Oracle clusterware 11gR2 requires time synchronization across all nodes within a cluster when Oracle RAC is deployed. Through two ways you can do time synchronization across the cluster.
1) An operating system configured network time protocol (NTP)
2) Oracle Cluster Time Synchronization Service (ctss).
While installing Oracle Grid Infrastructure, if you don't have NTP daemons then (Oracle Cluster Time Synchronization daemon)ctssd starts up in active mode and synchronizes time among cluster members without contacting an external time server.
When Oracle sees there is NTP, then the Oracle Cluster Time Synchronization daemon (ctssd)
starts up in observer mode and no active time synchronization is performed by
Oracle Clusterware within the cluster.
In fact Oracle Cluster Time Synchronization Service is designed for organizations whose cluster servers are unable to access NTP services. For example your RAC database does not have way to access any external time server, then you can use ctssd for your clusterware installation.
If you have NTP daemons on your server but you cannot configure them to synchronize time with a time server, and you want to use Cluster Time Synchronization Service to provide synchronization service in the cluster then do the following steps.
1) Stop the existing ntpd service
# /sbin/service ntpd stop
2) Disable ntpd service from the initialization sequences
# chkconfig ntpd off
3) Remove or move the ntp.conf file.
# rm /etc/ntp.conf
or,
# mv /etc/ntp.conf /etc/ntp.conf.org
4) Also remove the ntpd.pid file which maintains the pid for the NTP daemon.
# rm /var/run/ntpd.pid
However if you are using NTP and you you prefer to continue using it instead of Cluster Time
Synchronization Service, then you need to modify the NTP configuration to set the -x flag, which prevents time from being adjusted backward.
1) Edit the /etc/sysconfig/ntpd file to add the -x flag
# vi /etc/sysconfig/ntpd
OPTIONS="-x -u ntp:ntp -p /var/run/ntpd.pid"
2) Restart the NTP service.
# /sbin/service ntp restart
Note that on SUSE systems, you have to modify the configuration file /etc/sysconfig/ntp with the
following settings:
NTPD_OPTIONS="-x -u ntp"
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