All trend logs use circular storing. How many records a trend log can store before overwriting old ones depends on the trend log configuration.
For example, a trend log is configured to store 10 records. The logging starts and the first record x1 is stored. After 10 records the trend log is full. The next record x11 overwrites the oldest record which in our case is x1. The next logging x12 overwrites the oldest record x2, and so on.
Additional internal records can be generated by the log itself. These records may not be visible. Internal records are a part of the log usage.
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Figure:
Circular storing, schematic