Choosing Single Or Dual Controllers - HP P2000 G3 MSA Technical White Paper

Technical white paper | HP P2000 G3 MSA
Figure 2. Read I/O Processing with ULP

Choosing single or dual controllers

Although you can purchase a single-controller configuration, it is best practice to use the dual-controller configuration to
enable high availability and better performance. However, under certain circumstances, a single-controller configuration can
be used as an overall redundant solution.
Dual controller
A dual-controller configuration improves application availability because in the unlikely event of a controller failure, the
affected controller fails over to the surviving controller with little interruption to the flow of data. The failed controller can be
replaced without shutting down the storage system, thereby providing further increased data availability. An additional
benefit of dual controllers is increased performance as storage resources can be divided between the two controllers,
enabling them to share the task of processing I/O operations. Controller failure results in the surviving controller doing the
following:
Taking ownership of all RAID sets
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Managing the failed controller's cache data
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Restarting data protection services
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Assuming the host port characteristics of both controllers
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The dual-controller configuration takes advantage of mirrored cache. By automatically "broadcasting" one controller's write
data to the other controller's cache, the primary latency overhead is removed and bandwidth requirements are reduced on
the primary cache. Any power loss situation will result in the immediate writing of cache data into both controllers' compact
flash devices, reducing any data loss concern. The broadcast write implementation provides the advantage of enhanced
data protection options without sacrificing application performance or end-user responsiveness.
Note
When using dual controllers, it is highly recommended that dual-ported hard drives be used for redundancy. If you use
single-ported drives in a dual controller system and the connecting path is lost, the data on the drives would remain
unaffected, but connection to the drives would be lost until the path to them is restored.
Single controller
A single-controller configuration provides no redundancy in the event that the controller fails; therefore, the single
controller is a potential Single Point of Failure (SPOF). Multiple hosts can be supported in this configuration (up to two for
direct attach). In this configuration, each host can have access to the storage resources. If the controller fails, the host loses
access to the storage.
The single-controller configuration is less expensive than the dual-controller configuration. It is a suitable solution in cases
where high availability is not required and loss of access to the data can be tolerated until failure recovery actions are
complete. A single-controller configuration is also an appropriate choice in storage systems where redundancy is achieved
at a higher level, such as a two-node cluster. For example, a two-node cluster where each node is attached to a P2000 G3
FC with a single controller and the nodes do not depend upon shared storage. In this case, the failure of a controller is
equivalent to the failure of the node to which it is attached.
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