Cluster Advantages And Disadvantages - Oracle ZFS Storage Appliance Administration Manual

Cluster Advantages and Disadvantages

It is important to understand the scope of the Oracle ZFS Storage Appliance clustering
implementation. The term 'cluster' is used in the industry to refer to many different technologies
with a variety of purposes. We use it here to mean a metasystem comprised of two appliance
controllers and shared storage, used to provide improved availability in the case in which
one of the controllers succumbs to certain hardware or software failures. A cluster contains
exactly two appliances or storage controllers, referred to for brevity throughout this document
as controllers. Each controller may be assigned a collection of storage, networking, and other
resources from the set available to the cluster, which allows the construction of either of two
major topologies. Many people use the terms active-active to describe a cluster in which
there are two (or more) storage pools, one of which is assigned to each controller along with
network resources used by clients to reach the data stored in that pool, and active-passive to
refer to which a single storage pool is assigned to the controller designated as active along
with its associated network interfaces. Both topologies are supported by the Oracle ZFS
Storage Appliance. The distinction between these is artificial; there is no software or hardware
difference between them and one can switch at will simply by adding or destroying a storage
pool. In both cases, if a controller fails, the other (its peer) will take control of all known
resources and provide the services associated with those resources.
As an alternative to incurring hours or days of downtime while the controller is repaired,
clustering allows a peer appliance to provide service while repair or replacement is performed.
In addition, clusters support rolling upgrade of software, which can reduce the business
disruption associated with migrating to newer software. Some clustering technologies have
certain additional capabilities beyond availability enhancement; the Oracle ZFS Storage
Appliance clustering subsystem was not designed to provide these. In particular, it does not
provide for load balancing among multiple controllers, improve availability in the face of
storage failure, offer clients a unified filesystem namespace across multiple appliances, or
divide service responsibility across a wide geographic area for disaster recovery purposes.
These functions are likewise outside the scope of this document; however, the Oracle ZFS
Storage Appliance and the data protocols it offers support numerous other features and
strategies that can improve availability:
Replication of data, which can be used for disaster recovery at one or more geographically
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remote sites
Client-side mirroring of data, which can be done using redundant iSCSI LUNs provided by
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multiple arbitrarily located storage servers
Load balancing, which is built into the NFS protocol and can be provided for some other
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protocols by external hardware or software (applies to read-only data)
Redundant hardware components including power supplies, network devices, and storage
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controllers
Fault management software that can identify failed components, remove them from service,
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and guide technicians to repair or replace the correct hardware
Shutting Down a Clustered Configuration (CLI)
Configuring the Appliance 203