In Greenplum Database, the segments are where the database data is stored and where the majority of query processing takes place. User-defined tables and their indexes are distributed across the available number of segments in the Greenplum Database system, each segment containing a distinct portion of the data. Segment instances are the database server processes that serve segments. Users and administrators do not interact directly with the segments in a Greenplum Database system, but do so through the master.

Data Redundancy - Mirror Segments

Greenplum Database provides data redundancy by deploying mirror segments. Mirror segments allow database queries to fail over to a backup segment if the primary segment becomes unavailable. A mirror segment always resides on a different host than its corresponding primary segment. A Greenplum Database system can remain operational if a segment host, network interface or interconnect switch goes down as long as all portions of data are available on the remaining active segments.

During database operations, only the primary segment is active. Changes to a primary segment are copied over to its mirror using a file block replication process. Until a failure occurs on the primary segment, there is no live segment instance running on the mirror host -- only the replication process.

In the event of a segment failure, the file replication process is stopped and the mirror segment is automatically brought up as the active segment instance. All database operations then continue using the mirror. While the mirror is active, it is also logging all transactional changes made to the database. When the failed segment is ready to be brought back online, administrators initiate a recovery process to bring it back into operation.