Understanding the VVR Snapshot Feature

VVR enables you to create an image of the online data volumes, at a given point in time and such an image is referred to as a snapshot. The data in the original volume may change; however, the snapshot can still be used as a stable and independent copy for various purposes, including the following:

• to restore data both on the Primary and Secondary if the original data gets corrupted because of logical errors, administrative errors, or media errors such as disk failures.
• to verify the Disaster Recovery (DR) readiness of the DR site or perform firedrill activities.
• to create a copy of the data for application development or testing.
• to support off-host processing for applications such as Decision Support Systems (DSS) or for report generation.
• to perform online data verification of the volumes in an RVG when replication is in progress.
• to retain a consistent copy of the Secondary data volumes during Data Change Map (DCM) resynchronization.

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  Note    You can use the snapshot feature on the Primary and the Secondary host.

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The snapshot feature in VVR is the same as the snapshot feature in VxVM, because VVR is fully integrated with VxVM. In VVR, an RVG is a group of VxVM volumes; therefore, taking a snapshot of an RVG is the same as taking a snapshot of the data volumes in the RVG.

VVR provides two methods of creating snapshots which are:

• Traditional Snapshot Feature
• Instant Snapshot Feature

Traditional Snapshot Feature

The traditional snapshot feature of VVR enables you to create snapshots of all the data volumes in an RVG at a single point in time, by breaking off the volume plexes. You can create snapshots when the volume plexes are completely synchronized with the data volume.

This method requires you to create and attach the appropriate snapshot plexes that are the same size as the original volumes, before taking the snapshot. For more information on creating the plexes, refer to the VERITAS Volume Manager Administrator's Guide.

After creating and attaching the snapshot plexes, they must be synchronized with the data volume. The time required for synchronizing the plexes in the beginning is directly proportional to the size of the volume. Thus, depending on the size of the volume it may take a long time before the plexes are ready to be used.

When the plexes are synchronized, you can then create snapshots after freezing replication using the IBC commands or after pausing replication.

For more information on traditional snapshots, refer to the section Using the Traditional Snapshot Feature.

Instant Snapshot Feature

The instant snapshot feature enables you to take instant full snapshots, instant space-optimized snapshots, or instant plex-breakoff snapshots. The advantages of this method compared to the traditional method are:

• the plexes or snapshot volumes do not require synchronization before taking the snapshots.
• the snapshots are instantly available.

The instant snapshot feature provided by VVR can be used either from the Primary or the Secondary. VVR also provides you with the option to take space-optimized snapshots.

The types of instant snapshots that you can create are:

• Instant Full Snapshots
• Instant Space-Optimized Snapshots
• Instant Plex-Breakoff Snapshots

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  Note    If an RVG consists of only VxVM ISP volumes, you can perform only the instant full snapshot operation and the instant space-optimized snapshot operations. When creating the instant space-optimized snapshot, you must create the VxVM ISP cache object before proceeding with the operation as VVR does not support the cachesize parameter for an RVG with VxVM ISP volumes.

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Instant Full Snapshots

The instant full snapshot feature of VVR enables you to create a full snapshot of all the data volumes in an RVG without any delay. In this case, the snapshot plexes do not require synchronization before creating the snapshot. Therefore, the required data is available instantly after the snapshot is created. However, this method requires the snapshot volumes to be created with the appropriate naming convention, before you proceed with the snapshots. For more information on creating instant full snapshots, refer to the section Instant Full Snapshot.

Instant Space-Optimized Snapshots

VVR also enables you to take instant space-optimized snapshots. Unlike instant full snapshots, the instant space-optimized snapshots require less storage space than the original volume because space-optimized snapshots store only the data that has changed between the original volume and the snapshot. Typically, the data that has changed between the original volume and the snapshot volume over the lifetime of the snapshot is minimal compared to the total data on the volume. Thus, the space-optimization achieved can be significant. The snapshot data is managed by VVR using a cache object which functions as a space-optimized persistent store. You must create the cache object before you take the instant space-optimized snapshots or specify the size of the cache object. Multiple snapshots can be created on the same cache object. The cache object can be created with an autogrow option set to on to allow it to grow automatically if the cache volume size is not enough for the specified writes. For more information on the cache object, refer to the section Preparing the RVG Volumes for Snapshot Operation. For more information on creating instant space-optimized snapshots, refer to the section Creating Instant Space-Optimized Snapshots.

Instant Plex-Breakoff Snapshots

Similar to the traditional plex-breakoff snapshot feature, this method also requires the plexes to be attached to the source volume before taking the snapshots. Although the synchronization of the plexes may still take a long time, the major difference between the traditional snapshots and the instant plex-breakoff snapshots is that you can instantly start performing the operations such as refresh, restore, and snapback on the instant plex-breakoff snapshots.

The instant plex-breakoff snapshots operation requires the plexes to be named using the plexprefix attribute if you want to use specific plexes. Otherwise, VVR uses the plexes that are in the SNAPDONE state. For more information on creating the instant plex-breakoff snapshots, refer to the section Instant Plex-breakoff Snapshots.