A system can have a single instance or can have multiple instances (four or more).
- Every instance must meet the minimum RAM, CPU, and disk space requirements.
- Three instances are sufficient to perform leader election for distributing work. However, a multi-instance system needs a minimum of four instances because, with the minimum hardware requirements, three instances are not sufficient for running all HCI services at their recommended distributions.
- Hitachi Vantara has qualified HCI systems with up to 16 instances.
One instance
A single-instance system is useful for testing and demonstration purposes. It needs only a single server or virtual machine and can perform all product functionality.
However, a single-instance system has these drawbacks:
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Only a single point of failure. If the instance hardware fails, you lose access to the system.
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With no additional instances, you cannot choose where to run services. All services run on the single instance.
Multiple instances
A multi-instance system is suitable for use in a production environment because it offers these advantages over a single-instance system:
- You can control how services are distributed across the multiple instances, providing improved service redundancy, scale out, and availability.
- A multi-instance system can survive instance outages. For example, with a four-instance
system running the default distribution of services, the system can lose one instance and
still remain available.Note: For a search index to survive an instance outage:
- The system must have at least two instances running the Index service.
- The Index Protection Level for the index must be at least 2.
For more information, see the HCI Administrator Help, which is available in the Admin App.
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Performance is improved as work can be performed in parallel across instances.
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You can add additional instances to the system at any time.
By adding additional instances to a single-instance system, your system still has only one master instance, meaning there is still a single point of failure for the essential services that only a master instance can run.
For information about adding instances to an existing HCI system, see the Content Intelligence Administrator Help, which is available from the Admin App.
Two-instance system considerations
Two-instance systems are a viable option for the HCM use case, but not recommended for Hitachi Content Search.
Three-instance system considerations
Three-instance systems should have only a single master instance. If you deploy a three-instance system where all three instances are masters, the system might not have enough resources to do much beyond running the master services.