Storage modes
- Last UpdatedMar 07, 2025
- 4 minute read
Depending on a tag's definition, Historian uses one of these storage modes to retain the values received for that tag:
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No storage - No values are stored.
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Forced storage - All collected values are stored.
For comparison's sake, this is what forced storage looks like. The red dots represent collected values. All of these values are stored by Historian.
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Cyclic storage - Only values that occur at a specified time interval are stored. Using the same collected values as shown above, cyclic storage retains only the values represented by red dots.
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Delta storage - Only changed values are stored.
Types of delta storage
Delta storage, as a rule, requires Historian to store any value that is different than the previously received value. Time or deadband rules can be applied for delta storage to further constrain what values are stored.
These are all types of delta storage:
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Time-enforced delta storage -- Any changed value is recorded. Additionally, a record must be stored after a given amount of time. This storage mode is used by the Historian SDK.
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Time deadband delta storage -- Only changes outside of a particular time deadband are stored.
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Value deadband delta storage -- Only changes outside of a particular value deadband are stored.
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Rate of change (swinging door) deadband storage -- Only changes outside of a particular rate-of-change (swinging door) deadband are stored.
Measuring change with deadbands
Delta storage retains only those values that have significantly changed from the previously stored value.
For example, if you had a discrete (binary) tag that reflected the state of a power switch, you may not want to record every time the system checks to see that it is switched on. You might really be interested only in when it switches off when it is supposed to be running, and when it gets switched on again.
For analog (numeric) tags, you may only care only about large changes, but not tiny ones. Or, you may want a snapshot of values at certain intervals, and not every one that is reported. You can filter out extraneous value with deadbands.
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A time deadband is a time filter. It marks the minimum time (milliseconds) between stored values for a single tag. Any value changes that occur within the time deadband are not stored. For example, these red and blue points are all the values reported for a certain tag. The orange bars represent the time deadband, which starts over with every reported value. Only the red points (P2, P4, P7, P8, P9, P11) are stored. The other points are excluded because they fall within a deadband or outside of the time period. |
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A value deadband is a filter that marks the percentage of the difference between the minimum and maximum engineering units for the tag. Any data values that change less than the specified deadband are not stored. Here, the orange bars represent the value deadband, which starts over with every reported value. Only the red points (P2, P5, P6, P7, P10, P11) are stored. The other points are not stored because they fall within a deadband or outside of the time period. P9 is not stored because P8 was discarded and it is within the percentage deviation. |
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A swinging-door deadband marks a rate of change deadband, based on changes in the slope of the received values. For example, specifying a swinging door deadband value of 10 percent means that values are stored if the percentage change in slope of the consecutive data values exceeds 10 percent. |
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For more information on delta storage modes, see About Delta Storage Mode in the AVEVA Historian Administration Guide.