VMware vSphere ^TM

Replicates powered-on virtual machines over the network from one vSphere host to another without needing storage array-based native replication. vSphere Replication provides a number of unique advantages

• Reduce bandwidth needs
• Eliminate storage lock-in
• Build flexible disaster recovery configurations

vSphere Replication is a VMware proprietary replication engine that copies only changed blocks to the recovery site, ensuring both lower bandwidth utilization and more aggressive recovery point objectives compared with manual full system copies of virtual machines.

• Leverage “seed copy” of virtual machine data during initial synchronization
• Ensure efficient network utilization by tracking changed disk areas and only replicating deltas

vSphere Replication operates at the individual virtual machine disk (VMDK) level. This allows replication between datastores hosted on any storage.

• Repurpose older storage at protection site
• Leverage different storage technologies at opposite sites (i.e SAN to NAS, FC to iSCSI, etc)
• Use secondary storage only for protected virtual machines, not entire environment

vSphere Replication can be configured to ensure consistent application data along with virtual machine data with one simple click when configuring a virtual machine for replication.

• Automatic integration with Microsoft’s Volume Shadow Copy Service (VSS) to ensure consistent recovery copies
• Flexible RPOs of 15 minutes to 24 hours
• Control replication at virtual machine level via vCenter Server
• Scales to hundreds of virtual machines per cluster

vSphere Replication is included in all editions except Essentials.

vSphere Replication is a deeply integrated component of the vSphere platform – it is the only truly “hypervisor level” replication engine available today. Changed blocks in the virtual machine disk(s) for a running virtual machine at a primary site are sent to a secondary site, where they are applied to the virtual machine disks for the offline (protection) copy of the virtual machine.

Figure 1 - Replicate virtual machines' changed blocks

vSphere Replication consists of an agent that a part of the core vSphere 5 installation package on each host, and a set of virtual appliances that are deployed from the management interface. From a conceptual perspective, the agent is responsible for sending changed data from a running virtual machine, and the appliance receives the replication at a remote site and applies it to the offline disk files for the virtual machine. The vSphere Replication appliance is also responsible for managing replication, which gives the administrator visibility into the status of protection of the virtual machines as well as the ability to recovery virtual machines with a few simple clicks.

Configuring replication for up to 500 virtual machines through the same management interface that is used for all vCenter operations is a simple process of right-clicking on a virtual machine and selecting the destination for its replica.

Part of this process is to select a “Recovery Point Objective” which will tell vSphere Replication how old the copy of the virtual machine is allowed to get. It will then attempt to replicate data to meet the Recovery Point Objective at all times, ensuring your virtual machine data is never older than the defined policy for each virtual machine configured for replication.

Figure 2 - Selecting an RPO for multiple virtual machines

vSphere Replication will do an initial full synchronization of the source virtual machine and its replica copy; if desired, a seed copy of data can be placed at the destination to reduce the amount of time and bandwidth required for the initial replication. A seed copy of a virtual machine consists of a virtual machine disk file that can be placed at the target location through just about any mechanism. Seeding is not a necessary process, and vSphere Replication will create an initial copy at the target location whether or not a seed is present. If a user has given a seed for replication, the data therein will be used to reduce the amount of replication needed to initially synchronize the primary disk and its replica. A seed can be created manually, or can be copied into place by any mechanism the administrator chooses such as offline copying, FTP, “sneakernet”, or even using an ISO or a clone of a virtual machine.

After baseline synchronization is complete, vSphere Replication switches to transferring only the blocks of data that have changed. The vSphere kernel itself tracks unique writes to the disk files of protected virtual machines, and identifies and replicates only those blocks that have experienced unique writes during the configured recovery point objective. This ensures a minimized amount of data is sent over the network to the target, and allows for aggressive recovery point objectives. Once unique data is sent, it doesn’t need to be sent again. Only changes will be replicated, and the blocks sent to the target location’s vSphere Replication Appliance.

Figure 3 - Data flow of replication from vSphere agent to VR Appliance

At the target location, the data is received and checked within the vSphere Replication Appliance: Only fully consistent data is then written (via Network File Copy) to the target cluster’s vSphere hosts and thereby to disk. This manner of waiting for a completely consistent block group ensures recoverability of the replica virtual machine at all times, even if data is lost during transit or a crash occurs at any point during the transfer.

From the perspective of the protected virtual machine, this entire process is completely transparent and requires no changes to configuration or ongoing management. The replication is non-intrusive and irrespective of the operating system within the virtual machine.