A service's availability is the measure of its ability to respond predictably to requests. The availability of a service relies on the health of a number of components, including:

• The network and the server computers that host the service.
  
  
• Components such as network cards, power supplies, and hard disk drives.
  
  

The MOSS application model focuses only on the availability options for the server roles included in a MOSS server farm; it does not address the availability of other components.

Availability is usually expressed as a service's "uptime percentage." The following table correlates the uptime percentage with its implications for service outages.

Downtime Allowed for Standard Uptime Percentages

Acceptable uptime percentage	Downtime per day	Downtime per month	Downtime per year
95%	72 minutes	36 hours	18.26 days
99%	14.40 minutes	7 hours	3.65 days
99.9%	86.40 seconds	43 minutes	8.77 hours
99.99%	8.64 seconds	4 minutes	52.6 minutes
99.999%	0.86 seconds	26 seconds	5.26 minutes

The current MOSS application model addresses the first three levels of availability (95 percent, 99 percent, and 99.9 percent). The last two levels are not addressed in this release.

The level of availability you require affects the number and arrangement of servers and other components of your topology. For example, you can increase your level of availability by adding redundant servers and by changing the configuration of the servers.

To help you determine your general availability requirement, study the following questions.

Situation	Response
Is your availability requirement 99 percent or greater because of contract or other requirements?	Yes or No
If the service becomes unavailable, will employees of your organization be unable to reasonably perform their expected job responsibilities?	Yes or No
If the service becomes unavailable, will business and customer transactions be halted, leading to loss of business and/or customers?	Yes or No

If your answer to all three is yes, then your availability need is greater than 99 percent.

The following elements are available in the High Availability section of the Application page of the MOSS Model Wizard.

Do you require high availability in the Web Front-end (>99%)?

Yes. Capacity Planner will configure your capacity model with redundant front-end Web servers.No. Capacity Planner will not consider availability for your capacity model.

Do you require high availability in the database back end (>99%)

Yes. Capacity Planner will configure your capacity model to have the appropriate availability configuration for SQL Server. If you select this option, you must select the particular type of configuration to use.No. Capacity Planner does not need any further information about availability.

Which high availability solution do you plan to deploy for SQL Server?

SQL Server Failover Clustering (Active-Passive). Capacity Planner does not model hardware for the fail-over server. The additional disk space is shared between the primary and fail-over servers.SQL Server Database Mirroring. Capacity Planner will model additional hardware.SQL Server Log Shipping. Capacity Planner will model additional hardware.

How much disk space (GB) do you need for your content database?

Specify the amount of space you need for SQL Server database storage. You can specify up to 2 terabytes of disk space.Capacity Planner allocates disk space storage for other components as a percentage of the total SQL Server disk space. In addition, the Model Wizard increases storage space slightly to provide for basic logging and indexing features.

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How to Use the MOSS Model Wizard

Reference

Hardware - MOSS
Model Summary - MOSS

Other Resources

Microsoft Office SharePoint Server Model Wizard