PowerFlex: How to Calculate Spare Capacity

How to calculate  and set spare capacity for each pool.
How many nodes/devices can fail, before the system won't be able to rebuild it self.

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Spare capacity in ScaleIO has implication on how many nodes/devices can fail, before the system won't be able to rebuild it self.
Spare capacity is set per storage pool.
By default, the installer will calculate and set the spare capacity for each storage pool in a way, were all the pools will survive a single node failure.
This article will describe, how to calculate  and set spare capacity for each pool.

Steps

1. Select a storage pool.
2. In case of 10 or more SDSs belong to the storage pool, it is advised to have 10% spare capacity.
3. In case, the storage pool has less then 10 SDSs, calculate which SDS has the largest capacity in specific storage pool. For example SDS sds1, has the largest capacity, which is 2TB in storage pool SP1.
4. Calculate the percentage of the the capacity from step 3, in total pool capacity. For example, SP1 has 10TB of total capacity, then 2TB are 20%.
5. Decide, how many nodes you would like to be able to loose, and multiply the percentage from the previous step by this number. For example, in case you would like to loose 2 nodes, the percentage should be 2*20=40%.
6. Set the spare capacity by using the command:

    scli --modify_spare_policy --protection_domain_name <protection domain name> --storage_pool_name <storage pool name> --spare_percentage <percentage>