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Uptime is a measure of the time a computer system has been "up" and running. It came into use to describe the opposite of downtime, times when a system was not operational. The uptime and reliability of computer and communications facilities is sometimes measured in nines (similar to the unit of metallic purity). "Five nines" means 99.999% availability, which translates to a total downtime of approximately five minutes and fifteen seconds per year.

It is often used as a measure of computer operating system reliability and stability, in that this time represents the time a computer can be left unattended without crashing, or needing to be rebooted for administrative or maintenance purposes. Long uptime can also indicate negligence as many critical updates require reboot on some operating systems.

The uptime(1) command on Unix systems will show the current time, the uptime, the number of users and load averages for the past 1, 5 and 15 minute intervals.

The Uptime-Project, collected data on uptimes from users until 1 March 2007, and the current record for longest uptime is 11 years, 303 days, 20 hours and 57 minutes on a computer running OpenVMS. Rumours mention in January 2008 that Irish Rail had an OpenVMS machine up for 18 years[1], and was restarted just for Y2K tests. The uptime of a personal computer is sometimes displayed as a badge of honour on an email signature or web site/forum. This was especially true in the Windows 9x days[citation needed], where Windows NT and Windows 2000 users would boast of uptimes of more than 30 days, whereas many real-world Windows 9x installations crashed more often. In more recent times very long uptimes for home users with Windows NT and Windows 2000 machines are less striking because the Windows 9x line has been replaced by the Windows NT-based Windows XP.

Netcraft maintains the uptime records for many thousands of web hosting computers.

Users of Windows XP Professional, Windows Server 2003 and Windows Vista systems can type systeminfo at the Command Prompt to display all system information, including the System Up Time.

From Wikipedia, the free encyclopedia
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For other uses, see Downtime (disambiguation).
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.

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.



souce: wikipedia.org , linfo.org


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