6.7
Table Of Contents
- vSphere Availability
- Contents
- About vSphere Availability
- Business Continuity and Minimizing Downtime
- Creating and Using vSphere HA Clusters
- Providing Fault Tolerance for Virtual Machines
- How Fault Tolerance Works
- Fault Tolerance Use Cases
- Fault Tolerance Requirements, Limits, and Licensing
- Fault Tolerance Interoperability
- Preparing Your Cluster and Hosts for Fault Tolerance
- Using Fault Tolerance
- Best Practices for Fault Tolerance
- Legacy Fault Tolerance
- Troubleshooting Fault Tolerant Virtual Machines
- Hardware Virtualization Not Enabled
- Compatible Hosts Not Available for Secondary VM
- Secondary VM on Overcommitted Host Degrades Performance of Primary VM
- Increased Network Latency Observed in FT Virtual Machines
- Some Hosts Are Overloaded with FT Virtual Machines
- Losing Access to FT Metadata Datastore
- Turning On vSphere FT for Powered-On VM Fails
- FT Virtual Machines not Placed or Evacuated by vSphere DRS
- Fault Tolerant Virtual Machine Failovers
- vCenter High Availability
- Plan the vCenter HA Deployment
- Configure the Network
- Configure vCenter HA With the Basic Option
- Configure vCenter HA With the Advanced Option
- Manage the vCenter HA Configuration
- Set Up SNMP Traps
- Set Up Your Environment to Use Custom Certificates
- Manage vCenter HA SSH Keys
- Initiate a vCenter HA Failover
- Edit the vCenter HA Cluster Configuration
- Perform Backup and Restore Operations
- Remove a vCenter HA Configuration
- Reboot All vCenter HA Nodes
- Change the Appliance Environment
- Collecting Support Bundles for a vCenter HA Node
- Troubleshoot Your vCenter HA Environment
- Patching a vCenter High Availability Environment
- Using Microsoft Clustering Service for vCenter Server on Windows High Availability
The basis for vSphere HA admission control is how many host failures your cluster is allowed to tolerate
and still guarantee failover. The host failover capacity can be set in three ways:
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Cluster resource percentage
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Slot policy
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Dedicated failover hosts
Note vSphere HA admission control can be disabled. However, without it you have no assurance that
the expected number of virtual machines can be restarted after a failure. Do not permanently disable
admission control.
Regardless of the admission control option chosen, a VM resource reduction threshold also exists. You
use this setting to specify the percentage of resource degradation to tolerate, but it is not available unless
vSphere DRS is enabled.
The resource reduction calculation is checked for both CPU and memory. It considers a virtual machine's
reserved memory and memory overload to decide whether to permit it to power on, migrate, or have
reservation changes. The actual memory consumed by the virtual machine is not considered in the
calculation because the memory reservation does not always correlate with the actual memory usage of
the virtual machine. If the actual usage is more than reserved memory, insufficient failover capacity is
available, resulting in performance degradation on failover.
Setting a performance reduction threshold enables you to specify the occurrence of a configuration issue.
For example:
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The default value is 100%, which produces no warnings.
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If you reduce the threshold to 0%, a warning is generated when cluster usage exceeds the available
capacity.
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If you reduce the threshold to 20%, the performance reduction that can be tolerated is calculated as
performance reduction = current utilization * 20%. When the current usage minus the
performance reduction exceeds the available capacity, a configuration notice is issued.
Cluster Resources Percentage Admission Control
You can configure vSphere HA to perform admission control by reserving a specific percentage of cluster
CPU and memory resources for recovery from host failures.
With this type of admission control, vSphere HA ensures that a specified percentage of aggregate CPU
and memory resources are reserved for failover.
With the cluster resources percentage option, vSphere HA enforces admission control as follows:
1 Calculates the total resource requirements for all powered-on virtual machines in the cluster.
2 Calculates the total host resources available for virtual machines.
3 Calculates the Current CPU Failover Capacity and Current Memory Failover Capacity for the cluster.
vSphere Availability
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