How to set up Amazon RDS parameter group for Postgres?

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.

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.