In-Band Control Messaging Overview

When you take a snapshot on the Secondary, it contains a point-in-time copy of the data on the Primary. Because the Secondary may be behind the Primary, it is not known at exactly what time this point-in-time copy was made.

IBC messaging enables you to send a message in the replication stream to notify the Secondary that an event has occurred on the Primary. In the case of a file system, you can use the sync command on the Primary, and then send an IBC message. When this message arrives on the Secondary, the data on the Secondary is consistent at the file system level and replication stops. You then split off a mirror, which now contains a consistent image of the file system, and unfreeze replication.

The model with IBC Messaging is that a process on the Secondary waits for the IBC Message, and a process on the Primary sends the message when the desired event has occurred.

VVR provides the following options for IBC Messaging:

• Single command to perform off-host processing operations---vradmin ibc

This chapter explains the function of IBC Messaging and how to use the command vradmin ibc for off-host processing.

• IBC Messaging command-line utility---The vxibc utility

For more information on how to use the command-line utility vxibc, see Using the In-Band Control Messaging Utility vxibc and the IBC Programming API.

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  Note    If you choose not to use IBC Messaging, the data on the Secondary is consistent and can be recovered by the application but it is out-of-date.

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How to Use the Data on the Secondary

Using Snapshots

A snapshot is an image of the online data volumes at a specific point-in-time. Use snapshots of the Secondary data volumes to perform off-host processing operations, instead of directly using the Secondary data volumes. The data on the original volumes may change but the data on the snapshot can still be used as a stable and independent copy for various purposes.

VVR provides two methods of creating snapshots: instant snapshots and traditional snapshots. The instant snapshot feature is a separately licensed feature of VVR. For more information on instant snapshots, see Using the Instant Snapshot Feature. VVR also provides sample IBC scripts that can be used for creating snapshots. For more information, see Sample Scripts.

With the traditional snapshot feature, depending on the size of the volume, the time required for initial synchronization of the plexes can be very large. For more information on traditional snapshots, see Using the Traditional Snapshot Feature.

Before you can use the snapshot, some application-dependent recovery procedure has to be performed. For example, if the volume contains a file system, run the fsck program before mounting the file system.