To replicate virtual machines efficiently, before configuring a replication you can determine the storage and network bandwidth requirements for vSphere Replication.
Storage and network bandwidth requirements can increase when using vSphere Replication. The following factors play a role in the amount of network bandwidth that vSphere Replication requires for an efficient replication.
- Between the host running the replicated virtual machine and the vSphere Replication server.
- Between the vSphere Replication server and a host with access to the replication target datastore.
- Between the host and storage.
- Between storage and the host, during redo log snapshots.
Network-based storage is a concern when you are replicating virtual machines within a single vCenter Server instance, that shares the network for the levels of traffic listed. When you have two sites, each with a vCenter Server instance, the link speed between the two sites is the most important as it can slow down the replication traffic between the two sites.
vSphere Replication might not replicate every virtual machine or 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 2 TB of virtual machines on the datastores and use vSphere Replication to replicate half of these virtual machines. You might only replicate a subset of the VMDKs and the maximum amount of data for replication is 1 TB.
Data Change Rate and Recovery Point Objective
Recovery point objective (RPO) affects the data change rate. 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. 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.
If you have to transfer an average replication bundle of 4 GB 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 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.
There is no hard requirement about the minimal latency across the Wide Area Network (WAN) caused by the geographic distance between the data centers. However, when the WAN connecting the two data centers has latency, out-of-order or dropped packets, the replication throughput can be affected resulting in RPO violations.