In order to ensure continuous performance and minimize service downtime, services can be switched from the active server of a high-availability cluster to the redundant, passive server. This feature is called "Switchover," or in some cases "Failover." This section explains the basics of these two concepts.
Switchover refers to a switch from the active server to the redundant, passive server of a high-availability cluster. Under normal conditions, the active server manages all services. However, in the event the active server malfunctions, you can initiate a switchover and transfer all services to the passive server, assuming the passive server is functioning normally. In this case, the active server assumes the role of the passive server, and the passive server assumes the role of the active server.
When a switchover is initiated, the active server first assumes the role of passive server, and then the passive server becomes the active server. Thus, there will be a short period of time when both servers become passive and will not be able to provide services normally. The time to complete a switchover varies depending on the number and size of volumes or iSCSI LUNs (Block-Level), or the number and total load of services on the high-availability cluster.
When certain errors occur, the system can automatically initiate a switchover of services in order to maintain performance. This event is called "failover." The system might initiate failover in the following situations:
Switchover or failover might not succeed in the following situations:
If you manually initiate a switchover, but services cannot be switched to the passive server, then the system will attempt to switch services back to the active server.
In the event failover is initiated, but neither server is able to assume the role of active server due to errors, the high-availability cluster will not be able to provide service normally. If the servers do not automatically return to Standalone status, please press the Reset button on both servers to restore default settings. Once default settings have been restored you can combine the servers and create a high-availability cluster again.
High-availability clusters can be connected to uninterruptible power supply (UPS) units in order to reduce the risk of unexpected power loss. If power is lost, servers will receive backup power from any UPS units connected to them. When the UPS unit connected to the active server begins running low on power, the system will attempt to switch over services to the passive server. Please see the guidelines below.
Synology NAS supports connecting to local and SNMP UPS devices. For more information, please see the help for Control Panel > Hardware > UPS