6.5

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.
You can assume that only about 70% of a link is available for traffic replication. This means that on a
10Mb link you obtain a link speed of about 3GB per hour. On a 100Mb link you obtain a speed of about
30GB per hour.
To calculate the bandwidth, see Calculate Bandwidth for vSphere Replication.
Calculate Bandwidth for vSphere Replication
To determine the bandwidth that vSphere Replication requires to replicate virtual machines efficiently, you
calculate the average data change rate within an RPO period divided by the link speed.
If you have groups of virtual machines that have different RPO periods, you can determine the replication
time for each group of virtual machines. For example, you might have four groups with RPO of 15
minutes, one hour, four hours, and 24 hours. Factor in all the different RPOs in the environment, the
subset of virtual machines in your environment that is replicated, the change rate of the data within that
subset, the amount of data changes within each configured RPO, and the link speeds in your network.
Prerequisites
Examine how data change rate, traffic rates, and the link speed meet the RPO. Then look at the
aggregate of each group.
Procedure
1 Identify the average data change rate within the RPO by calculating the average change rate over a
longer period then dividing it by the RPO.
2 Calculate how much traffic this data change rate generates in each RPO period.
3 Measure the traffic against your link speed.
For example, a data change rate of 100GB requires approximately 200 hours to replicate on a T1
network, 30 hours to replicate on a 10Mbps network, 3 hours on a 100Mbps network.
VMware vSphere Replication Installation and Configuration
VMware, Inc. 19