5

I'm trying to create a parameter group in Amazon Web Services for an Postgres 9.5 RDS instance. I'm a little confused by the units being used for things such as shared_buffers and min_wal_size. Typically these would be specified in postgresql.conf with units, like 4GB or 128kB. If I enter the same in AWS I get an error that the value I specified was invalid, but strictly numeric values seem to be fine. Does anyone know what units these parameters are expecting?

5

Amazon RDS documentation provides a table of units for each parameter under the "Working with PostgreSQL Parameters" topic. Units are also usually included in the description of each parameter in the parameter group edit UI from the Amazon RDS web console.

min_wal_size (Postgres 9.5 Parameter Group Family)

  • Default: 16
  • Units: MB
  • Allowed Values: 2-201326592
  • Description: (16MB) Sets the minimum size to shrink the WAL to.

shared_buffers (Postgres 9.5 Parameter Group Family)

  • Default: {DBInstanceClassMemory/32768}
  • Units: 8kB
  • Allowed Values: 16-1073741823
  • Description: (8kB) Sets the number of shared memory buffers used by the server.

Regarding shared_buffers specifically, the documentation has this to say:

Note that some parameters use units that you might not be familiar with; for example, shared_buffers sets the number of 8 KB shared memory buffers used by the server.

2

Anthony's answer explains pretty well how RDS use those parameters. I just would like to add more comments to it.

You should follow table of contents for RDS.

The units may be seemed little bit weird to you if you compare it with postgresql.conf units. I just want to give an example;

Let's say you want to set shared_buffers to 1GB.

The unit of the shared_buffers in RDS is 8KB. Which means you should put n numbers of 8kb to make it 1 GB.

Which means; 1 GB -> 1024 MB -> 1048576 KB.

So you should set the value to 1048576 KB / 8 KB which is 131072

So the value should be 131072 in RDS parameter groups.

I hope it helps.

  • When dealing with memory, 1 GB -> 1024 MB -> 1048576 KB. – RDFozz Mar 28 '18 at 15:05
  • @RDFozz you are totally right. I just wanted to show how it needs to be set. – orhankutlu Mar 29 '18 at 11:11

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.