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Administrators in the modern data center face a dilemma: Virtualization allows more workloads to run on fewer physical servers, and as the server count drops, the business becomes more reliant on those hosts that are left. A single system failure can affect 10, 20, 30 or even more VMs. To mitigate the risk of workload disruption, the emphasis of IT management has slowly shifted from recovery to business continuity -- implementing technologies to ensure workload availability during a system failure. One such technology is the Fault Tolerance (FT) feature of VMware vSphere, which synchronizes duplicate VMs to ensure workloads will continue to run without interruption.
VMware vSphere 5.5 supports up to 32 host servers and up to 4,000 VMs per cluster. This represents a sizable cluster, and few but the largest organizations would likely approach its limits. However, vSphere imposes stricter limitations on Fault Tolerance clusters, supporting only four FT VMs per host system. The goal is to keep the overall number of FT VMs small to ensure plenty of computing resources and prevent contention that might cause unacceptable synchronization delays. Since FT is intended to protect mission-critical workloads, there is usually no shortage of available server capacity within a cluster.
Logs are used to record and synchronize activity between the original and duplicate VMs, but there is latency -- called logging time delay -- between the original and duplicated VMs. In most cases, the latency is less than one millisecond, but the actual figure will depend on the network performance between both host systems. A congested or troubled network will increase the latency. Even a few milliseconds of lag generally will not impact the integrity of the duplicate VM, because the duplicate VM has to be "caught up" with the logs to be fully synchronized with the original VM.
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