Disk Architecture for Optimal Performance

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To obtain maximum performance from your disk subsystem in ClearSCADA, you need to consider the location of the following:

• Operating system and ClearSCADA database

• Historic data files (the Historic directory)

• Event Journal files (the Event Journal directory)

The location of these files can play an important role in the performance of your system. If they are all located on the same disk, access to that disk is more likely to be slow. This is because some system activity may require different areas of the disk to be read simultaneously, which will cause slow performance. Another issue with a single disk is that the disk is likely to have less than 50% of its capacity available - disks with less than 50% capacity available are more likely to become fragmented. Data access takes a longer amount of time on fragmented disks.

By separating the three groups of files onto separate physical disks, you can improve the performance of your system. However, you should also note that the type of disks you use will affect system performance.

For optimal performance, you should aim to have three high performance disks (or preferably, three RAID arrays): one disk to store the operating system program installation and ClearSCADA database, one disk to store the historic data files and another disk to store the Event Journal data. This allows for faster access times as each disk is dedicated to a single purpose. It also reduces the chances of disk fragmentation as it becomes less likely that any of the disks will have less than 50% of their capacity available. Of course, with high performance disks or RAID arrays, this architecture becomes even more efficient.

However, if you cannot implement a high performance system with three RAID arrays or high performance disks, you can use the same principles to make the most of your existing hardware. By separating the most active directory from the other parts of your system, you should be able to improve the disk access speed (and therefore, system performance).

If your system only has two disks, you need to determine which is the most active directory - the Historic directory or the Event Journal directory. To do this:

• Access the Server Status Tool and display the Historic>Historian statistics.

• Examine the Total File entries for the Event data and the Raw data:

• If the total number of Event data files is significantly higher than the total number of Raw data files, you should move the Event Journal directory to a separate disk.

• If the total number of Raw data files is significantly higher than the total number of Event data files, you should move the Historic data directory to a separate disk.

• If the total number of Raw data file and Event data files are not significantly different, you can choose whether you want to move Historic data or Event Journal data to another disk.

For information on changing the location of the files, please refer to the Server Administration Guide in ClearSCADA Help.

If your system only has a single disk, you may experience slow performance due to an excessive amount of data needing to be written or read on the disk at the same time. This will depend on the amount of activity on your system and the size of the system as well as the type of disk being used. While partitioning the disk can have some benefits, they are negligible when compared to the effects of adding another physical disk. Partitioning the disk correctly also requires an expert knowledge of partitioning and can be time consuming. Using multiple disks will have a much more noticeable effect on your system performance than partitioning a disk. That's why we always recommend that you resolve single disk performance problems by adding separate physical disks. When you have added one or two separate, physical disks, you can then move the Historic files and/or Event Journal files to the new disks.

If you must use a single disk system, a higher performance disk may resolve any noticeable  performance issues. At the time of writing - May 2007 - we make the following recommendations:

For desktop PCs, we recommend hard disks with a seek time of 4 ms and spindle speed of 10,000 rpm as high performance.

For a high performance enterprise class or RAID addition class disk, we recommend disks with a seek time of 3 ms and spindle speed of 15,000 rpm.

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