db instance ping - cheeyoung/sqlplus-public GitHub Wiki
In an Oracle RAC environment, every few seconds the PING process of each instance checks the response of the interconnect to all instances of the same database.
It sends two messages. One message is 500 bytes in size (referred to in the column descriptions as 500B), and the other is 8 kilobytes in size (referred to in column descriptions as 8K).
For each message sent to each instance, the amount of time it took to get a response back is measured (in microseconds). The view records the latest measurements as well as cumulative data since instance startup.