An Overview of Redundancy

Redundancy is the ability of a system to distribute the load from a failed system across the remaining pieces of equipment without any performance issues or overload. One major issue with colocation facilities is there is no industry standard, regulations or rankings. As a result, it is often difficult to detect the quality of the redundancy systems.

Redundancy Calculation

Redundancy can be calculated with the formula n+1 which indicates that a system is meeting its optimal capacity plus one unit. For instance, if two UPSs are in the overall power system, each should be running at 50 percent to balance the load. If one fails, the other system can maintain the load. If there are three UPSs then each manages 33 percent of the load and if one fails, they each handle half of the third.

In an ideal situation at a colocation facility, systems will be paralyzing redundant with many units running at the n+1 setting with each unit being able to manage the remaining load if all other systems fail.

Powering Redundant Servers

The most important aspect of redundant servers is power. Without it, the equipment will not work thus creating downtime. Power outages can stop the cooling systems (potentially resulting in equipment damage), fire suppressant systems and Internet connections which can be disastrous for the customer.

How to Maintain Redundancy

To maintain redundancy at the server level it is important to keep the colocation facility powered with generators in case of an outage, ensure the generators can handle a large power load and inspect circuit breakers and switches regularly. Switches tend to be the most common point of failure.

The basic premise is that colocation facilities should have backups of everything; backup and redundant servers, backup generators, backup of the backup generators, backup cooling and backup Internet connections. Although it may seem excessive, the clients that rely on these facilities to operate their business expect the best and close to 100 percent uptime. Maintain redundancy at a colocation facility takes a great deal of work to be secure.

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Availability

Often referred to as uptime, the availability of a system describes its accessibility and ability to provide a level of redundancy that works to prevent failure at any single point.  For instance, if only one server is responding to HTTP requests, the server qualifies as a single point of failure.  If there are two web servers in a load balancing scheme, then the load balancer itself is the single point of failure.  Most high availability systems are equipped with efficient fault-tolerance mechanisms to ensure the necessary redundancies.  Because 100% availability is costly and rather difficult to achieve, the industry standard in the web hosting business is at around 99.9%.

Load Balancing

Load balancing is a method web hosting providers employ to more evenly distribute the system load over a group of available servers.  This technology is used to ensure that all servers process their fair share of requests without being overworked and more susceptible to failing.  There are many types of load balancers and these mechanisms come in both software and hardware varieties.

Response time and Transaction Load

The performance of a server is associated with the response time of a given transaction load.  In regard to a website, it is measured by the time it takes to load web pages and process requests to the web server.  The performance of large, high traffic websites can be greatly enhanced by upgrading hardware, residing on a dedicated server or utilizing load balancing technology.  In many cases, fine tuning software packages such as MySQL and PHP can also increase response times.

Scalability

The scalability of a system generally refers to the property that enables it to service any given load by upgrading hardware or incorporating additional services in a predetermined manner.  It essentially describes an instance in which performance and throughput can still be maintained once the load increases.  Scalability is a great quality to have in a web hosting solution as it can support the growth and increasing needs of your website.

Session State

Session state is often needed to maintain the connection of a client program and a given server.  This typically needs to be done to accommodate a user who is logged into the system.  If the topology consists of multiple servers, maintaining session state is crucial and something that must be taken care.  Most load balancing software applications on the market provide facilities that enable this to done.  The most common methods are implementing cookies and rewriting URLs with the server details included.  load_balance.htm

Throughput

Though closely tied to performance, throughput is best described as the number of concurrent transactions the server is able to process.  The higher the throughput, the more processing power to the system.

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