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Using the automatic synchronization feature

The Automatic Synchronization feature enables you to transfer the data on the Primary to the Secondary over the network. You can synchronize the Secondary using automatic synchronization either when the application is active or inactive.

The Automatic Synchronization procedure transfers data in the Primary data volumes to the Secondary by reading the Primary data volumes from start to finish and sending the data to the Secondary.

  Note   Automatic Synchronization does not maintain the order of writes; therefore, the Secondary is inconsistent until the process is complete.

The Secondary becomes consistent after the automatic synchronization completes. To use Automatic Synchronization successfully, the network must be sized appropriately. Note that the synchronization will complete only if the Primary receives writes at a lesser rate than they can be sent to the Secondary. If the Primary receives writes at a faster rate than they can be sent to the Secondary, the synchronization might never complete, especially if the writes are dispersed widely in the volume.

This feature enables you to synchronize multiple Secondary hosts at the same time. When performing automatic synchronization to multiple Secondary hosts, synchronization proceeds at the rate of the slowest network.

VVR pauses synchronization if the Secondary fails or the network disconnects. If the Primary fails while synchronization is in progress, the synchronization continues from the point at which it had stopped when the Primary recovers.

Prerequisite for using Automatic Synchronization

• Each data volume in the Primary RVG must have a DCM associated to it. If data volumes do not have DCMs, an attempt to automatically synchronize a Secondary fails.

The vradmin startrep command when used with the option -a enables you to start replication and automatically synchronize the Secondary data volumes with the Primary data volumes in an RDS; it brings the Secondary data volumes up-to-date with the Primary data volumes. You can use this command to synchronize the Secondary when the data volumes contain data and when the application is active or inactive. Replication to another Secondary can be started only after this automatic synchronization completes.

The vradmin startrep command can be issued from any host in the RDS. To check the status and progress of the automatic synchronization, use the vxrlink status command on the Primary RLINK. For more information, see Displaying the status of a Secondary.

To synchronize the Secondary and start replication using automatic synchronization, issue the following command:

# vradmin -g diskgroup -a startrep local_rvgname sec_hostname

The argument local_rvgname is the name of the RVG on the local host and represents its RDS.

The argument sec_hostname is the name of the Secondary host displayed in the output of the vradmin printrvg command. If the RDS contains only one Secondary, the sec_hostname is optional.

Example—Using the Automatic Synchronization Feature

In this example, the data volumes in the Primary RVG hr_rvg on host seattle contain valid data and the application is active. To start replication and synchronize the Secondary RVG hr_rvg on host london, issue the following command:

# vradmin -g hrdg -a startrep hr_rvg london

Notes on using automatic synchronization

• If you associate a new volume to an RDS while automatic synchronization is in progress, VVR does not automatically synchronize the newly associated data volume.
• In an RDS containing multiple Secondaries that have SRL overflow protection set to dcm, more than one Secondary may require the use of the DCM. If one Secondary is undergoing automatic synchronization and the RLINK of another Secondary is about to overflow, the Automatic Synchronization is abandoned and the DCM becomes active for the overflowing RLINK.
• If you try to automatically synchronize a new RLINK while an existing RLINK is using the DCM mechanism, the automatic synchronization fails.
• To remove a Secondary from a DCM resynchronization process, detach the corresponding Primary RLINK.
• If you try to dissociate a DCM from a data volume while the DCM is in use, the operation fails.
• If the DCM is detached because of I/O errors while the DCM is in use, the resynchronization is abandoned and the RLINKs that are being synchronized are detached.