Home > Storage > PowerFlex > White Papers > Dell PowerFlex: Maintenance Modes > Concurrent operations in a single Protection Domain
Within a given Protection Domain, all SDSs concurrently in, or concurrently entering PMM, must belong to the same Fault Set. If you do not use fault sets, only one node (which constitutes a fault unit) can be in maintenance mode at a time.
If there is enough spare capacity, it is possible to put several nodes in a fault set into PMM simultaneously. Nevertheless, for simplicity’s sake, our suggested guideline is to do nodes one at a time.