Planning Storage Environments › Sample Calculations › Storage Capacity for Two Sets of Recovery Data, One Full and One Incremental Backup

Storage Capacity for Two Sets of Recovery Data, One Full and One Incremental Backup

Assume the following:

• You have to do a full backup of 1 Terabyte of user data per week.
• You have to do daily incremental backups.
• About 10% of the data changes daily.
• The data from the last two backup cycles are available, online, for fast recovery.
• You are using LTO tape drives with 2:1 compression in a library with 20 slots.
• All media are used as efficiently as possible.

First, calculate the amount of capacity you need to store the output of the current backup operations. LTO media elements have a raw capacity of 200 GB with 2:1 compression. After you deduct 10% for overhead, the real capacity is close to 180 GB. The 1 Terabyte full backup thus requires:

1 Terabyte ÷ 180 GB / media element = 6 media elements

Using the above equation, you can also calculate the safety margin as follows:

(6 X 180 - 1000) / 1000 = 8%

Because six tapes (1 Terabyte) provide an 8% safety margin, you do not need to add extra tapes. In this example, you need only 6 LTO tapes to store a full backup. Based on the rate of change you estimated, the incremental backups amount to:

1 Terabyte × 10% changed / incremental × 5 incrementals = 500 GB changed

Therefore, at a minimum, you need the following:

500 GB ÷ 180 GB / media element = 3 media elements

Because three tapes (500 GB) provide a 9% safety margin, you do not need to add extra tapes. You need only three tapes to store a single set of incremental backup data.

Next, calculate the amount of storage space you need for your online recovery data. You need to retain the last two backup sets in the library, so you need 9 tapes for the oldest set of recovery data and 9 tapes for the newest set. To store your recovery data you need 18 tapes.

Therefore, your total storage requirement is as follows:

9 tapes for current backup + 18 tapes for recovery = 27 tapes

Next, you calculate the capacity of the library by deducting cleaning slots:

20 slots/library - 1 cleaning slot = 19 available slots

Therefore, you have a deficit of 27 - 19 = 8 slots and must do one of the following:

• Add a library.
• Compress the stored data.
• Store only one set of recovery data online.