Raid provides
  + Economy - several inexpensive disks.
    
  + Performance and capacity - multiple disks in parallel.

  + Fault tolerance - disk failure protection (except RAID 0)
      and hot-swappable possible - SAS (serial SCSI) and SATA.

  - Multiple disks increase chance of failure.
    * While RAID protect against immediate loss of data
    *   Drive health needs to be monitored (or alarmed)

  - Optimal performance requires expensive controller.
      
  - Except for Raid 0, totol disk size > available storage size.
     Some storage lost to provide fault tolerance.