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High Availability
High availability (HA) is a characteristic of a system which aims to ensure an agreed level of operational performance, usually uptime, for a higher than normal period. Modernization has resulted in an increased reliance on these systems. For example, hospitals and data centers require high availability of their systems to perform routine daily activities. Availability refers to the ability of the user community to obtain a service or good, access the system, whether to submit new work, update or alter existing work, or collect the results of previous work. If a user cannot access the system, it is – from the user's point of view – ''unavailable''. Generally, the term '' downtime'' is used to refer to periods when a system is unavailable. Principles There are three principles of systems design in reliability engineering which can help achieve high availability. # Elimination of single points of failure. This means adding or building redundancy into the system so tha ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Uptime
Uptime is a measure of system reliability, expressed as the percentage of time a machine, typically a computer, has been working and available. Uptime is the opposite of downtime. It is often used as a measure of computer operating system reliability or 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. Conversely, long uptime may indicate negligence, because some critical updates can require reboots on some platforms. Records In 2005, Novell reported a server with a 6-year uptime. Although that might sound unusual, that is actually common when servers are maintained under an industrial context and host critical applications such as banking systems. Netcraft maintains the uptime records for many thousands of web hosting computers. A server running Novell NetWare has been reported to have been shut down after 16 years of uptime due to a failing hard disk ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Microsecond
A microsecond is a unit of time in the International System of Units (SI) equal to one millionth (0.000001 or 10−6 or ) of a second. Its symbol is μs, sometimes simplified to us when Unicode is not available. A microsecond is equal to 1000 nanoseconds or of a millisecond. Because the next SI prefix is 1000 times larger, measurements of 10−5 and 10−4 seconds are typically expressed as tens or hundreds of microseconds. Examples * 1 microsecond (1 μs) – cycle time for frequency (1 MHz), the inverse unit. This corresponds to radio wavelength 300 m (AM medium wave band), as can be calculated by multiplying 1 μs by the speed of light (approximately ). * 1 microsecond – the length of time of a high-speed, commercial strobe light flash (see air-gap flash). * 1 microsecond – protein folding takes place on the order of microseconds. * 1.8 microseconds – the amount of time subtracted from the Earth's day as a result of the 2011 Japanese earthquake. * 2 ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Nine (purity)
Nines are an informal, logarithmic notation for proportions very near to one, or equivalently percentages very near 100%. Their common uses include grading the purity of materials. Put simply, "nines" are the number of consecutive nines in a percentage such as 99% (two nines) or a decimal fraction such as 0.999 (three nines). The number of nines of a proportion is \mathrm = -\log_(1-x) However, there are different conventions for stating a non-integer number of nines. 99.5% could be expressed as "two nines five" or 2.3 nines, as outlined below. A completely pure material ( = 1) would have an infinite number of nines. Precious metals The exact purity of very fine precious metals such as platinum, gold and silver can be of great interest. Based on the system of millesimal fineness, a metal is said to be ''one nine'' or ''one nine fine'' if it is 900 fine, or 90% pure. A metal that is 990 fine is then described as ''two nines fine'' and one that is 999 fine is described as ''thr ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Floor And Ceiling Functions
In mathematics and computer science, the floor function is the function that takes as input a real number , and gives as output the greatest integer less than or equal to , denoted or . Similarly, the ceiling function maps to the least integer greater than or equal to , denoted or . For example, , , , and . Historically, the floor of has been–and still is–called the integral part or integer part of , often denoted (as well as a variety of other notations). Some authors may define the integral part as if is nonnegative, and otherwise: for example, and . The operation of truncation generalizes this to a specified number of digits: truncation to zero significant digits is the same as the integer part. For an integer, . Notation The ''integral part'' or ''integer part'' of a number ( in the original) was first defined in 1798 by Adrien-Marie Legendre in his proof of the Legendre's formula. Carl Friedrich Gauss introduced the square bracket notation in h ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Unavailability
Unavailability, in mathematical terms, is the probability that an item will not operate correctly at a given time and under specified conditions. It opposes availability. Numerical values associated with the calculation of availability are often awkward, consisting of a series of 9s before reaching any significant numerical information (e.g. 0.9999999654). For this reason, it is more convenient to use the complement measure of availability, namely, unavailability. Expressed mathematically, unavailability is 1 minus the availability. Therefore, a system with availability 0.9999999654 is more concisely described as having an unavailability of 3.46 × 10−8. Calculations using unavailability Often fault trees and reliability block diagrams will use unavailability of the various components in the calculation of the top level failure rates through AND gates or parallel redundant components. Repairable Model Unavailability (Q), using the repairable model, may be expressed mathe ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Mathematical Coincidence
A mathematical coincidence is said to occur when two expressions with no direct relationship show a near-equality which has no apparent theoretical explanation. For example, there is a near-equality close to the round number 1000 between powers of 2 and powers of 10: :2^ = 1024 \approx 1000 = 10^3. Some mathematical coincidences are used in engineering when one expression is taken as an approximation of another. Introduction A mathematical coincidence often involves an integer, and the surprising feature is the fact that a real number arising in some context is considered by some standard as a "close" approximation to a small integer or to a multiple or power of ten, or more generally, to a rational number with a small denominator. Other kinds of mathematical coincidences, such as integers simultaneously satisfying multiple seemingly unrelated criteria or coincidences regarding units of measurement, may also be considered. In the class of those coincidences that are of a ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Service-level Agreement
