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.
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?
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.
Consider furthermost snapshot is 8 hours old and closest one is 1 hour old. The closest snapshot (1 hour old) will be retained for 8 hours and the capacity of that snapshot needs to support 8 hours data change which is 20 GB in this case.
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.
We need to have 8 snapshot at any point in time. When a new snapshot is created, it should support 8 hours data change which is 20 GB. For example, Once the furthermost snapshot i.e 8 hours old snapshot is deleted at 9th hour, a new Snapshot should be created and it should support 8 hours data change. This process continues, so we need 8 snapshots and each one should have 20 GB capacity to support 8 hours data change.
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.
The Pointer-based Snapshot is a space optimal solution i.e it does not require the same size of source volume. But it requires a small fraction of the size of the source volume and this space is referred as save location. When a write is issued to the source volume for the first time, the original data in the source volume at that location is copied to this save location. So, when a new snapshot is created, it should support 8 hours data change which is 20 GB (save location size).
Please refer the topic “Pointer-based Virtual Replication – Snapshot” in Module 14-Replication.