Service Level Agreement (SLA) is a commonly misused notion in server hosting and colocation business. Our article presents a down-to-earth view of this important aspect.
Most of the server hosting clients require SLA (Service Level Agreement) prior to signing a colocation contract.
SLA for Server Hosting
SLA specifies in % the total uptime / downtime of a server availability through the provider´s network on one side, along with provider´s penalization due to failing in network availability. The higher the SLA the higher the price of such service. No surprise if a 0.5% higher SLA resulting in double the price.
Most of the server colocation customers are satisfied with a SLA of some 99 point something per cent. Lets look at a typical server colocation SLA: „..our common Service Level Agreement covers 99.0 % availability, upgradable to 99.6 or up to 99.96%“.
Working few seconds with a calculator is enough to see how these high availability numbers fade away in reality. With 99.0% availability tech provider is allowed for a 7hrs 12min outage in a months. With SLA of 99.6% it is slightly better, but still not many customers will be satisfied with 2hrs 52min downtime per month. And even the „flagship“ SLA of 99.96% still permits the provider for 18min down time per months without any penalty.
It is necessary to say that there is no provider of absolutely zero down time. To look properly after the network some planned outages are just unavidable. These instances (network maintainance, router installation etc.) are normaly not covered by SLA, under the provision of advance warnings and carefull planning to minimize impact on customer servers by shortening the outage and planning for low traffic periods.
Just to give a concrete example: at the Coolhousing server hosting data center during the last 12 months (June 2007 – 2008) there was none of network outage except of 3 maintainance instances of a total of 40 minutes. This represents a 100% availability, or 99,9999761% availability when including the planned service outages.
Author: Jirka Dvořák