By Dan Barber

Backup to disk is typically going to be done on lower-speed, higher-capacity storage. Most companies use 1TB or higher SATA drives, which makes sense because they’re inexpensive and you can use them to easily add a lot of capacity to an array. There’s no good reason to buy high-speed drives because you’re not going to be able to take advantage of that speed—you simply can’t back up that fast. Your backup servers aren’t powerful enough, and you don’t want to bog down your production SAN by using too much I/O bandwidth on backup instead of on requests.

1. Have A Strategy.

In planning for capacity, you first need to determine your backup strategy. For example, you may have a strategy that dictates backups every three hours, once a day for seven days, once a week for four weeks, once a month for 12 months, and one yearly backup.

2. Determine Your Capacity.

By starting with that information, you can determine the backup/restore capacity you’ll need. To do that, determine your average change rate, which is the amount of data that changes over a typical day. At Reliant, here’s how we advise customers to find their rate: Back up for a week, every night. Do a full backup on Sunday. Do an incremental backup each day, Monday through Friday. Then average the size of those backups to get a daily change figure. For example, if you have 100TB in your environment and 1TB of it changes every day, that’s your change rate.

3. Decide How Many Backups You Want To Store.

You also need to decide how many backups you want to store. If you want to store one copy for seven days and 1TB changes every day, you’ll need at least 7TB of storage for daily backups. Add one backup for each week, that’s another four copies at 7TB each, so that’s another 28TB. You can do these calculations to find the amount of capacity you’ll need to accommodate your specific backup strategy. 

4. Consider Backup/Restore Performance.

In addition to capacity, backup/restore performance is also a key consideration. Imagine an entire file server goes down and you lose the 20TB of data it was storing. You don’t want the restore to take four days; you want it back quickly. You need to ask yourself, “How much time can we be down while doing a restore?” That will determine the performance you need from your backup systems. For example, you may be able to buy 15 3TB disks, but then you learn that it will take five hours to restore each 3TB. Buying more of the smaller disks will enable you to do your restore faster, which may be a better way to minimize the risk of harm to your business from unplanned downtime.