With segment mirroring enabled, Greenplum Database automatically fails over to a mirror segment instance when a primary segment instance goes down. Provided one segment instance is online per portion of data, users may not realize a segment is down. If a transaction is in progress when a fault occurs, the in-progress transaction rolls back and restarts automatically on the reconfigured set of segments. The gpstate utility can be used to identify failed segments. The utility displays information from the catalog tables including gp_segment_configuration.
If the entire Greenplum Database system becomes nonoperational due to a segment failure (for example, if mirroring is not enabled or not enough segments are online to access all user data), users will see errors when trying to connect to a database. The errors returned to the client program may indicate the failure. For example:
ERROR: All segment databases are unavailable
On the Greenplum Database master host, the Postgres
postmaster process forks a fault probe process,
ftsprobe. This is also known as the FTS (Fault Tolerance Server) process. The
postmaster process restarts the FTS if it fails.
The FTS runs in a loop with a sleep interval between each cycle. On each loop, the FTS probes each primary segment instance by making a TCP socket connection to the segment instance using the hostname and port registered in the
gp_segment_configuration table. If the connection succeeds, the segment performs a few simple checks and reports back to the FTS. The checks include running a
stat system call on critical segment directories and checking for internal faults in the segment instance. If no issues are detected, a positive reply is sent to the FTS and no action is taken for that segment instance.
If the connection cannot be made, or if a reply is not received in the timeout period, then a retry is attempted for the segment instance. If the configured maximum number of probe attempts fail, the FTS probes the segment’s mirror to ensure that it is up, and then updates the
gp_segment_configuration table, marking the primary segment “down” and setting the mirror to act as the primary. The FTS updates the gp_configuration_history table with the operations performed.
When there is only an active primary segment and the corresponding mirror is down, the primary goes into the Not In Sync state and continues logging database changes, so the mirror can be synchronized without performing a full copy of data from the primary to the mirror.
There is a set of server configuration parameters that affect FTS behavior:
In addition to the fault checking performed by the FTS, a primary segment that is unable to send data to its mirror can change the status of the mirror to down. The primary queues up the data and after
gp_segment_connect_timeout seconds pass, indicates a mirror failure, causing the mirror to be marked down and the primary to go into
Not In Sync mode.
Parent topic: Enabling High Availability and Data Consistency Features