1.4.5   Disk Space

Data Log Trend recording

Data Log Trend tags 10 bytes per sample on the SCADA node.  If you are sampling 6000 tags every 1-second and you are using NO deadband, then it will be:

6000 tags * 10 bytes/sample * 3600 samples/hour * 24 hours/day * 365 days/year  * 1/1024 *1/1024 *1/1024  = 1.77 Gigabytes in a year.

If a deadband is used, it will be less. If you assume the tags exceed deadband only 75 percent of the time it will be

1.77 * .75 = 1.33 Gigabytes

Data Log Trend files are typically located at C:\WebAccess\node\project_node\log


You have to make an estimate of the number of graphics on the SCADA Node, Project Node and Clients. Assume 200 Kbytes/graphic, 20 tags per graphic and 300 graphics

300 graphics * 200 Kbytes/graphic  * 1/1024 * 2 = 5 Megabytes in /bgr and 5 Megabytes in DRW = 10 Megabytes

Around 32 Megabytes is required for all the default symbol libraries, widgets and system drawing files on the Project Node and SCADA node(s).  

The entire Symbols and widgets are not downloaded to the Clients.  Only the individual symbol or widget is downloaded to a client if it is requested while using DRAW. Similarly, only the individual Graphic File(s) requested by a Client is downloaded and cached on the Client PC.

Data Logging

WebAccess collects each tags DATA in a separate file that allows remixing tags on a DATALOG trend without losing past data.

Data Log DB is the Dead Band used to reduce file size of the Data Log.  The Tag's value must change more than the Dead Band in order for a record to be added to the Data Log File. DATA LOG Deadband is in percent of SPAN (% of SPANHI - SPANLO).

You can Data Log Analog and Discrete (e.g. Digital) Tags.  You cannot Data Log Text-type Tags.

To view Data  Log Trends, you should create at least One Data Log Trend Group.  A maximum of 12 tags can be viewed in one Trend Group simultaneously; but, you can add or replace Tags "on the fly" in any Trend Group without losing historical data.  The Data Log Trend Group uses a pre-built template display.  You need only enter the Tag names in a list to build a Trend Group.

You should also check that the Deadband for Data Logging is set appropriately. Too large of a Deadband will prevent any data from being logged. Too small a Deadband will result in unnecessary recording of values that have not changed and will waste disk space.

The data log files are located on the SCADA node. Typically, the default path is


It is recommended to specify another path to reduce the time for re-installing software. This is done is SCADA Node Properties, Data Log Folder.  WebAccess modifies the security settings of all files in drive:\WebAccess during Node software installation.  A large number of Data Log files will greatly increase software installation.

A file is created for each tag allowing tags to be added or removed from Data Log Trends without loosing historical data.

It is highly recommended to use a Data Log deadband to reduce Disk Access. This will reduce the size of the Data Log Trend files and time used to write to the Hard Drive.  The Hard Drive is usually the bottle neck in Data Logging.  An IDE Hard Drive at 7200 rpm can handle about 800 Tags per second of Data Logging. Using Disable Data Log Seconds recording can also greatly reduce Disk Space and system throughput if one-second data is not needed.

Log Data Maintenance should also be used to delete and, optionally Archive, Data Log Trend files.  A large number of files in the Data Log Folder will reduce Disk Access time and slow you system greatly.  You should also Defragment your hard drive frequently (once per day is the Microsoft recommendation) if you are data logging