IBM E16RMLL-I Implementation Guide - Page 87
Backup set volumes
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We have a total of 49 on-site, off-site, and scratch tapes. Fifteen percent of this is nine tapes. Backup set volumes It is worth calculating backup set tape space requirements separately from regular file-based processing due to the nature of backup sets. Note that although the backup set is a server-initiated procedure, our calculations are based on previous file-backup operations. Note that this step may not be necessary, since you may create backup sets onto disk and copy them onto media such as CD or use another tape format that is common to both the server and the client. For this calculation we are assuming the backup set consists of the entire client system's file; however, you may only use a subset of its file spaces. To estimate the number of backup set volumes required: 1. Using Table 2-1 on page 18, multiply the total storage used by (1 - the data compression rate) to give the total gigabytes for each client. Round up to the next multiple of the native tape capacity, since backup sets, like server database backups, cannot stack onto tapes. 2. Multiply the number of backup sets by the total gigabytes for that client to give the client backup set size. 3. Multiply the client backup set size by the yearly retention ratio (that is, desired monthly retention/12 months) for that client. Sum all client backup set sizes to give the total backup set requirement. 4. Divide the total backup set requirement by the device capacity to find the number of tape cartridges required. Backup set calculation To calculate: 1. Multiplying 1200 by (1 - 0.5) gives 600, and multiplying 400 by (1 - 0.66) gives 136. Assuming a native tape capacity of 400 GB, this rounds up to 800 and 400 GB, respectively. 2. From previous calculations, client backup set sizes are 6 * 800 (4800 GB) and 6 * 400 (2400 GB). 3. Assuming retention periods of 4/12 and 2/12, this gives 4800*0.33 + 2400*0.16 = 2000 GB as backup set space requirement. 4. If the tape device has a capacity of 400 GB, then dividing 2000 by 400 gives 5 cartridges required to store all backup set data during the specified time frame. Chapter 2. Implementation planning 57