6.7

Problem
When a Secondary VM resides on a host that is heavily loaded, the Secondary VM can affect the
performance of the Primary VM.
Cause
A Secondary VM running on a host that is overcommitted (for example, with its CPU resources) might not
get the same amount of resources as the Primary VM. When this occurs, the Primary VM must slow down
to allow the Secondary VM to keep up, effectively reducing its execution speed to the slower speed of the
Secondary VM.
Solution
If the Secondary VM is on an overcommitted host, you can move the VM to another location without
resource contention problems. Or more specifically, do the following:
n
For FT networking contention, use vMotion technology to move the Secondary VM to a host with
fewer FT VMs contending on the FT network. Verify that the quality of the storage access to the VM is
not asymmetric.
n
For storage contention problems, turn FT off and on again. When you recreate the Secondary VM,
change its datastore to a location with less resource contention and better performance potential.
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To resolve a CPU resources problem, set an explicit CPU reservation for the Primary VM at an MHz
value sufficient to run its workload at the desired performance level. This reservation is applied to
both the Primary and Secondary VMs, ensuring that both VMs can execute at a specified rate. For
guidance in setting this reservation, view the performance graphs of the virtual machine (before Fault
Tolerance was enabled) to see how many CPU resources it used under normal conditions.
Increased Network Latency Observed in FT Virtual Machines
If your FT network is not optimally configured, you might experience latency problems with the FT VMs.
Problem
FT VMs might see a variable increase in packet latency (on the order of milliseconds). Applications that
demand very low network packet latency or jitter (for example, certain real-time applications) might see a
degradation in performance.
Cause
Some increase in network latency is expected overhead for Fault Tolerance, but certain factors can add to
this latency. For example, if the FT network is on a particularly high latency link, this latency is passed on
to the applications. Also, if the FT network has insufficient bandwidth (fewer than 10 Gbps), greater
latency might occur.
Solution
Verify that the FT network has sufficient bandwidth (10 Gbps or more) and uses a low latency link
between the Primary VM and Secondary VM. These precautions do not eliminate network latency, but
minimize its potential impact.
vSphere Availability
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