| Can you see what's coming next? What happens if | | | | capabilities in the software, usually the operating |
| the controller fails? Ha! - we're ready for that! We put | | | | system.The software is set up to take note of errors |
| in another controller, and connect the second string of | | | | detected by the hardware, and automatically 'mark |
| cables to this one instead! Redundant controllers.Other | | | | down' a specific component, or access path, |
| configurations are also possible, using both interfaces | | | | depending on a preset 'threshold' of error tolerance. At |
| on both controllers, however these are mainly | | | | the same time, the system operations and |
| concerned with system throughput, or the ability to | | | | maintenance personnel are alerted, so that the |
| switch drives between two or more systems. The | | | | perceived problem can be rectified.The provision of |
| extra redundancy provided in these cases is more of | | | | redundancy in a system gives the maintenance |
| a bonus than a necessity.Moving further up the data | | | | engineer the ability to work on the failing component or |
| chain, we need a path between the controller and the I | | | | path while the system continues operation. We can |
| O (input/output) section of the mainframe. By now you | | | | also test multiple paths to a component to eliminate |
| will see that we will use two paths to provide | | | | possible causes of a failure which, we have seen, |
| redundancy. You can also work out that, if the I/O unit | | | | could lie anywhere in the chain from the drive back to |
| fails, we have problems, not only in talking to our disk | | | | the I/O unit.A further redundancy commonly built in to |
| drive but to tapes, printers, datacomms etc.This | | | | large systems is the power supply. The system power |
| possibility of any one component in the system being | | | | is provided by a generator, driven by a motor, which is |
| able to affect the whole system, or a significant | | | | driven from mains power. In the event of a power |
| section of it, is known as 'Single Point Sensitivity'. A | | | | supply failure, a diesel engine takes over driving the |
| Single Point of Failure is one which has the potential to | | | | generator to ensure continuous power. The |
| affect operations to some extent.The way to avoid | | | | change-over period is covered by batteries, which are |
| this, of course, is to duplicate everything, CPUs, I/Os, | | | | kept charged during normal operation. In another |
| Controls etc. The most difficult component to duplicate | | | | version, the batteries drive the generator and isolate |
| is memory, as this is the basic part of the system, | | | | the system from power surges.The complete system |
| where everything is controlled.Multi processors have | | | | can be duplicated in this way, with more resources |
| been in operation since the 1960s and dual paths have | | | | being placed in susceptible areas, such as disks and |
| been used to access subsystems. In the case of | | | | tapes. The cost of additional components is weighed |
| larger systems, for example a 6x4 (6 CPUs, 4I/Os) for | | | | against the requirement for continuous operation, and |
| paths were provided to disk and tape controllers to | | | | what effect a catastrophic failure might have. The |
| increase throughput and allow 2 or more operating | | | | ultimate redundancy uses a completely separate |
| systems to run on selected components. Units can be | | | | duplicate system located at a remote area, possibly |
| combined or removed 'on the fly' and systems carry | | | | hundreds of miles distant (of course, with duplicated |
| on working with more or less resources.We have | | | | links!)Tony is an experienced computer engineer. He is |
| talked about redundancy in hardware, or the physical | | | | currently webmaster and contributor to . A set of |
| components of the system, by providing alternative | | | | diagrams accompanying these articles may be seen |
| hardware. There is also the provision of redundant | | | | at . |