Downtime
From Wikipedia, the free encyclopedia
Downtime or outage refers to a period of time or a percentage of a timespan that a system is unavailable or offline. This is usually a result of the system failing to function because of an unplanned event, or because of routine maintenance.
The term is commonly applied to networks and servers. The common reasons for unplanned outages are system failures (such as a crash) or communications failures (commonly known as network outage).
The opposite of downtime is uptime.
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[edit] Characteristics
Unplanned downtime may be the result of a software bug, human error, equipment failure, malfunction, high bit error rate, power failure, overload due to exceeding the channel capacity, a cascading failure, etc.
See also: Planned downtime
[edit] Impact
Outages caused by system failures can have a serious impact on the users of computer/network systems, in particular those industries that rely on a nearly 24-hour service:
- medical informatics
- nuclear power and other infrastructure
- banks and other financial institutions
- aeronautics, airlines
- news reporting
- e-commerce and online transaction processing
- persistent online games
Also affected can be the users of an ISP and other customers of a telecommunication network.
Corporations can lose business due to network outage or they may default on a contract, resulting in financial losses.
Those people or organizations that are affected by downtime can be more sensitive to particular aspects:
- some are more affected by the length of an outage - it matters to them how much time it takes to recover from a problem
- others are sensitive to the timing of an outage - outages during peak hours affect them the most
The most demanding users are those that require high availability.
[edit] Cost
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| Business | Average US$ cost per hour due to network outage |
|---|---|
| Brokerage Operations | $5.45 Million |
| Credit Card Authorisations | $2.5 Million |
| ATM Fees | $14,000 |
| Tele Ticket Sales | $69,000 |
[edit] Famous outages
AT&T lost its frame relay network for 26 hours on April 13 1998[1]. This affected many thousands of customers, and bank transactions were one casualty. AT&T failed to meet the service level agreement on their contracts with customers and had to refund[2] 6600 customer accounts, costing millions of dollars.
[edit] Service levels
In service level agreements, it is common to mention a percentage value (per month or per year) that is calculated by dividing the sum of all downtimes timespans by the total time of a reference time span (e.g. a month). 0% downtime means that the server was available all the time.
For Internet servers downtimes above 1% per year or worse can be regarded as unacceptable as this means a downtime of more than 3 days per year. For e-commerce and other industrial use any value above 0.1% is usually considered unacceptable.
[edit] Response and reduction of impact
It is the duty of the network designer to make sure that a network outage does not happen. When it does happen, a well-designed system will further reduce the effects of an outage by having localized outages which can be detected and fixed as soon as possible.
A process needs to be in place to detect a malfunction - network monitoring - and to restore the network to a working condition - this generally involves a help desk team that can troubleshoot a problem, one composed of trained engineers; a separate help desk team is usually necessary in order to field user input, which can be particularly demanding during a downtime.
A network management system can be used to detect faulty or degrading components prior to customer complaints, with proactive fault rectification.
Risk management techniques can be used to determine the impact of network outages on an organisation and what actions may be required to minimise risk. Risk may be minimised by using reliable components, by performing maintenance, such as upgrades, by using redundant systems or by having a contingency plan or business continuity plan. Technical means can reduce errors with error correcting codes, retransmission, checksums, or diversity scheme.
[edit] Planning
A planned outage is the result of a planned activity by the system owner and/or by a service provider. Such activities can include changes or upgrades, and they are often scheduled as maintenance windows.
Outages can also be planned as a result of a predictable natural event, such as Sun outage.
Maintenance downtimes have to be carefully scheduled in industries that rely on computer systems. In many cases, system-wide downtimes can be averted using what is called a "rolling upgrade" - the process of incrementally taking down parts of the system for upgrade, without affecting the overall functionality.
[edit] Other usage
Downtime can also refer to time when human capital or other assets go down. For instance, if employees are in meetings or unable to perform their work due to another constraint, they are down. This can be equally expensive, and can be the result of another asset (i.e. computer/systems) being down. This is also commonly known as "dead time".
[edit] See also
- Myth of the nines for a discussion of downtime and availability
- High availability
- Uptime
- mean down time
