Release 10.1B: Progress Fathom Replication
User Guide
After-image extent sizing on a source database
After you estimate the volume of after-image data your database generates, you can resize the after-image extents for use with Fathom Replication. Progress Software recommends that the size of one after-image extent have enough capacity to store four hours of typical database activity when running Fathom Replication. Additionally, Progress further recommends that the total size of all after image extents have the capacity to store one week of typical database activity. These recommendations are based on the following:
- The total capacity of your source database after-image extents must include additional capacity to allow for some target database downtime.
- Most simple hardware failures can be resolved within a four-hour period. For example, if a drive controller or network card fails, it is a simple operation to have another installed.
- In certain situations, it may be necessary to have your target machine out-of-service for an extended length of time. The total after-image storage allocated must have the capacity to store large numbers of transaction-log records.
There are additional transaction-log records written by the RDBMS when a database is enabled as a source database. These additional transaction-log records require additional after-image space, referred to as replication overhead. In order to calculate the replication overhead, the estimated after-image size information (obtained in the "Calculating current after-image volume" section) must be multiplied by 1.5.
The following examples demonstrate how to calculate the source database after-image extent sizes and total capacity when using fixed after-image extents. The calculated after-image extent size and total after-image capacity are shown in bold at the bottom of each of the tables.
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