Identifying the most up-to-date Secondary

Use the vxrlink updates command to identify the most up-to-date Secondary in a Volume Replicator configuration. The vxrlink updates command can be issued only on a Secondary.

Syntax for vxrlink updates command

For multiple Secondaries, the vxrlink updates command enables you to determine the Secondary that contains the most up-to-date data and hence the most suitable replacement for the Primary in the case of a take over.

For a single Secondary, the vxrlink updates command can be used to determine the extent to which the Secondary is behind the Primary. You can decide whether or not to take over the Primary role by looking at the update ID of the Secondary and the number of updates by which the Primary is ahead of the Secondary.

If the Secondary is paused and is behind the Primary, the vxrlink updates command may show inaccurate values as long as the Replicator Log is being written to, because the status is the same as it was before the pause. However, if the Replicator Log overflows and the DCM is activated then the vxrlink updates command output displays the correct value by which the Secondary is behind. When the Primary switches to the DCM mode, it reconnects the Secondary RLINK and also sends updated information. This information also includes the last update sequence number on the Primary and the time that is associated with this update. Hence, the latest values are displayed.

To display the output only in terms of an update ID, use the vxrlink updates command without the -T option. The output displays the update ID as a sequence number. A sequence number is a 64-bit value that increases incrementally and hence is unique for each new update and is assigned for every new update that arrives at the Primary. The output of the vxrlink updates command displays the 64-bit number as two 32-bit sequence numbers that are separated by a dot. For example,

To display the exact time on the Primary at which the Secondary is up-to-date use the vxrlink updates command with the -T option. The -T option displays the exact time in hours by which the Secondary is behind.

The output of the vxrlink -T updates command is displayed in a three-column structure with two rows; ID and Time. The ID row displays the update IDs.

If the local time of the Primary node has been adjusted to accommodate the daylight savings or for any other reason, then the updates in the Replicator Log may still have the time stamp based on the earlier clock settings. This may appear incorrect. However, the new updates are time stamped based on the changed time settings.

The following table describes the values of the most up-to-date Secondary.

Table: Most up-to-date secondary status

The time stamp in the Time row indicates the time at which the update was written on the Primary. The first column displays the last update ID and the time at which it was written on the Primary.

The second column displays the last update ID that has been received on the Secondary and the time when it was written on the Primary. If the Secondary is up-to-date then the ID and the time in this column is the same as that in the first column. However, if the Secondary is behind, then the ID and the time is different from that in the first column.

The third column indicates the exact number of updates by which the Secondary is behind. This value is obtained as a difference between the second and first column.