When you create an array-based replication protection group that contains datastore groups or a vSphere Replication protection group that contains individual virtual machines, Site Recovery Manager creates a placeholder virtual machine at the recovery site for each of the virtual machines in the protection group.
A placeholder virtual machine is a subset of virtual machine files. Site Recovery Manager uses that subset of files to register a virtual machine with vCenter Server on the recovery site.
The files of the placeholder virtual machines are very small, and do not represent full copies of the protected virtual machines. The placeholder virtual machine does not have any disks attached to it. The placeholder virtual machine reserves compute resources on the recovery site, and provides the location in the vCenter Server inventory to which the protected virtual machine recovers when you run recovery.
The presence of placeholder virtual machines on the recovery site inventory provides a visual indication to vCenter Server administrators that the virtual machines are protected by Site Recovery Manager. The placeholders also indicate to vCenter Server administrators that the virtual machines can power on and start consuming local resources when Site Recovery Manager runs tests or runs a recovery plan.
When you recover a protected virtual machine by testing or running a recovery plan, Site Recovery Manager replaces the placeholder with the recovered virtual machine and powers it on according to the settings of the recovery plan. After a recovery plan test finishes, Site Recovery Manager restores the placeholders and powers off the recovered virtual machines as part of the cleanup process.
About Placeholder Virtual Machine Templates
When you protect a template on the protected site, Site Recovery Manager creates the placeholder template by creating a virtual machine in the default resource pool of a compute resource and then by marking that virtual machine as a template. Site Recovery Manager selects the compute resource from the set of available compute resources in the data center on the recovery site to which the folder of the virtual machine on the protected site is mapped. All the hosts in the selected compute resource must have access to at least one placeholder datastore. At least one host in the compute resource must support the hardware version of the protected virtual machine template.
About Placeholder Datastores
If you use array-based replication to protect datastore groups, or if you use vSphere Replication to protect individual virtual machines, you must identify a datastore on the recovery site in which Site Recovery Manager can store the placeholder virtual machine files.
Placeholder virtual machine files are very small, so the placeholder datastore does not need to be large enough to accommodate the full virtual machines.
To enable planned migration and reprotect, you must select placeholder datastores on both sites.