The extended trend log contains approximately 50% of the configured storage capacity.
For example, only 250 entries are in the Trend log view even though the ETL is configured for 500 entries.
After the ETL runs long enough to fill up the 500 entry capacity, ONLY 250 entries are shown in a Trend log list view. (see below)
EcoStruxure Building Operation
Building Operation Enterprise Server
An extended trend log (ETL) includes an equal number of internal entries hidden from view; these entries are overhead incurred in the operation of the ETL. For example, each time a transfer takes place, an internal entry is added.
Depending on the specifics of the configuration (i.e., Maximum Transfer Interval), the percentage of internal entries for a given ETL relative to its full size varies.
From EBO 3.2.3, the internal entries of an ETL are hidden, and no information is provided to give any visibility into how much storage is being used by internal entries.
The solution is to increase the size of the ETL to ensure there is enough storage capacity for data and overhead.
Typically, the size must be doubled (in this case, 1000) in the particular ETL to make sure 500 entries of data can be stored.
Although these transfer events are hidden, it is possible to identify these events by adding the Sequence number column in the trend log list. It is also possible to decrease the overhead amount stored in the ETL by increasing the "Maximum transfer interval" in the ETL properties, as shown in the screenshots.
Here it can be seen that by increasing the Maximum transfer interval to 1 hour for an interval log of 5 minutes that ONLY 1 'overhead' record (sequence # 565) is added per 10 entries.
Using this or a similar setting will allow to reduce the storage requirement of the ETL.