| 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. |
