Cluster Disadvantages

When deciding between a clustered and standalone Oracle ZFS Storage Appliance configuration, it is important to weigh the costs and benefits of clustered operation. It is common practice throughout the IT industry to view clustering as an automatic architectural decision, but this thinking reflects an idealized view of clustering's risks and rewards promulgated by some vendors in this space. In addition to the obvious higher up-front and ongoing hardware and support costs associated with the second head, clustering also imposes additional technical and operational risks. Some of these risks can be mitigated by ensuring that all personnel are thoroughly trained in cluster operations; others are intrinsic to the concept of clustered operation. Such risks include:

• The potential for application intolerance of protocol-dependent behaviors during takeover,

• The possibility that the cluster software itself will fail or induce a failure in another subsystem that would not have occurred in standalone operation,

• Increased management complexity and a higher likelihood of operator error when performing management tasks,

• The possibility that multiple failures or a severe operator error will induce data loss or corruption that would not have occurred in a standalone configuration, and

• Increased difficulty of recovering from unanticipated software and/or hardware states.

These costs and risks are fundamental, apply in one form or another to all clustered or cluster-capable products on the market (including the Oracle ZFS Storage Appliance), and cannot be entirely eliminated or mitigated. Storage architects must weigh them against the primary benefit of clustering: the opportunity to reduce periods of unavailability from hours or days to minutes or less in the rare event of catastrophic hardware or software failure. Whether that cost/benefit analysis will favor the use of clustering in an Oracle ZFS Storage Appliance deployment will depend on local factors such as SLA terms, available support personnel and their qualifications, budget constraints, the perceived likelihood of various possible failures, and the appropriateness of alternative strategies for enhancing availability. These factors are highly site-, application-, and business-dependent and must be assessed on a case-by-case basis. Understanding the material in the rest of this section will help you make appropriate choices during the design and implementation of your unified storage infrastructure.