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The fundamental requirement when designing a cluster is that there be at least two servers. The servers must be the same platform (that is, the same processor type, and ideally the same model to reduce complexity), and should be running the same operating system release with the same patch releases. The servers should offer similar performance, via identical memory and processor speed. Ideally, the servers in the failover pair are completely identical. Using identical servers will help reduce the cluster s complexity, conferring all the benefits of simple systems ease of management, brief learning curve, fewer parts to break, and so forth. While it is not an absolute requirement, it is generally preferable if clustered servers have similar performance parameters. If one server in a cluster, ozzie, is noticeably faster than its partner, harriet, then after ozzie fails over to harriet, users may complain about the decrease in performance. This may lead the administrator to manually switch operations back to ozzie before the problem that caused ozzie to fail in the first place is completely fixed, or at a time when the brief outage might inconvenience the user community. By configuring the systems with identical performance parameters, you can increase availability simply by reducing the number of times that a failover must take place.
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Some vendors release variations of their CPUs that have subtle differences. Each generation may have unique instructions that allow for different optimizations in applications, or may require operating system support in terms of a kernel architecture. Sun s earlier SPARC chip families, for example, had several kernel architectures, requiring minor variations in the SunOS or Solaris operating system. When a processor supports special memory access modes or fast block copying, and an application such as a database takes advantage of them, you must be certain you have identical processors in each server or you ll need differently optimized versions of the database on each node. While these differences are hard to detect, there are no guarantees that these differences won t, at some point in the future, make two machines incompatible in the same failover pair. This unlikely incompatibility becomes a moot point if the hardware is all the same. In the Microsoft world, there may be similarly subtle differences between the various Intel architecture CPUs that run Windows operating systems. Processors made by Intel and AMD are supposed to be perfectly compatible, but it is not hard to imagine that there could be minor differences that won t appear for years after their initial implementation. Not all Windows NT systems are binary-compatible: HP/Compaq/DEC s old Alpha chip could not run the same code that runs on an Intel Pentium processor, for example. The same vendor may have several chip lines running different operating systems,
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Local Clustering and Failover
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or the same operating system on multiple chips. The matched set of operating system and processor must be consistent if you want to be sure you re getting a consistent applications platform on all nodes in your cluster. Vendors may have dozens of different system models. As with patch releases, it is unlikely that any vendor has tested every single combination of systems that can possibly be built into a failover pair. Never be the first to do anything when production machines are involved. Stick with simple configurations; use combinations that are well known and well tested.
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It is amazing how often system administrators suggest creating clusters composed of incompatible systems, such as a Windows server and an HP-UX server. Why The argument goes, Since we already have both systems, why can t we connect them together and fail back and forth There is an apparent cost savings if you can combine two systems that are already on hand. This is, unfortunately, a surefire recipe for failure. In order to successfully failover between incompatible hardware, a number of issues would need to be addressed: Failover software. Both systems would need to be able to run compatible versions of the failover management system that could communicate with each other, and we are not aware of any examples of commercial failover software that work across multiple OSs, even under the best of conditions. The FMS would have to handle a multitude of combinations of hardware. Each server type has many variations (kernel architectures, as described previously, are just one example), and when you add another whole class of server, the permutations increase dramatically. Applications and data formats. The application would need to function identically on both servers, despite differences in hardware architecture. It would also need to read data written by the other server, on a potentially incompatible filesystem, or in an incompatible format or style (word size, big-endian versus little-endian, and so forth). An application vendor who is willing to give a discount for an extra failover server license may be less willing to do so when the failover server is from a different vendor, assuming that their application even runs on both platforms. Network interfaces. The heartbeat networks become more complicated when the NICs on each server are of drastically different types. If the network technologies are incompatible, then a network bridge or other hardware may be required that adds complexity and potential failure points. Disks. In addition to all the other disk requirements, the shared disks must also be physically compatible with both servers. While interface cards don t need to be the same for both servers, they must be able to operate in a dual-hosted environment, with the other, probably
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