Disponibilidad de Datos

El factor de disponibilidad de un equipo o sistema es una medida que nos indica cuanto tiempo está ese equipo o sistema operativo respecto de la duración total durante la que se hubiese deseado que funcionase. Típicamente se expresa con un porcentaje. No debe de ser confundida con la rapidez de respuesta. 

Ejemplo

• Necesitamos que una máquina funcione 12 horas al día, pero ésta falla una hora cada día.
  Para repararla, se necesita además media hora adicional.
  Por lo tanto, su disponibilidad es de (12-1-0.5)h/12h *100% = 87.5 %.

Se pueden realizar estudios más rigurosos para tomar en cuenta el tiempo de llegada de los técnicos, el tiempo de diagnóstico, la logística de las piezas de recambio... etc.

Ejemplo típico de disponibilidad de un "servidor cambalandés"

Availability 

The most simple representation for availability is as a ratio of the expected value of the uptime of a system to the aggregate of the expected values of up and down time, or

Example

If we are using equipment which has mean time between failure (MTBF) of 81.5 years and mean time to recovery (MDTR) of 1 hour:

MTBF in hours = 81.5*365*24=713940

Availability= MTBF/(MTBF+MDTR) = 713940/713941 =99.999859%

Unavailability = 0.000141%

Outage due to equipment in hours per year

U=0.01235 hours per year.

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Availability of the module is the percentage of time when system is operational. Availability of a hardware/software module can be obtained by the formula given below.

Availability is typically specified in nines notation.

For example 3-nines availability corresponds to 99.9% availability.

A 5-nines availability corresponds to 99.999% availability. 
Percentage calculation
Availability is usually expressed as a percentage of uptime in a given year. The following table shows the downtime that will be allowed for a particular percentage of availability, presuming that the system is required to operate continuously. Service level agreements often refer to monthly downtime or availability in order to calculate service credits to match monthly billing cycles. The following table shows the translation from a given availability percentage to the corresponding amount of time a system would be unavailable per year, month, or week

Availability %	Downtime per year	Downtime per month*	Downtime per week
90%	36.5 days	72 hours	16.8 hours
95%	18.25 days	36 hours	8.4 hours
98%	7.30 days	14.4 hours	3.36 hours
99%	3.65 days	7.20 hours	1.68 hours
99.5%	1.83 days	3.60 hours	50.4 minutes
99.8%	17.52 hours	86.23 minutes	20.16 minutes
99.9% ("three nines")	8.76 hours	43.2 minutes	10.1 minutes
99.95%	4.38 hours	21.56 minutes	5.04 minutes
99.99% ("four nines")	52.6 minutes	4.32 minutes	1.01 minutes
99.999% ("five nines")	5.26 minutes	25.9 seconds	6.05 seconds
99.9999% ("six nines")	31.5 seconds	2.59 seconds	0.605 seconds

* For monthly calculations, a 30-day month is used.
It should be noted that uptime and availability are not synonymous. A system can be up, but not available, as in the case of a network outage.

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The availability of a repairable system is defined as "the probability that the system is operating at a specified time t."  --Barlow and Proschan [1975]

The qualitative definition of availability is defined as "a measure of the degree of a system which is in the operable and committable state at the start of mission when the mission is called for at an unknown random point in time."  --Blanchard [1998] & MIL-STD-721

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High availability 
Reliability and Availability Basics