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SmartPools Virtual Hot Spare (VHS) helps ensure that node pools maintain enough free space to successfully reprotect data if a drive fails. Though configured globally, VHS operates at the node pool level so that nodes with different size drives reserve the appropriate VHS space. This functionality helps ensure that, while data might move from one disk pool to another during repair, it remains on the same class of storage. VHS reservations are clusterwide and configurable as either a percentage of total storage (0 to 20 percent) or several virtual drives (one to four). The mechanism of this reservation is to allocate a fraction of the node pool’s VHS space in each of the node pool’s constituent disk pools.
No space is reserved for VHS on SSDs unless the entire node pool consists of SSDs, which means that a failed SSD could have data moved to HDDs during repair. However, without adding additional configuration settings, the alternative is reserving an unreasonable percentage of the SSD space in a node pool.
The default for new clusters is for VHS to have both Reduce amount of available space and Deny new data writes enabled with one virtual drive. On upgrade, existing settings are maintained. All customers are encouraged to enable VHS.