When you configure a virtual machine for replication, vSphere Replication starts an initial configuration task. During this task a replica virtual machine is created on the target site, and data synchronization occurs between the source and the target vCenter Server sites.

The speed of data synchronization depends on the availability of information about the block allocation of the VMDK files. vSphere Replication uses this information to find empty regions of the disks and accelerate the sync operations by skipping these regions. The speed of data synchronization also depends on the site for which block allocation information is available.
  • If the allocation information is available at both sites, data synchronization occurs at the highest possible speed.
  • If the allocation information is available only at the source or the target site, vSphere Replication skips the empty regions on the VMDK disks at that site, but processes the entire disk at the site where the allocation information is not available. Therefore, data synchronization is slower.
  • If the allocation information is not available at either site, data synchronization is done by comparing all blocks between the source site and the target site, even if many of the blocks are not allocated on the disk by the guest OS. This is the slowest method for data synchronization.
Note: The availability of block allocation information has little effect on the speed of data synchronization for VMDK disks that are almost full.

Factors That Affect the Availability of Block Allocation Information

The availability of allocation information and the degree to which vSphere Replication can use it to accelerate data synchronization depend on:

  • The ESXi versions.
  • The vSphere Replication Management server versions.
  • The type of VMDK disks, and the type of volumes on which the disks reside.
Version Support
Table 1. Product Versions at the Source and the Target Site
Source Site Target Site Result
ESXi Host vSphere Replication Management Server ESXi Host vSphere Replication Management Server
6.x or later 6.x or later 6.x or later 6.x or later The acceleration of initial synchronization is supported.
6.x or later 6.x or later Earlier than 6.x Earlier than 6.x The allocation information is available only on the source site.
6.x or later 6.x or later Earlier than 6.x 6.x or later
6.x or later 6.x or later 6.x or later Earlier than 6.x
The Type of the Datastore
Disks on VMFS or vSAN datastores provide full allocation information.
NFS datastores cannot provide allocation information for the disks that are located on them.
Replication disks on the source and the target site can be on different datastore types. The acceleration of the initial synchronization depends on whether both sites can provide allocation information, or only one site. If none of the sites can provide allocation information, no acceleration occurs.
The Type of Virtual Disk
Lazy zeroed thick disks, thin disks, and vSAN sparse disks, Space-Efficient sparse disks, and VMDK sparse snapshots provide allocation information.
Eager zeroed thick disks do not provide allocation information.
Virtual disks that are based on vVols are native to the volume. vSphere Replication 8.5.x can get allocation information from them only when they are on the target site. For this reason, the acceleration of the initial synchronization is partial.