Microway Application Note 17

Need for CMOS RTC (Real Time Clock) - OS clock sychronization for clusters

Procedure

Need for CMOS RTC (Real Time Clock) - OS clock sychronization for clusters

    There are problems with SMP kernels, MPI, and NFS file systems in some cases if the CMOS RTC and the OS clock on the cluster are not all synchronized with at least a master (in a master-slave cluster architecture) or with each other in a peer-to-peer architecuture. As is indicated in Microway Application Note 16 an essential prerequisite in an NFS mount for a cluster is this step. Problems may also arise if a benchmark test like NetPerf is being run and the clocks are not in sync.

Procedure

   The best way is to use an utility called xntp readily available an an RPM package on the  Red Hat installation CDROM in the directory RedHat/RPMS. It is called xntp3-5.93-2.i386.rpm. Here is how to use xntp.

1) Set all system clocks to the current time.

2) Write the time to the CMOS RTC using the command clock -w.

3) Mount the Red Hat installation CDROM on each computer in the cluster. (mount /dev/cdrom /mnt)

4) cd to /mnt/RedHat/RPMS.

5) As root (you may need to su) enter rpm -ivh --replacepkgs xntp3-5.93-2.i386.rpm.

6) Now edit the file /etc/ntp.conf

    ON ALL SYSTEMS (COMPUTERS IN THE CLUSTER) comment out the lines as shown:

    #multicastclient

   # listen on default 224.0.1.1

   #broadcastdelay 0.008

   ON NON-HOST SYSTEMS (every one but the host you will use to sync all the other nodes)

   Edit the lines:

   server HOSTNODE # local clock

  #fudge 127.127.1.0 stratum 0

  (where HOSTNODE is the name got on host node by typing hostname.)

  Close the /etc/ntp.conf file on each node.

7) Start xntpd on all the systems, "/sbin/xntpd".

    NOTE:

• You can start this each time you boot by adding this to your /etc/rc.d/rc.local file.
• It will take some time for synchronization, but you can see messages from xntpd in /var/log/messages.
• What you have just done is to tell the host node to run xntp and use the local system clock as reference. In  addition, all the other nodes in the cluster will get their time from the host.
• Although xntpd is supposed to keep the system clock and the RTC in sync, it is probably a good idea to sync them once a day. You can do this by (as root) going to /etc/cron.daily and creating a file called "sync_clocks" that contains the following:    

          # Assume ntp is running, so sync the CMOS RTC to OS system clock  

          /sbin/clock -w

• Now all the clocks in the cluster should be sync'ed and use the host as reference. If you wish to use an external reference consult the xntpd documentation.      

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