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Identifying the most up-to-date Secondary

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

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 takeover.

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, number of updates the Primary is ahead of the Secondary, and how long you expect the Primary to be unavailable.

Issue the following command on the Secondary.

# vxrlink -g diskgroup -T updates rlink_name

To display only the update ID in the output, 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 therefore is unique for each new update. The output of the vxrlink updates command displays the 64-bit number as two 32-bit sequence numbers separated by a dot. For example,

high_seq_num . low_seq_num

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. Note that the update information may be inaccurate if:

• the Secondary has been rebooted and even before it comes up, the Primary becomes unavailable.
• the Secondary reboots and the RLINK gets disconnected.

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. The timestamp in the Time row indicates the time at which the update was written on the Primary. The time is displayed in Mon date time format, where Mon is a locale abbreviated month name followed by the date and the time in the locale's appropriate time format.

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 and also the time in the locale's appropriate time format by which it is behind. This value is obtained as a difference between the second and first column.

  Note   If the system time is reset to a value different from that of the current system time, the output of the vxrlink -T updates command appropriately shows a negative or an inaccurate value, until the updates that were completed before resetting the system time are replicated.

Example—to determine the most up-to-date Secondary

This example shows how to determine the most up-to-date Secondary in an RDS containing Primary seattle and the Secondaries london and newyork. This example displays the last update ID received by the Secondary and the last known update ID on the Primary.

1. On the Secondary london, enter the following command:

   # vxrlink -g diskgroup updates to_seattle

   The output resembles:

   Secondary has received an update ID of 37364.104, last known update ID on Primary is 99 updates ahead.

2. On the Secondary newyork, enter the following command:

   # vxrlink -g diskgroup updates to_seattle

   The output resembles:

   Secondary has received an update ID of 37364.118, last known update on Primary is 95 updates ahead.

   Compare the output on london and newyork. The host newyork has received the later update with the update ID 37364.118, whereas the other Secondary london is behind newyork by 14 updates. The host newyork is more up-to-date than the host london.

This example shows how to determine the status of the Secondary in an RDS containing Primary seattle and the Secondary london, using the -T option with the vxrlink updates command.

1. On the Secondary london, enter the following command:

   # vxrlink -g diskgroup -T updates to_seattle

   If the Secondary is up-to-date the output displays the following:

   Last update Secondary Secondary

   on Primary up-to-date as of behind by

   ID 34666.0 34666.0 0

   Time Oct 16 11:17:44    Oct 16 11:17:44          00:00:00

   If the Secondary is not up-to-date the output displays the following:

   Last update Secondary Secondary

   on Primary up-to-date as of behind by

   ID 34666.640 34666.592 48

   Time Oct 16 11:17:44    Oct 16 11:17:42        00:00:02