Drives

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Backup Window and Amount of Data

One of the most critical steps in evaluating the storage media requirements is determining the actual backup window. This must be the actual amount of time you are allowed to have backups running, while at the same time, controlling the backup hardware, using a major part of the network, and using the resources on the systems being backed up. The size of your backup window is becoming a much harder measurement to define. You must be able to determine the number of hours in a day and in a week that can be dedicated to backups, as this is an integral part of the equation to determine media requirements.

Based on the data we collected in the earlier chapters, you should now have a very good idea of how much data needs to be backed up each day and each week. You need to know how much data needs to be backed up during each window. Generally, the largest backups will be the full backups. In the past, most administrators performed daily incremental backups and did all their full backups over the weekend when most people were not working. This concept is changing. It is very common now for a percentage of the systems, say, one-fifth, to have full backups done each day and the remaining systems to have incremental backups each day. The weekends are saved to do maintenance or to catch up. If this is closer to your model, then your window would be the time each day when backups are performed, and the amount of data would be the average sum of the total data that would be backed up, roughly a fifth of your total data. If you do not have specific operational information on the amount of data that will make up your incremental backups, you can estimate using a percentage of change to calculate the amount of data. It is common to use 20 percent, unless you have a more accurate measurement. The goal here is to try to get as close as possible to your actual environment.

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