VMware vSphere Fault Tolerance 4 - Manual (Page 6)

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To ensure that the secondary virtual machine runs as fast as the primary, it is recommended that:
• The hosts in the FT cluster are homogenous, with similar CPU make, model, and frequency. The CPU frequency difference should
not exceed 400 MHz.
• Both the primary and secondary hosts use the same power management policy.
• CPU reservation is set to full for cases where the secondary host could be overloaded. The CPU reservation setting on the primary
applies to the secondary as well, so setting full CPU reservation ensures that the secondary gets CPU cycles even when there is
CPU contention.
2.4. I/O Latencies
All incoming network packets to the primary, and all disk reads at the primary, are immediately sent to the secondary. However, as
explained in section 1.4, network transmits and disk writes at the primary are held until the secondary acknowledges that all events
that precede the packet transmit or disk write. As a result, the round-trip network latency between the primary and the secondary
affects the I/O latency of disk writes and network transmit operations. Since the round trip latency in a LAN environment is usually in
the order of a few hundred microseconds, and disk I/O latencies are usually on the order of a few milliseconds, this delay does
not impact disk write operations. One may, however, notice delays in network ping responses if the response time is shown in
microseconds. For best performance, it is recommended that the round-trip network latency between the primary and secondary
host be less than 1 millisecond.
2.5. Network Link
Since the primary and secondary virtual machines proceed in vLockstep, the network link between the primary and the
secondary host plays an important role in performance. A Gigabit link is required to avoid congestion. In addition, higher bandwidth
network interfaces generally have lower transmission latency. If the network is congested and the primary host is not able to send
traffic to the secondary (i.e. when the TCP window is full), then the primary virtual machine will make little or no forward progress. If
the network connection between the primary and secondary hosts goes down, either the current primary or the current secondary
virtual machine will take over, and the other virtual machine will die.
2.6. NIC assignments for Logging Traffic
FT generates two types of network traffic:
• Migration traffic to create the secondary virtual machine
• FT logging traffic
Migration traffic happens over the NIC designated for VMotion and it causes network bandwidth usage to spike for a short time.
Separate and dedicated NICs are recommended for FT logging traffic and VMotion traffic, especially when multiple FT virtual
machines reside on the same host. Sharing the same NIC for both FT logging and VMotion can affect the performance of FT virtual
machines whenever a secondary is created for another FT pair or a VMotion operation is performed for any other reason.
VMware vSwitch networking allows you to send VMotion and FT traffic to separate NICs while also using them as redundant links for
NIC failover. See
KB article 1011966 for more information.
Adding multiple uplinks to the virtual switch does not automatically result in distribution of FT logging traffic. If there are multiple FT
pairs, then traffic could be distributed with IP-hash based load balancing policy, and by spreading the secondary virtual machines to
different hosts.
2.7. Virtual Machine Placement
FT logging traffic is asymmetric: the bulk of the traffic flow happens from the primary to the secondary hosts and the secondary host
only sends back acknowledgements. If multiple primary virtual machines are co-located on the same host, they could all compete for
the same network bandwidth on the logging NIC. Idle virtual machines consume less bandwidth, but I/O-intensive virtual machines
can consume a lot of network bandwidth. It can be helpful to place the primary of one FT pair and the secondary of another FT pair
on the same host to balance the traffic on the FT logging NIC.