As we have only one instance in postgres, but greenplum is a combination of many postgres instances sewn together.So setting up the shared buffer with "Alter system" will set the value for the master or all the segments? For example setting a value of 125 MB for 8 segments will make shared_buffer 125*8= 1GB or will 125 MB be for all the segments
The shared_buffers configuration is a per-segment configuration. Setting a value of 125MB for 8 segments will allocate 1GB for all 8 segments. Here is a snippet from the documentation.
Sets the amount of memory a Greenplum Database segment instance uses for shared memory buffers. This setting must be at least 128KB and at least 16KB times max_connections.
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Each Greenplum Database segment instance calculates and attempts to allocate certain amount of shared memory based on the segment configuration.