text: When the process to save critical data starts, the system stops handling I/O requests from the 
  host applications, and Metro Mirror and Global Mirror relationships go offline. The system 
  powers off when the saving of the critical data completes. If AC power is restored after the 
  system state dump starts, the dump continues to completion. The system then restarts. 
  
  **Note:**Storwize V7000 expansion canisters do not cache volume data or store state 
  information in volatile memory. Therefore, they do not require battery power. If AC power to 
  both power supplies in an expansion enclosure fails, the enclosure powers off. When AC 
  power is restored to at least one of the power supplies, the controller restarts without 
  operator intervention. 
  
  In a fully redundant system with two batteries and two canisters, there is enough charge in the 
  batteries to support saving critical data from both canisters to a local drive twice. In a system 
  with a failed battery, there is enough charge in the remaining battery to support saving critical 
  data from both canisters to a local drive once. 
  
  If both node canisters shut down without writing the cache and state data to the local drive, 
  the system is unable to restart without an extended service action. The system configuration 
  must be restored. If any cache write data is lost, volumes must be restored from a backup. 
  Therefore, it is important not to remove the canisters or the power supply units from the 
  control enclosures unless directed to do so by the service procedures. Removing either of 
  these components might prevent the node canister from writing its cache and state data to 
  the local drive. 