Dell PowerProtect DD Management Center (DDMC) 7.9 Installation and Administration Guide

Space projection algorithm for DDMC

DDMC uses a sophisticated algorithm to project growth in space usage and to predict when a DD system will run out of space. This algorithm was developed and verified using years of autosupport reports and should be accurate.

For this algorithm, DDMC uses a seven-day moving average instead of actual measured values. This smooths out the effects of file system cleaning and other activities that repeat every week (for example, deleting an old full backup and creating a one every weekend).

The goal of this algorithm is to compute a linear projection of space growth using an optimal set of recent data points. The data history is scanned to find the projection with the best fit, that is, the regression with the highest R² value.

The R² value is a measure of how close the regression fits the actual measurements. A value of "1" means that the fit was perfect. A value of "0" means that there was no fit at all. A value of "0.8" means that DDMC found a projection that matches the measurements closely enough to be meaningful and not misleading.

After the best fit is determined, the projection must pass the following validation tests to ensure that the prediction is accurate:

1. DDMC must have at least 15 days of historical data.
2. The regression R² value must be at least 0.8 or higher.
3. Time-to-full must be less than 10 years in the future.
4. The system must be at least 10% full.
5. The most recent data sample must be within 5% of the projection.

Combining all these validation criteria accounts for typical system usage behavior, such as space becoming free after a cleaning cycle, jumps in usage as new backup loads are stored on the system, and space becoming free when backups are deleted.