

-- 
  Bruce Momjian  <bruce@momjian.us>        https://momjian.us
  EDB                                      https://enterprisedb.com

  Only you can decide what is important to you.

On Tue, Jan 16, 2024 at 04:19:55PM -0500, Bruce Momjian wrote:
> On Wed, Jan 17, 2024 at 02:42:16AM +0530, Atul Kumar wrote:
> > Hi,
> > 
> > I am new to RDS postgres, I have version 14 running on it with m7g.large
> > 
> > I found that lots of parameters has DBInstanceClassMemory written, so what
> > exactly is the value of this variable ?
> > 
> > How should I calculate it?
> 
> We can't answer RDS-specific questions here.  I suggest you ask the
> vendor.

Okay, it seems someone here _did_ answer the question.  :-)

-- 
  Bruce Momjian  <bruce@momjian.us>        https://momjian.us
  EDB                                      https://enterprisedb.com

  Only you can decide what is important to you.

> On Jan 16, 2024, at 4:19 PM, David G. Johnston <david.g.johnston@gmail.com> wrote:
>
> On Tuesday, January 16, 2024, Atul Kumar <akumar14871@gmail.com> wrote:
> Hi,
>
> I am new to RDS postgres, I have version 14 running on it with m7g.large
>
> I found that lots of parameters has DBInstanceClassMemory written, so what exactly is the value of this variable ?
>
> How should I calculate it?
>
>  IIRC it’s the amount of RAM on your instance.  You look it up in a table usually.  Or check the web console.


Yes, I’m pretty sure that it’s the amount of RAM *in bytes*.

Although I like the flexibility of having that expressed in a variable, it wan’t always as easy to use as I wanted it
tobe. Specifically, I found that differences in units made it a little difficult to figure out how AWS was configuring
things. 

For example, on an old instance we had, shared_buffers was defined as {DBInstanceClassMemory/10922}. It took me a while
tofigure out that that translates to “calculate 75% of available memory, and express that value in 8Kb blocks”. How?
Well,10922 = 8 * 1024 * 1.33333. shared_buffers is expressed in 8Kb blocks, so converting from units of bytes
(DBInstanceClassMemory)to 8kB blocks (shared_buffers) requires dividing by 8 * 1024. And dividing by 1.3333 is the same
asmultiplying by 3/4, which is 75%.  

This may have been explained in AWS documentation but I couldn’t find it at the time and it took some work on my part
tofigure out the logic behind 10922 and some other config magic numbers. Maybe this will save you some time. 

Cheers
Philip

On 1/16/24 3:20 PM, Bruce Momjian wrote:
>> We can't answer RDS-specific questions here.  I suggest you ask the
>> vendor.
> Okay, it seems someone here_did_  answer the question.  🙂

Yeah, but still best that people go to vendors for stuff that's not 
community. If for no other reason than one of the best ways to get 
documentation improved is for support to get tired of answering the same 
question over and over again ;p
-- 
Jim Nasby, Data Architect, Austin TX