History count capacity in Niagara G3 stations and ComTrol CWEB / CES stations

[Product_Support]

Issue

How many histories can be created in a station and how many records are allowed per history?

Environment

I/A Series G3 ENC, ENS or SoftJace
CComTrolCCWEBor CES

Cause

Technical product information update.

Resolution

Regardless of platform type, histories are stored under the station's file system (file:^history/). Stations running on a PC-based system (Enterprise Server or SoftJACE) store the histories in subdirectories using segmented files; this provides improved search capabilities over a flat file system. Stations running on a QNX-based system (ENC) store all of the stations histories in a single compressed file (history.zip). A running station opens the history.zip file in the ENC's ram disk space, the histories are compressed and saved to flash whenever the station is saved. Opening the history.zip file to the ram disk eliminates the need to write to the flash drive each time a record is added to the history file.

When the station needs to write a record to a history, it opens the specific history into the running station (either from the file system if PC-based, or from the ram disk if QNX-based). The open history does contribute to the heap.used of the station. The history service has one property 'max open time' (default 5 minutes) which causes the station to close the history from the running station heap if no records have been written to the history for a time period exceeding the 'max open time'.

Each history extension which is added to the station does add slightly to the station resources because the extension is represented in the bog file. When the history is first enabled, the station creates the history file. The initial file includes a 1600 byte header (defines the configuration parameters) and a single page file of 4096 byte size. When triggered by either the collection interval or COV/COS, a record is added to the page file. When the page file becomes full, additional page files are created based on the capacity configuration of the history.

History Record Size by Point Type

Point Type	bytes/record
boolean	13
enum	16
single precision numeric	16
double precision numeric	20
string	variable length *
*NOTE: A single 'string' record cannot exceed one page (4096 bytes)

Since histories count as file descriptors, there is a physical limitation on the number of histories allowed in a QNX-based ENC station. It is recommended not to exceed 800 histories, given the file descriptor limit of 1000.

Note: The platform services view of the QNX-based JACE station includes a property to modify the size of the ram disk within defined limitations. Note that the alarm database is also maintained in the same ram disk space. Exercise caution when modifying the default ram disk size; note that allocating additional memory to the ram disk will limit the memory available for other functions by the QNX OS. Reducing the ram disk size too much could result in history or alarm records not being stored properly and erratic station behavior.