The amount of memory required by the historic granule index is determined by four factors:
- The total number of historic aggregates. Typically, this is the number of points and accumulators that have historic enabled. However, there are also other historic aggregates.
- The number of weeks covered by the historic data in the primary index. This amount is defined by the Index After setting, (see Reduce the Size of the Historic Granule Index).
- The number of weeks covered by the historic data in the secondary index. This amount is equal to the difference between the Keep Online setting and the primary index period (defined by the Index After setting).
- The granule size. This varies between 32-bit and 64-bit servers:
- 68 bytes for 32-bit servers
- 96 bytes for 64-bit servers.
You can determine the number of historic aggregates by using the Server Status Tool:
- Access the ClearSCADA Server Status tool, see Run the Server Status Tool.
- Expand the Historic branch for the required server.
- Select the Historianentry.
- Examine the Total Stream entry for the Raw row. This is the total number of historic items within the Historic aggregate, (see Historic Types and Streams).
NOTE: You can also use the Total Primary File and Total Secondary File entries to calculate the current (rather than maximum) memory usage. Also, the Total Memory Usage entries indicate the amount of memory used by the Historian, including the granule index.
- Use the following calculation to determine the maximum amount of memory required for the historic granule index:
(Total Historic Items × Primary Weeks × Primary Granule Size) + (Total Historic Items × Secondary Weeks × Secondary Granule Size)
Example:
A 64-bit system has 100,000 historic items that are kept online for 5 years. They spend 1 year (52 weeks) in the primary index and 4 years (209 weeks) in the secondary index.
(100000 × 52 × 96) + (100000 × 209 × 8) = 635.5MB
This means up to 635.5MB memory is required, (see Reduce the Size of the Historic Granule Index).
When granules in the secondary index are queried, they are promoted into the primary index. Additional memory is required while the granules remain in the loaded granule cache. The size of the cache and the queries being performed will determine how much additional memory is required .
As time passes, new granules are created. They are added to the primary index and so additional memory is required until the server is next restarted.
All granules in a mounted archive volume are added to the primary index. Additional memory is required until the archive volume is dismounted.