Colocation and Redundancy

October 7, 2012 by  Filed under: Web Hosting 

Colocation is a form of web hosting that is very popular with businesses and those with more demanding hosting needs. Colocated hosting is where space in a data centre is rented out; individuals and companies can then use this space to store their web servers. The data centre provides the bandwidth and the power that the server needs and, if you choose a managed plan, can monitor and maintain the server for you. What many prospective hosting customers look for when they are trying to choose a colocation provider is a high level of uptime. Customers do not want their websites to crash or be inaccessible as this could mean a loss of business or income. This means, therefore, that most colocation centres will look for ways to ensure that their systems remain operational almost 24/7. This is where the concept of ‘redundancy’ comes in.

In data centres redundancy is the practice of duplicating crucial components or parts of the system. This is done in order to improve the overall reliability of the system and to protect against failure. The idea is that if one component fails, breaks or needs servicing, another component can immediately step in to take up the slack. Redundancy in colocation uses what is called ‘N+1 redundancy’ or ‘parallel redundancy’; this means that any particular part of the system has the required components, power or capacity plus one additional unit. If you have two units, they should both be running at 50% capacity or power so that if one fails the other can easily take over. If you have three units they should be running at 66% power or capacity and so on.

A good way of understanding this is to use suspension bridges as a real life example. Suspension bridges are supported by a large number of cables, each of these cables have extra strength that is not being utilised in most circumstances. However, if a cable was to break, the extra strength in the other cables would mean that the bridge could still be supported – the bridge would not collapse if one cable broke. Data centres will have at least one extra component or unit in order to use redundancy, however spreading the power, capacity or function across three units is generally advised. There are a number of different ways that colocation facilities utilise redundancy.

Power failures and power surges can be damaging in colocation as they can cause loss of data and connectivity and can stop the cooling systems from working. In some cases they can even damage the servers. Therefore, in colocation centres it is highly important that redundancy is used to avoid power failures and protect against surges. Many colocation centres are connected to at least two independent power supplies; some are connected to as many as seven. This means that if there is a power outage or failure on one power line, the others can pick up the slack. In the unlikely event that all the power lines fail, colocation centres have ‘uninterruptible power supply’ units installed. These units, or UPSs, can temporarily power the centre until the power can be brought back on-line or until the generators can start-up. UPSs will also utilise redundancy so that if there is a power cut and one of the UPS units fails, the colocation centre will still be able to operate. As another form of power backup, generators will also use N+1 redundancy.

Ensuring that internet connectivity is not lost is another important facet of colocation and redundancy. Like with power supplies, a colocation centre will not just use one telecommunications provider or one fibre optic cable for their internet connection, They will utilise a number of providers and redundant connections in case of any problems. Using redundancy in connections to the internet will also provide the centre with low levels of latency. Another way that redundancy can be used is to have the servers connected to the network both through a wired connection and wirelessly. This means if one method of connection encounters problems, internet connectivity is not lost. It is crucial that colocation centres try to maintain connectivity at all times as losing connections to the internet can also lose them customers.

As colocation centres are basically just storage areas filled with machines it can easily get very hot. Servers in particular also need to be kept at specific temperatures in order to function at their best. This means that the cooling systems in place in colocation centres are very important and must be kept on-line as much as possible. The majority of centres will use Computer Room Air Conditioner units (or CRAC) in order to keep cool. The CRAC units will be set up, like the power supply and internet connection, in an N+1 formation. But it is not just the units themselves that will use redundancy – the pipes supporting the units will have multiple backup sets as will the ‘chillers’ that support the CRAC units.

Redundancy, then, is hugely important in colocation. When deciding between providers it can be helpful to look at whether the centre utilises redundancy and if it does whether it covers internet connectivity, cooling and power. Many colocation providers will claim extremely high levels of uptime, but if they do not use redundancy then they cannot follow through on their claims.

© Izzy Evans 2012

If you would like to find out more about colocated hosting then you can visit Colocation.

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