I'm sorry, I've renamed pg_startup.log for when postgresql was restarted, file would be recreated, but it only appends information about that the output of the register will appear in the directory pg_log .. it doesn't return to show server configuration data, so I suppose that 128MB of shared_buffers is an entry about postgresql was created for first time.
Although, I continue not being sure about if it is getting assigned 2048MB .. because I run top command and it shows little memory used (total 8GB, used 340MB, free 7.6GB).
With 2048MB as shared_buffers, it should assined initially when postgresql server starts or it starts with a memory and it can grow until 2048MB?
Thanks beforehand.
Cheers...