What is VVR?

VVR is an extension of the logical volume management capability of VxVM and is included with VxVM. You can start using VVR in a VxVM configuration by installing a valid VVR license.

VVR replicates initially synchronized volumes from a source location to one or more remote locations across any distance. It provides an exact or consistent copy of application data at the remote locations. VVR provides several methods to initialize the application data between the primary location and the remote location, such as using the network, using tape backup, and moving disks physically.

How VVR Processes Application Writes

This section helps you understand how application writes are directed when VxVM is not being used, when VxVM is added, and when VVR is added.

When VxVM/VVR is not being used

When VxVM is not being used, the application writes to a file system placed on a disk partition. In the case of applications or databases on raw devices, the database writes directly to the disk partition, instead of to a file system. In either case, the application, that is, the database or a file system, sends data to the operating system to be written to disk and the operating system communicates directly with the disks.

How Application Writes are Processed When VxVM and VVR are not Used

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When VxVM is present

When VxVM is being used, the applications write to logical devices called volumes, which have been created on Volume Manager disks (VM disks) rather than physical disks. A volume is a virtual disk device that appears to applications, such as databases, and file systems as a physical disk. However, a volume does not have the limitations of a physical disk.

How Application Writes are Processed When VxVM is Used

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When VVR is enabled

When VVR is added, it resides between the application and the underlying VxVM volumes. All writes to these replicated volumes are intercepted and replicated to the Secondary host in the order in which they were received at the Primary. Writes are also applied to the local volumes. However, reads are directly processed using the local volumes.

How Application Writes are Processed When VxVM and VVR are Used

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VVR sends writes to the Secondary in the order in which they were received on the Primary. The Secondary receives writes from the Primary and writes to local volumes.

The data volumes on the Secondary cannot be used while replication is in progress: that is, you cannot run an application directly on the data volumes on the Secondary. If an application on the Secondary attempts to access the data volumes, it will see inconsistent data because the application on the Primary is constantly changing the data on the volumes. To use the data on the Secondary while the Primary is active, use the snapshot feature to create a version of the data which is not being changed.

If a disaster occurs on the Primary, the applications that were running on the Primary can be brought up on the Secondary.