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ITIL :: View topic - Availability Calculations
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Availability Calculations

 
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kirti
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Joined: Jan 21, 2008
Posts: 3

PostPosted: Tue Feb 05, 2008 9:13 pm    Post subject: Availability Calculations Reply with quote

following is the question on availability

What will be the optimal availability that we can provide to a Customer based upon the following results?

workstation (98%) - link (97%) - router (99%) - WAN (99.95%) - router (98%) - link (97.8%) - server (99.2%)
options are

97%
98%
The product of all percentages
99.95%

I thought the ans to be product of all percentage but its not, its 97%, how do we calculate availablity?
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UKVIKING
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Joined: Sep 16, 2006
Posts: 3294
Location: London, UK

PostPosted: Wed Feb 06, 2008 12:11 am    Post subject: Reply with quote

Why would you give that information to a customer

1 - they will hold it against you

2 - they will hold it against you

3 - they will hold it against you

I dont think you did the number correctly

I did the server * etc acrocss and came up with 89.4319 %

First

Availability is just not raw numbers. It needs to be done in context to the SLA

For example

The working hours for the tool / service you are using is 0800 - 1600 local business time Monday to Friday. The service is expected to be available 95 % of time or .95 of 8 hours per day for a 5 day period.

So that would mean 5 % of 480 minutes which is 24 minutes per day of potential service unavailability or 2 hours per week.

The SLA would determine whether the working hours local includes holidays etc for the local as well as global holidays if the server is not in the region where the user is

That is for the customer

For you as AM and taking your example you need to adjsut you calculation areas a bit

Start with the application supplying the service
Is it on 1 machines (Single point of failure)
or is it spread over 10 machines in a cluster. At what point is the #-
of servers being unavailable have an impact on the application and users

Is the network connections from the server / server farm a SPOF or are there redundant links from the server farm to the network so that users can access the equipment.

Look at how the network is structured - router pairs, switch pairs

So then look at the the users of the service

Can the service be available but users can not get to it ? Yes

The Web site / servers are up 100% availability but MY PC can connect because I have a bad Network card. I have an availability issue not the service

In order to have AM and to have it usefull for you and your customers, Service level management and the definition of the service is requried
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Change Management is POWER & CONTROL. /....evil laughter
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Mark-OLoughlin
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Joined: Oct 12, 2007
Posts: 306
Location: Ireland

PostPosted: Wed Feb 06, 2008 7:26 am    Post subject: Reply with quote

Is this an exam style question or a real world question?
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Mark O'Loughlin
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dboylan
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Joined: Jan 03, 2007
Posts: 189
Location: Redmond, WA

PostPosted: Wed Feb 06, 2008 8:59 am    Post subject: Re: Availability Calculations Reply with quote

kirti wrote:

I thought the ans to be product of all percentage but its not, its 97%, how do we calculate availablity?


Who told you that it is 97% and how did they arrive at that figure.

Using the method described in ITIL of multiplying them all together gives you 89.43% and that is the answer I would stick to.
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Mark-OLoughlin
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Joined: Oct 12, 2007
Posts: 306
Location: Ireland

PostPosted: Wed Feb 06, 2008 9:06 am    Post subject: Reply with quote

One way of looking at it may be that there is a link showing as "link (97%) "

So this is the weakest "link" in the chain provided. This may be the "minimum" availability of this "service" when taken the other %'s into consideration and assuming that this part of the chain has no failover etc. Maybe the question asked meant this instead of optimal.

Just presenting a "possible" answer to the question.
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nasz
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Joined: Jun 18, 2007
Posts: 10
Location: Adelaide, South Australia

PostPosted: Thu Apr 15, 2010 11:13 am    Post subject: Reply with quote

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

A=MTBF/(MTBF+MTTR)

Example 01:
If we are using equipment which has ‘Mean Time Between Failure’ (MTBF) of 81.5 years and ‘Mean Time To Recovery’ (MTTR) of 1 hour:
MTBF in hours = 81.5*365*24=713940
Availability= MTBF/(MTBF+MDT) = 713940/713941 =99.999859%
Unavailability = 0.000141%
Outage due to equipment in hours per year
Unavailability =0.01235 hours per year.

Whereas, availability of an IS service or system (a combination of hardware/software components) is the percentage of time when the service is operational. Availability of an IS service can be obtained by the formula given below:

A=Uptime/(Uptime +Downtime)

Example 02:
If an IS Service has an agreed uptime of 200 hours for a 30 day period (M-F 0800-1800hrs) and was not available for 3 hours during that time the availability of that IS Service is 98.52% for that period.

200/(200 +3)=0.9852 or 98.52%

However, when an IS Service is utilised by several different business units; and each unit has an individual SLA on place with IS Operations, the following formula can provide a means of calculating the end-user availability of a specific group, region or location, where the IS Service is delivered:

(Agreed Hours* No.of Users-Sum of (incident x Duration* % of users affected))/(Agreed Hours* No.of Users)*100

Example 03:
If an IS Service has an agreed uptime of 200 hours for a 30 day period (M-F 0800-1800hrs) and has a user community of 1000 users in four locations (Each location has 250 users) and one location experiences a single outage of 3 hours. Therefore the end-user availability of that IS Service is 99.625% for that period.

(200 *1000-(3 x 25%) )/(200*1000)*100=99.625 or 99.635%

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.

Availability will be shown in service terms, as opposed to component, system or server terms as the business does not see the component level. All they know is that the email service is unavailable for example. This change in perception of unavailability will help drive IS to be more customer focused.
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