Update September 2012: With the introduction of vSphere 5.1, vSphere Replication is an integral part of VMware vSphere. vSphere replication can be used for free, although SRM is still a seperate add-on product.
Also read: Comparing Disaster Recovery options for VMware vSphere – About VR, ABR, SRM, Veeam & Zerto
With the introduction of VMware Site Recovery Manager 5 (SRM), VMware also released a new replication option: vSphere Replication.
With previous versions of SRM, it was mandatory to use the replication and snapshot technology provided by your storage vendor. E.g. when using NetApp, you had to use snapmirror (for replication) and flexclone (for failover testing).
vSphere Replication is a new option, which enables storage replication through the ESXi hypervisor. vSphere Replication is part of SRM and available at no extra cost.
Although vSphere Replication looks the same at first sight, there are some differences between vSphere Replication and Array Based replication. For reference I have summed up the differences in the following table:
|vSphere Replication||Array Based Replication|
|Replication using the vSphere/hypervisor layers.||Replication using the storage layer.|
|Incoporated in the SRM license, available at no additional costs.||A replication and snapshot license is mandatory. Costs depend on the storage vendor.|
|A different storage solution (e.g. Netapp & HP) on both locations is no problem. vSphere replication can replicate between these solutions.||You have to use the same storage solution, at least it should be the same brand.|
|Replication of local or direct attached storage is possible.||You can only replicate the data on the centralized storage solution (SAN)|
|RPO of 15 minutes – 24 hours||RPO of 0 minutes is possible (synchronous replication) till 24 hours, depending on the scheduling options of your storage solution.|
|A maximum of 500 virtual machines can be replicated.||‘Unlimited’ number of virtual machines can be replicated depending on the capacity, distance between the locations and available bandwith.|
|You cannot replicate physical servers.||You can replicate physical servers.|
|Replication can be configured per virtual machine.||Replication is configured per LUN/VMFS or NFS volume.|
|Configured by the VMware vSphere administrator||Configured by the storage administrator|
|Raw device mapping are not supported||Raw device mapping are supported|
|Virtual machines using Fault Tolerance cannot be replicated||Virtual machines using Fault Tolerance can be replicated|
|Linked cloned virtual machines (View, Cloud Director) cannot be replicated||Linked cloned virtual machines (View, Cloud Director) can be replicated|
|Clustered virtual machines (e.g. Microsoft Clustering Services) cannot be replicated||Clustered virtual machines (e.g. Microsoft Clustering Services) can be replicated|
|Automatic reprotection and a failback is not available. Note: In SRM 5.1 reprotection and failback is supported. See this article for more information.||Automatic reprotection and a failback is available.|
|vApp consistency (or consistency groups) are not available.||Consistency groups are available through your storage solution.|
|Application consistency can be achieved through VSS.||No application consistency options is available.|
I hope this table will help you to decide which replication technology to use.