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[rrd4j] Updated readme (#8822) 

Changed some wordings.

-Kept "DataSource" as the term for the way data is stored in a .rrd file. Such is more inline with the used Default Datasources (-numeric, ...).
-Removed the requirement for an everyMinute Strategy, inserted a explanation for relationship between Sample Intervall and Strategy instead.

Signed-off-by: Jürgen Baginski <opus42@gmx.de>
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Jürgen Baginski 2020-10-30 23:26:21 +01:00 committed by GitHub
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bundles/org.openhab.persistence.rrd4j/README.md
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@ -36,7 +36,7 @@ Once the rightmost box in the drawer is full, the leftmost box is emptied, the c
## Datasources
For every persisted Item, a separate database file is created in the `userdata/persistence/rrd4j` folder.
These database files are called datasources, which contain the archives of different granularities.
These database files contain the archives of different granularities.
By default, if `services/rrd4j.cfg` does not exist, or if an Item is not explicitly listed in a `<dsName>.items` property value in it, then the respective Item will be persisted according to the [default datasource settings](#default-datasource).
@ -83,7 +83,7 @@ It must be one of the following string values:
### `<heartBeat>` (Heart Beat)
The heartbeat parameter helps the database to detect missing values.
i.e. if no new value is stored after "heartBeat" seconds, the value is considered missing when charting.
i.e. if no new sample is stored after "heartBeat" seconds, the value is considered missing when charting.
It must be a positive integer value.
@ -101,6 +101,9 @@ The time interval (seconds) between reading consecutive samples from the OpenHAB
It must be a positive integer value.
Relation between the sample intervall and openHABs persistence strategy.
The persistence strategy determines on which events (time, change, startUp) a new value is provided to the persistence service, the sample intervall determines at which time steps the actually provided value is being read (i.e an everyMinute strategy provides a new value at every full minute, a sample intervall of 10 seconds would use such a provided value 6 times).
### `<consolidationFunction>` (Consolidation Function)
Determines the type of data compression to be used when more than one sample is to be stored in a single "storage box".
@ -165,7 +168,7 @@ So it covers `144 X 10 X 60` seconds of data (24 hours) at a granularity of ten
The service automatically creates three default datasources with the properties below.
There is no `.items` parameter for the default datasources.
This means that any Item with an allocated strategy in the `rrd4j.persist` file is persisted using the default settingswith the only exception if the Item is explicitly listed in an `.items` property value of a datasource in the `rrd4j.cfg` file.
This means that any Item with an allocated strategy in the `rrd4j.persist` file is persisted using one the default settings, unless the Item is explicitly listed in an `.items` property value of a datasource in the `rrd4j.cfg` file.
#### default_numeric
@ -255,8 +258,7 @@ Items {
```
**IMPORTANT:**
The strategy `everyMinute` (60 seconds) **must** be used, otherwise no data will be persisted (stored).
Other strategies can be used, too.
When creating a custom datasource in the `rrd4j.cfg` file the used [sample interval](#sampleinterval-sample-interval) should be 20 seconds or less in order to keep the granularity. The selection of the used strategy has no effect on the granularity.
---