The Cluster Time Synchronization Service (CTSS) is installed as part of Oracle Clusterware and runs in observer mode if it detects a time synchronization service or a time synchronization service configuration, valid or broken, on the system. For example, if the etc/ntp.conf file exists on any node in the cluster, then CTSS runs in observer mode even if no time synchronization software is running.
If CTSS detects that there is no time synchronization service or time synchronization service configuration on any node in the cluster, then CTSS goes into active mode and takes over time management for the cluster.
If CTSS is running in active mode while another, non-NTP, time synchronization software is running, then you can change CTSS to run in observer mode by creating a file called etc/ntp.conf. CTSS puts an entry in the alert log about the change to observer mode.
When nodes join the cluster, if CTSS is in active mode, then it compares the time on those nodes to a reference clock located on one node in the cluster. If there is a discrepancy between the two times and the discrepancy is within a certain stepping limit, then CTSS performs step time synchronization, which is to step the time, forward or backward, of the nodes joining the cluster to synchronize them with the reference.
Clocks on nodes in the cluster become desynchronized with the reference clock (a time CTSS uses as a basis and is on the first node started in the cluster) periodically for various reasons. When this happens, CTSS performs slew time synchronization, which is to speed up or slow down the system time on the nodes until they are synchronized with the reference system time. In this time synchronization method, CTSS does not adjust time backward, which guarantees monotonic increase of the system time.
When Oracle Clusterware starts, if CTSS is running in active mode and the time discrepancy is outside the stepping limit (the limit is 24 hours), then CTSS generates an alert in the alert log, exits, and Oracle Clusterware startup fails. You must manually adjust the time of the nodes joining the cluster to synchronize with the cluster, after which Oracle Clusterware can start and CTSS can manage the time for the nodes.
When performing slew time synchronization, CTSS never runs time backward to synchronize with the reference clock. CTSS periodically writes alerts to the alert log containing information about how often it adjusts time on nodes to keep them synchronized with the reference clock.
CTSS writes entries to the Oracle Clusterware alert log and syslog when it:
Detects a time change
Detects significant time difference from the reference node
The mode switches from observer to active or vice versa
Having CTSS running to synchronize time in a cluster facilitates troubleshooting Oracle Clusterware problems, because you will not have to factor in a time offset for a sequence of events on different nodes.