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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
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