In order to calculate the length of time any log data will be kept for, the individual log intervals shall be taken into account. If you log a value every 5 minutes then it will use more disk space (data) than if you logged it every hour.
It should be noted that the DLCLIENT actually declares the number of days per Mb as you set them up in the ObView engineering view for the log itself. As you change the interval, it will tell you how many days it will keep per Mb and it calculates this dynamically. This is the best way of setting it up.
The total period of data retention is also affected by the total number of Mb configured for the DLCLIENT on the main config page, namely, the 'Max rolling log size (Mb)', which is defaulted to 1Mb and therefore needs to be increased if you want to store more data. This setting is the overall storage and so base don a 30 minute interval for a log value, if that log stores 540 days of data, should you setup an additional log of the same type, the overall storage period will be reduced to 270 days and so on. Therefore, once you have calculate the storage period you want for all the configured logs, you can then use that information to calculate the total storage required to achieve the period of storage you require.
A log with 30 minute intervals will retain 540 days of data for 1Mb of storage. Therefore, please see below for the way of working out how to set the 'Max rolling log size (Mb)'.
1Mb/540 Days (for 30 minute readings) = 0.0018518Mb per day
0.0018518Mb x 365 Days = 0.6759Mb per value per year at 30 minutes.
If you have 20 electricity meters, each with 4 readings that is 80 values to be logged, all at 30 minute intervals.
So, 80 values x 0.6759Mb = 54.07256Mb per year for all 80 readings.
If you want to store for 3 years, that is 54.07256Mb x 162.21Mb
Therefore, you need to set the 'Max rolling log size (Mb)' to at least 162.21Mb but I would set it to 200Mb as a start.
The other relevant setting, which is the 'Max auto-archive log size (Mb)', which archives the data sets at a pre-defined size, which is defaulted off. This simply makes full backups of logs in chunks of data as specified, instead of or as well as rolling them over. When DLCENTRAL is used, this feature is normally turned off.