CompTIA Security_ Deluxe Study Guide_ SY0-201 - Emmett Dulaney [206]
Most ISPs and network providers have extensive internal fail-over capability to provide high availability to clients. Business clients and employees who are unable to access information or services tend to lose confidence. The trade-off for reliability and trustworthiness, of course, is cost: Fail-over systems can become prohibitively expensive. You’ll need to carefully study your needs to determine whether your system requires this capability.
For example, if your environment requires a high level of availability, your servers should be clustered. This will allow the other servers in this network to take up the load if one of the servers in the cluster fails.
FIGURE 8.1 Server clustering in a networked environment
Fault Tolerance
Fault tolerance is primarily the ability of a system to sustain operations in the event of a component failure. Fault-tolerant systems can continue operation even though a critical component, such as a disk drive, has failed. This capability involves over-engineering systems by adding redundant components and subsystems.
Fault tolerance can be built into a server by adding a second power supply, a second CPU, and other key components. Several manufacturers (such as HP, Unisys, and IBM) offer fault-tolerant servers; these servers typically have multiple processors that automatically fail over if a malfunction occurs.
In addition to fault-tolerant servers, you can have fault-tolerant implementations such as Tandem, Stratus, and HP. In these settings, multiple computers are used to provide the 100 percent availability of a single server.
There are two key components of fault tolerance you should never overlook: spare parts and electrical power. Spare parts should always be readily available to repair any system-critical component if it should fail. The redundancy strategy N+1 means that you have the number of components you need, plus one to plug into any system should it be needed. For example, a small company with five standalone servers that are all the same model should have a power supply in a box nearby to install in any one of the servers should there be a failure. (The redundancy strategy 1+1 has one spare part for every component in use.)
Since computer systems cannot operate in the absence of electrical power, it is imperative that fault tolerance be built into your electrical infrastructure as well. At a bare minimum, an uninterruptible power supply (UPS)—with surge protection—should accompany every server and workstation. That UPS should be rated for the load it is expected to carry in the event of a power failure (factoring in the computer, monitor, and any other device connected to it) and be checked periodically as part of your preventative maintenance routine to make sure the battery is operational. You will need to replace the battery every few years to keep the UPS operational.
A UPS will allow you to continue to function in the absence of power for only a short duration. For fault tolerance in situations of longer duration, you will need a backup generator. Backup generators run off of gasoline or diesel and generate the electricity needed to provide steady power. While some backup generators can come on instantly in the event of a power outage, most take a short time to warm up before they can provide consistent power, and thus you will find that you still need to implement UPSs within your organization.
Redundant Array of Independent Disks
Redundant Array of Independent Disks (RAID) is a technology that uses multiple disks to provide fault tolerance. There are several designations for RAID levels.
RAID stands for not only Redundant Array of Independent Disks, but also Redundant Array of Inexpensive Disks. While the latter term has lost its popularity, you might still encounter it in some texts.
The most commonly implemented