5.5

Dataset Size
vSphere Replication might not replicate every virtual machine nor every VMDK file in the replicated virtual
machines. To evaluate the dataset size that vSphere Replication replicates, calculate the percentage of
the total storage used for virtual machines, then calculate the number of VMDKs within that subset that
you have configured for replication.
For example, you might have 2TB of virtual machines on the datastores and usevSphere Replication to
replicate half of these virtual machines. You might only replicate a subset of the VMDKs and assuming all
the VMDKs are replicated, the maximum amount of data for replication is 1TB.
Data Change Rate and Recovery Point Objective
The data change rate is affected by the recovery point objective (RPO). To estimate the size of the data
transfer for each replication, you must evaluate how many blocks change in a given RPO for a virtual
machine. The data change rate within the RPO period provides the total number of blocks that
vSphere Replication transfers. This number might vary throughout the day, which alters the traffic that
vSphere Replication generates at different times.
vSphere Replication transfers blocks based on the RPO schedule. If you set an RPO of one hour,
vSphere Replication transfers any block that has changed in that hour to meet that RPO.
vSphere Replication only transfers the block once in its current state at the moment that
vSphere Replication creates the bundle of blocks for transfer. vSphere Replication only registers that the
block has changed within the RPO period, not how many times it changed. The average daily data
change rate provides an estimation of how much data vSphere Replication transfers or how often the
transfers occur.
If you use volume shadow copy service (VSS) to quiesce the virtual machine, replication traffic cannot be
spread out in small sets of bundles throughout the RPO period. Instead, vSphere Replication transfers all
the changed blocks as one set when the virtual machine is idle. Without VSS, vSphere Replication can
transfer smaller bundles of changed blocks on an ongoing basis as the blocks change, spreading the
traffic throughout the RPO period. The traffic changes if you use VSS and vSphere Replication handles
the replication schedule differently, leading to varying traffic patterns.
If you change the RPO, vSphere Replication transfers more or less data per replication to meet the new
RPO.
Link Speed
If you have to transfer an average replication bundle of 4GB in a one hour period, you must examine the
link speed to determine if the RPO can be met. If you have a 10Mb link, under ideal conditions on a
completely dedicated link with little overhead, 4GB takes about an hour to transfer. Meeting the RPO
saturates a 10Mb WAN connection. The connection is saturated even under ideal conditions, with no
overhead or limiting factors such as retransmits, shared traffic, or excessive bursts of data change rates.
VMware vSphere Replication Administration
VMware, Inc. 24