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4536
May 19th, 2018 05:00
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?
Geri_Thomas
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May 21st, 2018 07:00
Each snapshot is 8 hours of data which = 20 GB of data. They need 8 snapshots at any given time. So 8*20=160 GB sounds right to me.
Rivendell_86
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May 27th, 2018 08:00
Yes, however at any point in time there are no 8 snapshots which are 8 hours old. Only furthermost from any given point in time is 8 hours old and takes 20 GB. Other 8 are closer to the moment an hence their size is not 20 GB.
Rivendell_86
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May 30th, 2018 01:00
Yes, but the old 8 hours age snapshots are not kept. They are deleted. Once the closest snapshot becomes 2 hours old, the 8 hours old snapshot is deleted and therefore does not take up any space. At any point in time there are only 8 snapshots 8, 7, 6, 5, 4, 3, 2 and 1 hours old.
Rivendell_86
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May 31st, 2018 02:00
Yes, it should support 8 hours of changes, but why do you suggest that it instantly takes up 20 GB? Pointer based snapshots do not take up any capacity.