When you create 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 Datastores
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.