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Archive tapes
For archive tapes:
1. Using Table 2-1 on page 18, multiply the GB copied per archive by (1 - the
data compression rate), giving transferred archive data.
2. For each client, multiply the transferred archive data by the number of
archives kept in a year times the yearly retention ratio (that is, desired
monthly retention/12 months), and sum up, giving total archive data.
3. Divide the total archive data by the native tape capacity, giving the total
number of tapes required.
Archive tape calculation
To calculate:
1. The clients in our table have 200 GB and 80 GB of archive data. The
compression ratios are 0.5 and 0.66. This equals 200 *(0.5) and 80 * (0.34),
which equals 100 and 27.2 GB.
2. The number of archives kept are 12 and 12. The yearly retention ratios are
12/12 and 12/12. This equals 100*12* 1 + 27.2*12*1 = 1526.4 GB.
3. If the tape device has a capacity of 400 GB, then dividing 1526.4 GB by 400
gives 4 cartridges required to store all archive data.
Image backup tapes
Image requirements are calculated on total allocated file system space. For the
sake of simplicity, we are not considering any compression rating (you can use
the compression ratio for the client on the sum of file systems), and therefore the
number is already overestimated without any need for extra tapes:
1. Consider all client file spaces eligible for image backup. Sum them all, giving
transferred image size per client.
2. Using Table 2-1 on page 18, multiply the number of image backups by
transferred image size per client times yearly retention ratio (that is, desired
monthly retention/12 months), and sum them all giving total image size.
3. Divide the total image size by the native tape capacity, giving the total number
of tapes required.
Image backup tape calculation
To calculate:
1. In our example, we assume that the sum of all of client 3 eligible file spaces
for logical volume backup would be 10 GB * 4 = 40 GB.
2. The retention ratio is 1/12. Assuming the previous calculation, this gives 12 *
40 * 0.16 = 76.8 GB.
Chapter 2. Implementation planning
55
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