text: Be careful when you are using slower disk subsystems for the secondary volumes for 
  high-performance primary volumes because the Storwize V7000 cache might not be able 
  to buffer all the writes. Flushing cache writes to slower back-end storage might impact 
  performance of your hosts. 
  
  For more information about data migration using Storwize V7000, see 11.5, “Volume mirroring 
  and migration options” on page 512. 