Applies To:
  • CitectSCADA 1.00, 1.01, 1.10, 1.11, 1.20, 2.00, 2.01

Question: I want to store trend data for 1 month, is it better to create 31 x 1 day files or 4 x 1 week files or 1 x 1 month file? 

The optimum size of trend files for one month would be 4 x 1 week files. Each trend file has a fixed overhead in disk storage (256 byte header) as well as extra CPU overhead for Citect - it is therefore better to have fewer (but larger) trend files. A large number of small trend files will waste extra disk storage due to file headers, as well as increase disk fragmentation. Many small trend files means that Citect has to roll the files more often. When Citect fills one file and starts another, it must do a fair amount of disk access, which will slow Citect during this time. It is better to have fewer files.

There should however, be several files to minimise disk wastage when all the files are full. For example, to store 4 weeks of data you really need 5 x 1 week files. This happens because when the last file is full, Citect will start overwriting the first file again, so the existing data in that file is lost. If you have 5 files, then you will have from 4 to 5 weeks of storage. It is best not to create a single big file as you will need an extra file to cover the full period (this extra file will waste more storage). For example, 2 x 1 month files will give you between 1 and 2 months of storage - but will also take more disk space than 5 x 1 week files.