Boundaries, thresholds and supported limits
In SharePoint Server 2010, there are certain limits that are by design and cannot be exceeded, and other limits that are set to default values that may be changed by the farm administrator. There are also certain limits that are not represented by a configurable value, such as the number of site collections per Web application.
- Boundaries are absolute limits that cannot be exceeded by design. It is important to understand these limits to ensure that you do not make incorrect assumptions when you design your farm.An example of a boundary is the 2 GB document size limit; you cannot configure SharePoint Server to store documents that are larger than 2 GB. This is a built-in absolute value, and cannot be exceeded by design.
- Thresholds are those that have a default value that cannot be exceeded unless the value is modified. Thresholds can, in certain circumstances, be exceeded to accommodate variances in your farm design, but it is important to understand that doing this may affect the performance of the farm in addition to the effective value of other limits.The default value of certain thresholds can only be exceeded up to an absolute maximum value. A good example is the document size limit. By default, the default document size threshold is set to 50MB, but can be changed to support the maximum boundary of 2GB.
- Supported limits define the tested value for a given parameter. The default values for these limits were defined by testing, and represent the known limitations of the product. Exceeding supported limits may cause unexpected results, significant decrease in performance, or other harmful effects.Some supported limits are configurable parameters that are set by default to the recommended value, while other supported limits relate to parameters that are not represented by a configurable value.
An example of a supported limit is the number of site collections per Web application. The supported limit is 250,000, which is the largest number of site collections per Web application that met performance benchmarks during testing.
It is important to be aware that many of the limit values that are provided in this document represent a point in a curve that describes an increasing resource load and concomitant decrease in performance as the value increases. Therefore, exceeding certain limits, such as the number of site collections per Web application, may only result in a fractional decrease in farm performance. However, in most cases, operating at or near an established limit is not a best practice, as acceptable performance and reliability targets are best achieved when a farm’s design provides for a reasonable balance of limits values.
Thresholds and supported limits guidelines are determined by performance. In other words, you can exceed the default values of the limits, but as you increase the limit value, farm performance and the effective value of other limits may be affected. Many limits in SharePoint Server can be changed, but it is important to understand how changing a given limit affects other parts of the farm.
How limits are established
In SharePoint Server 2010, thresholds and supported limits are established through testing and observation of farm behavior under increasing loads up to the point where farm services and operations reach their effective operational limits. Some farm services and components can support a higher load than others so that in some cases you must assign a limit value based on an average of several factors.
For example, observations of farm behavior under load when site collections are added indicate that certain features exhibit unacceptably high latency while other features are still operating within acceptable parameters. Therefore, the maximum value assigned to the number of site collections is not absolute, but is calculated based on an expected set of usage characteristics in which overall farm performance would be acceptable at the given limit under most circumstances.
Obviously, if some services are operating under parameters that are higher than those used for limits testing, the maximum effective limits of other services will be reduced. It is therefore important to execute rigorous capacity management and scale testing exercises for specific deployments in order to establish effective limits for that environment.
Note: We do not describe the hardware that was used to validate the limits in this document, because the limits were collected from multiple farms and environments. For descriptions of the farms we used in testing, see?Performance and capacity test results and recommendations (SharePoint Server 2010) and?Performance and capacity technical case studies (SharePoint Server 2010).
The Equalizer Metaphor
You can consider thresholds and supported limits as sliders on a graphic equalizer, with each limit representing a certain frequency. In this metaphor, increasing the value of one limit may decrease the effective value of one or more other limits.
Imagine that one slider represents the maximum number of documents per library, a supported limit with a maximum tested value of around 30 million. However, this value depends on another slider, which represents the maximum size of documents in the farm, a threshold with a default value of 50 MB.
If you change the maximum size of documents to 1 GB to accommodate videos or other large objects, the number of documents your library can serve to users efficiently is reduced accordingly. For example, a given farm’s hardware configuration and topology may support 1 million documents up to 50 MB. However, the same farm with the same number of documents cannot meet the same latency and throughput targets if the farm is serving a larger average document size because the file size limit was set to 1 GB.
The degree to which the maximum number of documents is reduced in this example is difficult to predict and is based on the number of large files in the library, the volume of data that they contain, the farm’s usage characteristics, and the availability of hardware resources.
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