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ISM v3 excercise question

Hi all,

There is an exercise in ISM v3 book. I wonder if the answer provided in it is correct.

Short description (exercise is at the end of Module 14 Replication):

60 GB of data change every 24 hours. RPO is 1 hour. 8 hours of data is required for restoration.

Solution: pointer-based storage snapshots.

Calculate required storage for these snapshots. We require to maintain 8 snapshots at any time with one hour between the snapshots.

My calculations:

Every hour changes in data take 60/24 =2.5 Gb.

So, we keep furtherst snapshot for 8 hours, 8*2.5=20 GB. The second snapshot is 7 hours away 7*2.5=15.5 GB.

Required space can be calculated 2.5*(8+7+6+5+4+3+2+1)=90 GB.

The answer provided in the book, however is 8*20=160 GB. How can this be right?

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