Administering the SRL

The size of the SRL is critical to the performance of replication. When the SRL overflows for a particular Secondary, the Secondary becomes out of date until a complete resynchronization with the Primary is performed. Because resynchronization is a time-consuming process and during this time the data on the Secondary cannot be used, it is important to prevent the SRL from overflowing. Hence, when initially configuring VVR, determine an appropriate size for the SRL. The maximum size of the SRL can be derived from various criteria, however, the size of the SRL volume cannot be less than 110 MB. If the size that you have specified for the SRL is less than 110MB, VVR displays a message that prompts you to specify a value that is equal to or greater then 110 MB. For more information, refer to "Sizing the SRL" in the VERITAS Volume Replicator Planning and Tuning Guide.

It is possible that an SRL of an appropriate size overflows because of changes in the environment. This section describes how to protect from SRL overflows and administer VVR if the SRL overflows.

Protecting from SRL Overflow

To avoid complete synchronization of Secondary in the case of an SRL overflow, VVR provides autodcm or dcm mode of SRL protection. For more information, see "Protection Against SRL Overflow---srlprot attribute" in the VERITAS Volume Replicator Administrator's Guide.

To enable SRL protection, each data volume in the RDS must have an associated DCM. For more information, see "Associating a Data Change Map to a Data Volume" in the VERITAS Volume Replicator Administrator's Guide. To change the replication setting for SRL protection, see Changing the Replication Settings.

Incrementally Synchronizing the Secondary After SRL Overflow

The default protection mode for the SRL is autodcm and every data volume in the RVG must have a DCM. When the SRL fills up, whether the RLINK is connected or not, DCM logging is activated and a bit corresponding to the region of the update is turned on for every incoming update. When you are ready to replay the DCM, start the DCM resynchronization process. To start the resynchronization, use the Replay DCM wizard.

Note that you can also use the cache or cache size parameters with the Replay DCM wizard. If you specify these attributes, the wizard first creates a space-optimized snapshot of the Secondary data volumes before starting the resynchronization. Data is transmitted to the Secondaries only after all the RLINKs taking part in the resynchronization have connected. All the Secondaries taking part in the resynchronization must remain connected for the resynchronization to continue. The resynchronization will pause if any of the Secondary RLINKs are paused. During DCM resynchronization, VVR does not maintain the order of updates to the Secondary. As a result, the Secondary remains inconsistent until the resynchronization operation is complete. Note that if the Primary becomes unavailable during the time the resynchronization is taking place, the applications cannot be restarted on the Secondary.

If the Secondary data volumes are mirrored, you can break off mirrors to retain consistent (though out-of-date) copies of data until the resynchronization is complete. However, to overcome this problem, create snapshots of the Secondary data volumes before the resynchronization starts.

The cache attribute specifies a name for the precreated cache object, on which the snapshots for the volumes in the specified RVG will be created. The cachesize attribute specifies a default size for the cache object with respect to the source volume. You can specify only one of these attributes at one time with the Replay DCM wizard to create one cache object for each snapshot.

If you do not specify either of these options then the Replay DCM wizard resynchronizes the Secondary data volumes without creating the snapshots.

Replay DCM starts replay of a DCM that is active because of an SRL overflow or failback logging. Replay occurs on all RLINKs on which the dcm_logging flag is set. If any of these RLINKs are disconnected or paused, replay DCM is delayed until this condition clears on all RLINKs.

Prerequisite for incrementally resynchronizing the Secondary

  The RVG must have the dcm_logging flag set.

  To incrementally resynchronize the Secondary

1. From any view of the RDS for which the resynchronization is to be started, select Tools > Replay DCM.
2. Complete the Summary page as follows:

   Cache Name	Select Cache Name and specify the name for a pre-created cache object on which the snapshots for the data volumes in the Secondary RVG will be created.
   Cache Size	Select Cache Size and specify a default size for the cache object with respect to the source volume. The default is Megabytes. Select another unit from the drop-down menu, if necessary.
   none	Use this option if you have a consistent mirror of the Secondary data volumes and do not need to use snapshots, in which case break off the mirror first so that a consistent copy of the data is available in case something went wrong during the resynchronization.

3. Click OK to start the incremental synchronization.

   Refresh the RDS view to monitor the progress of the incremental synchronization. The RDS view shows how much data is left to send to each Secondary that is being resynchronized.

Resizing the Primary SRL

The Primary SRL can be increased while the application is active or while replication is in progress.

Prerequisite

  Make sure there is enough space in the disk group on the Primary to increase the size of the SRL.

  To resize the SRL

1. Select Configuration > Resize SRL.
2. Complete the Resize SRL dialog box as follows:

   Increase By	To increase the data volume by a specified size, select Increase By.
   Resize To	To resize the data volume to a specified size, select Resize To. The default size unit is megabytes. To specify a size unit, select either sectors, kilobytes, or gigabytes from the drop-down list. Depending on the selected option, specify the size in the provided field.

3. Click OK.