Rockwell Automation FactoryTalk Historian SE 3.0 H2H Interface User Guide - Manual (Page 20)

Principles of Operation
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In general the interface can sustain higher data rates (events/second) when configured for
exception data collection. Exception updates are queued in memory on the source Historian
Server. This is not the case for archive updates. Only the most recently accessed archive data
is stored in memory, specifically the archive data cache. When the interface requests archive
data it is likely a certain percentage is read from disk. Disk reads require more computer
resources than reading from memory. As the interface data rate increases, the source
Historian Server will work harder if the interface is requesting archive versus exception data.
History Recovery
History recovery is executed after each of the following events:

On interface startup.

After restoring a lost Historian Server connection.

Update overflow error recovery.

For each new point added to the interface.
History recovery is configured through interface startup parameters. Users can specify the
recovery period, time increments within the total recovery period, a pause time between
increments, and the maximum number of archive events returned per data request. These
parameters enable users to manage the performance impact on the source and receiving
Historian Servers by throttling the data collection rate.
The starting point for history recovery is set on a tag-by-tag basis. If the current value for a
tag is more recent than the starting point of the recovery period, history recovery will begin at
the tag‟s current value. This prevents data overlap, streamlines data retrieval, and prevents
out-of-order data. There is one exception:
Pt Created is the value given to a tag when it is
created. If the current value of an interface tag is Pt Created , the interface performs history
recovery for the total recovery period.
History recovery is performed independently for each scan class. The interface periodically
prints a message to indicate completion status. Every minute the interface calculates percent
completion. A message will be printed for each 10% completion increment or every 5
minutes, whichever comes first.
If a scan class is configured for exception data collection, the interface transitions from
archive to exception data on the last history recovery increment. As the interface cycles
through its tag list, each tag is added to the exception update list after obtaining its last
recovery increment. By default the interface breaks from history recovery to collect exception
updates every 5 seconds. This time interval is configurable through the
/rh_qcheck startup
parameter. As it nears the end of its tag list, more tags are in the update list and the time to
collect exception updates increases. This extra overhead can significantly increase the time
for complete history recovery.
The default behavior is for history recovery data to bypass compression on the receiving
Historian Server. Source data is written to the receiving archive in one of three modes;
append, replace or no replace. This is configured on a tag-by-tag basis. If the
/dc interface
startup parameter is specified, this data is subjected to compression. Historian 2 and Historian
3.2 servers will always apply compression to interface data. In this case the data write mode
is effectively disabled.
Time-range history recovery is configured through the interface startup configuration file.
When enabled, the interface starts up, recovers archive data for the specified start and end