ESXi replication requires a Virtual Appliance to be deployed on a source and target host.
ESXi hosts have no service console and the Virtual Appliance serves to replace this.
On the Virtual Appliance, a scratch disk is added to hold the files necessary for replication.
This knowledgebase article summarises those files.
The scratch disk is used to store two types of files:
vzmap files - block maps (in the form of a vzmap file) for the VMs replicated to the
destination host. This is block map information, and not actual data blocks. These maps are
compared to the source VM during each replication pass to identify the data blocks that have
changed since the last replication. The vzmap files make differential replication significantly
faster as they remove the need to scan the destination VM blocks for comparison with the
vzUndo files - As data is sent to the destination host (via the VA), blocks in the destination
disk are written to the undo file before they are overwritten by the changed data. If
replication fails and an undo becomes necessary, then the original destination disk blocks are
read from the undo file and written to the destination disk to rollback the failed replication.
This is a key function designed to provide resiliency in the face of a network failure; if there
is a network failure during the replication pass, the destination VM is not corrupted by
More Information about this topic can be found in the vRanger User Guide; 'About Replication' and 'Configuring the vRanger Virtual Appliance'