A service-level agreement (SLA) is a commitment between a service provider and a customer. Particular aspects of the service – quality, availability, responsibilities – are agreed between the service provider and the service user. The most common component of an SLA is that the services should be provided to the customer as agreed upon in the contract. As an example, Internet service providers and telcos will commonly include service level agreements within the terms of their contracts with customers to define the level(s) of service being sold in plain language terms. In this case, the SLA will typically have a technical definition of '' mean time between failures'' (MTBF), '' mean time to repair'' or '' mean time to recovery'' (MTTR); identifying which party is responsible for reporting faults or paying fees; responsibility for various data rates; throughput; jitter; or similar measurable details. Overview A service-level agreement is an agreement between two or ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Mainframes
A mainframe computer, informally called a mainframe or big iron, is a computer used primarily by large organizations for critical applications like bulk data processing for tasks such as censuses, industry and consumer statistics, enterprise resource planning, and large-scale transaction processing. A mainframe computer is large but not as large as a supercomputer and has more processing power than some other classes of computers, such as minicomputers, servers, workstations, and personal computers. Most large-scale computer-system architectures were established in the 1960s, but they continue to evolve. Mainframe computers are often used as servers. The term ''mainframe'' was derived from the large cabinet, called a ''main frame'', that housed the central processing unit and main memory of early computers. Later, the term ''mainframe'' was used to distinguish high-end commercial computers from less powerful machines. Design Modern mainframe design is characterized less ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Voltage Spike
In electrical engineering, spikes are fast, short duration electrical transients in voltage (voltage spikes), current (current spikes), or transferred energy (energy spikes) in an electrical circuit. Fast, short duration electrical transients (overvoltages) in the electric potential of a circuit are typically caused by * Lightning strikes * Power outages * Tripped circuit breakers * Short circuits * Power transitions in other large equipment on the same power line * Malfunctions caused by the power company * Electromagnetic pulses (EMP) with electromagnetic energy distributed typically up to the 100 kHz and 1 MHz frequency range. * Inductive spikes In the design of critical infrastructure and military hardware, one concern is of pulses produced by nuclear explosions, whose nuclear electromagnetic pulses distribute large energies in frequencies from 1 kHz into the gigahertz range through the atmosphere. The effect of a voltage spike is to produce a correspondi ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Brownout (electricity)
A brownout is a drop in voltage in an electrical power supply system. Unintentional brownouts can be caused by excessive electricity demand, severe weather events, or a malfunction or error affecting electrical grid control or monitoring systems. Intentional brownouts are used for load reduction in an emergency, or to prevent a total grid power outage due to high demand. The term ''brownout'' comes from the dimming of incandescent lighting when voltage reduces. In some countries, the term ''brownout'' refers not to a drop in voltage but to an intentional or unintentional power outage (or blackout). Effects Different types of electrical apparatus will react in different ways to a voltage reduction. Some devices will be severely affected, while others may not be affected at all. Resistive loads The heat output of any resistive device, such as an electric space heater, is equal to the power consumption, which is directly proportional to the square of the applied volta ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Power Outage
A power outage (also called a powercut, a power out, a power failure, a power blackout, a power loss, or a blackout) is the loss of the electrical power network supply to an end user. There are many causes of power failures in an electricity network. Examples of these causes include faults at power stations, damage to electric transmission lines, substations or other parts of the distribution system, a short circuit, cascading failure, fuse or circuit breaker operation. Power failures are particularly critical at sites where the environment and public safety are at risk. Institutions such as hospitals, sewage treatment plants, and mines will usually have backup power sources such as standby generators, which will automatically start up when electrical power is lost. Other critical systems, such as telecommunication, are also required to have emergency power. The battery room of a telephone exchange usually has arrays of lead–acid batteries for backup and also ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
List Of Unusual Units Of Measurement
An unusual unit of measurement is a unit of measurement that does not form part of a coherent system of measurement, especially because its exact quantity may not be well known or because it may be an inconvenient multiple or fraction of a base unit. This definition is not exact since it includes units such as the ''week'' or the ''light-year'', which are quite "usual" in the sense that they are often used, but can be "unusual" if taken out of their common context, as demonstrated by the furlong-firkin-fortnight (FFF) system of units. Many of the unusual units of measurements listed here are colloquial measurements, units devised to compare a measurement to common and familiar objects. Length Hammer unit Valve's Source game engine uses the ''Hammer unit'' as its base unit of length. This unit refers to Source's official map creation software, Hammer. The exact definition varies from game to game, but a Hammer unit is usually defined as a sixteenth of a foot (16 H ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
