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VMware Site Recovery Manager replicates virtual machines from one site to another for disaster recovery and planned migrations, with automated failover and failback. You have two options for SRM replication: vSphere Replication and storage array replication.
The choice between remote replication techniques in Site Recovery Manager (SRM) rests primarily on scalability and storage costs. In general, VMware advises SRM users with smaller infrastructures to use vSphere Replication, and those with larger infrastructures to replicate virtual machines (VMs) via storage, but don't take that as a hard and fast rule. You can also mix storage array-based replication with vSphere Replication in one environment. Here's a closer comparison of the SRM replication options.
Hypervisor-based vs. storage-based VM replication for SRM
|Feature||SRM vSphere Replication||Storage array-based replication|
|Replication location||At the hypervisor level||At the storage level|
|Storage at recovery site||Does not need to match active site||Must be the same array type as the active site|
|Supported storage types||Direct-attached storage, local storage and other low-end options, as well as iSCSI, Fibre Channel and NFS storage||ISCSI, Fibre Channel and NFS. The supported storage replication vendors are listed on VMware's website|
|Integration with Site Recovery Manager||VSphere Replication communicates with SRM via vCenter Server||Third-party storage arrays interface with SRM via storage replication adapters written by the replication vendor|
|Cost||Included as an appliance with SRM 5.1. Requires vSphere hosts and vCenter Servers to operate||Varies by storage vendor. The cost of storage-based replication will include storage array hardware, licensing and storage snapshot licensing|
|Use cases||Smaller infrastructures, remote offices of large companies and non-business-critical applications||Large and/or business-critical environments|
|Recovery point objective (RPO) times||Fifteen-minute to 24-hour RPOs||Zero-minute (synchronous replication) to 24-hour RPOs|
|Reprotect and failback||Yes. Reprotect and failback are supported by vSphere Replication only in SRM 5.1, not in older versions||Yes. In older versions of SRM, only storage-array replication supports these features|
|VM limits||Five hundred. Deploy no more than 100 VMs per Replication server in SRM. VMware limits SRM's number of Replication servers to 10||One thousand. If vSphere Replication and array-based replication are used together, the limit remains 1,000 total VMs|
|Protection groups||250||250. If vSphere Replication and array-based replication are used together, the limit remains 250 total VMs|
|Snapshots||Yes, but vSphere Replication will only recover the latest snapshot||Yes, with specific limitations detailed in VMware's SRM 5.1 documentation|
|Linked clones||Not supported||Supported as long as nodes in the snapshot tree are replicated|
|Raw device mapping (RDM) VMs||Virtual mode RDM only||Virtual and physical RDM formats|
|Application-consistent recovery||Supported for Windows environments with VSS||The storage vendor ensures application-consistent replication|
|Works with VMware Fault Tolerance||No||Yes|
Notes: SRM does not support server-based replication, so it was not included in this comparison.
VMware also offers vSphere Replication as a feature of vSphere Enterprise Plus without purchasing SRM. VMware vSphere Replication's features and limitations, when used without SRM, are not included in this comparison.