Configuring Appropriate Event and Historic Cache Sizes

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A common problem reported by ClearSCADA users is related to poor performance caused by inappropriate settings for the event journal and historic cache settings.  In the worst case scenario, poor configuration can lead to DBServer running out of available virtual memory and shutting down (crashing) due to lack of system resources.



The ClearSCADA database runs memory resident.  When insufficient resources are available to maintain the database in physical RAM, performance will be seriously degraded as Windows memory management kicks in and moves data in and out of memory, utilizing the page file on disk.



To run correctly, your server needs enough memory to run all programs and its operating system as well as the amount of memory you specify for the historic cache and event journal cache. So, if you set the historic and event journal caches too high, this will prevent the server from being able to run its programs (as it will require more memory than is available).



We recommend that before you configure the historic cache and event journal cache, you examine the amount of available physical memory on the server. To do this, set the server to go Main, wait 60 seconds for it to stabilize, then  access the Server Status Tool and look at the Available Physical Memory setting in the General/Information section. As a general rule, you can safely allocate up to 500MB of the available physical memory to the historic cache and event journal cache (in total). This of course assumes that the server has that much physical RAM available for use by historic data processing.  Most systems use around 25MB for the historic cache and use up to 475MB for the event journal cache.

The amount of memory you allocate for historic and event journal cache depends on your system and what types of data you are using.  The key thing to remember is that a historic record is 32 bytes in size, while an event journal record is 768 bytes.  So even though the event journal cache may be significantly larger, you will be able to search more historic data in a much smaller historic cache.

We also recommend that you use the Performance Monitor driver to monitor the server's performance counters. This allows you to view the amount of physical memory available as a value in ViewX. It also allows you to set the system to raise alarms if the memory drops below suitable thresholds (slow performance will usually occur when the amount of physical memory available drops to around 10MB, but we recommend that most systems raise alarms when the available memory drops below 200MB).



On large systems that run 32 bit operating systems, you should make sure that the total amount of cache size allocated does not cause the total amount of memory used by the server to exceed 3GB. In the normal course of events, DBServer can only use up to 2GB of RAM, however by changing some operating system settings you can use up to 3GB of RAM for DBServer on 32 bit systems. See Using more than 2GB Memory in Windows for more information.  The amount of memory being used by the server for processes is shown in the Available Virtual Memory entry on the Information section of the Server Status Tool (it shows the amount of virtual memory being used and the amount of virtual memory available). As long as the total of the virtual memory being used and the two caches does not exceed 3GB, the system should run without any major performance problems.



When you have worked out how much physical memory you can safely allocate to the caches, you can use the Server Configuration Tool to define the cache sizes (use the Global Parameters/Historic section to define the cache settings).



By configuring your cache settings appropriately and configuring your points and Logic programs within the suggested limits, you will reduce the amount of performance issues on your system.

32 Bit Operating Systems: Note that the  recommendations described here are relevant to ClearSCADA running on 32 bit operating systems.  64 bit operating systems with compatible versions of ClearSCADA will be able to exceed these recommendations.  Refer to the ClearSCADA online help for further information on appropriate settings for your combination of ClearSCADA and operating system.

 

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