Automatic unmounting is triggered if overwrite reserve utilization on the volume reaches the threshold specified by the fractional space reservation policy. SnapManager automatically unmounts the databases and stops the write operations to LUNs in that volume.
The threshold for overwrite reserve is specified by the policy for fractional space reservation. Another component of the fractional space reservation policy is the last-resort action that prevents further consumption of overwrite reserve; hence, automatic unmounting is always enabled.
SnapManager first uses backup set deletion to free some overwrite reserve. If this is not enough, unmounting the affected database prevents further consumption of overwrite reserve. This happens when both components of a fractional space reservation policy are enabled; and the unmounting of databases is triggered at a later level of overwrite reserve utilization than the level that is used to trigger the deletion of Exchange backup Snapshot copies.