To develop a model for capacity planning, there are two dimensions of backup capacity that need to be examined and understood--one related to performing daily backups and the other pertaining to retention and recall of data. The former is driven by the requirement that sufficient resources are available to meet daily demand. Think of that as a bandwidth consideration. The latter--relating to retention and recall--focuses on the availability of backup data for timely recovery, and is a policy and media capacity consideration. Let's look at the components of each of these in a traditional LAN-based tape-centric backup environment.
Bandwidth
Generally, bandwidth capacity for backups pertains to how much data must be moved within the backup period, and is based on a number of components along the data path. If we look at a traditional network-based backup, the bandwidth capacity is the slowest of the following:
- The primary disk from which data is read
- The backup client placing data on the network
- The network transporting backup data to the server
- The backup server reading, processing and writing data to the tape storage system
- The aggregate tape write performance of the tape storage system
While all of these are factors in backup performance analysis, from a backup capacity planning point of view, the last three items are the most relevant.
The first step is to understand current resource capacity. Here are the basic calculations:
- Network bandwidth = number of backup channels x network throughput
- Server bandwidth = number of media servers x I/O throughput capacity
- Tape drive bandwidth = number of tape drives x tape write rate per drive
- Backup window (in hours)
Your current daily backup capacity is the lowest of the first three items multiplied by the backup window. To make this number more realistic, I'd suggest factoring a reserve capacity, based on actual observed performance. A reasonable place holder is 10%.
Next, determine the current utilization rate. This entails measuring the current daily backup volume, both average and peak, and determining the percent of capacity in use. In many environments, a much higher volume of data is backed up on weekends or other periods of high activity. Be sure to factor this into your calculations.
Once this baseline is established, the next step is to estimate the rate of growth. This should include both the data requirements of current applications as well as those being planned in the future. Also, be sure to include other factors such as consolidation of multiple backup environments. Obviously, the accuracy of this forecast is critical to the validity of the capacity planning exercise. So I strongly recommend that these numbers be measured against actual data periodically and the forecast adjusted accordingly (see "Forecasting your tape drive needs").
