Computer Associates BABWBR1151S38 Implementation Guide - Page 37

Sample Calculations 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. Chapter 2: Planning Storage Environments 37