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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 have not been
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 Aect 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.
Product Versions at the
Source and the Target
Site
The acceleration of initial synchronization is supported only on ESXi hosts
6.0.x or later.
If the ESXi and the vSphere Replication Server on the source site are 6.x or
later, but the vSphere Replication Server or the hosts at the target site are
not 6.x or later, the allocation information will be available only on the
source site.
If the vSphere Replication Management servers at the source and at the
target site are both 6.x, but one or more ESXi hosts at the target site are
not 6.0 or later, if the vSphere Replication Management server selects a
target host that is not 6.0 or later, there will be no allocation information
available on the target site.
Note Because vSphere Replication Management server 6.x cannot select
only ESXi 6.0 hosts for the initial synchronization, the acceleration of the
operations might vary depending on the selected host. To achieve
maximum acceleration, all ESXi hosts at the target site that act as storage
servers for vSphere Replication should be ESXi 6.0 or later.
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.
Note 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.
Using VMware vSphere Replication
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