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Snap Pool Size Planning

Proper planning for Snap Pool size is critical in managing snapshot efficiency and storage. Key factors to consider include the number of planned snapshots for the master volume, its size, and the amount of changed data written between snapshots. If snapshot volumes are mounted for write access, it's important to estimate the unique data written to each. Use this equation to calculate needed space: (unique data to master per snapshot * # of snapshots) + (unique data to mounted snapshots * # of writes) + reserve space (600MB), multiplied by a safety factor of 1.25.

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Snap Pool Size Planning

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  1. Snap Pool Size Planning • Factors to consider • Number of snapshots planned for the master volume. • Size of the master volume • Amount of changed data written to the master volume between snaps • Specifically how much data is either new or modified • Will the snapshot volumes be mounted for write access? • If so, how much unique data will on average be written to each snapshot volume? • Based on this information, a customer can estimate how much snap pool space is needed for a master volume, using the general equation • ( (amount unique data written to master volume per each snapshot * # snapshots) • + (amount unique data written to each mounted snapshot volume * # snapshots written) • + snapshot function reserve space (plan on 600 MB) ) • * 1.25 (safety factor) Dot Hill NDA Material

  2. Snap Pool Size Planning - Example • Master volume = 100GB • Average 10% change rate = 10GB • Keep 5 snapshots all mounted • Average 2GB changed data on mounted snapshots • ( (amount unique data written to master volume per each snapshot * # snapshots) • + (amount unique data written to each mounted snapshot volume * # snapshots written) • + snapshot function reserve space (plan on 600 MB) ) • * 1.25 (safety factor) ((10GB * 5) + (2GB *5) + .6GB) * 1.25 ((50GB) + (10GB) + .6GB) * 1.25 ((60.6GB) * 1.25 If you have multiple volumes associated with the snap pool you need to calculate those individually and add them to the snap pool space. 75.75 GB Dot Hill NDA Material

  3. AssuredSnap Resources – Data • A snapshot may consume up to 200% of the size of the master • The preserved data is tracked by one top level page table • The write data is tracked by a different top level page table Since the preserved and write data is tracked separately, if every block is over-written on the master volume and every block is over written on the snapshot, the snapshot will consume 200% of the size of the master volume Dot Hill NDA Material

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