Disk tier Distributed RAID or DRAID Easy Tier Effective capacity Encryption key Encryption key server Encryption of data-at-rest Enhanced Stretched Systems Enhanced Stretched Systems intelligently route I/O traffic between nodes and controllers to  reduce the amount of I/O traffic between sites, and to minimize the effect on host application  I/O latency. Enhanced Stretched Systems include an implementation of additional policing  rules to ensure that the correct configuration of a standard stretched system is used. A stretched system is an extended high availability (HA) method that is supported by the SVC  to enable I/O operations to continue after the loss of half of the system. Enhanced Stretched  Systems provide the following primary benefits. In addition to the automatic failover that  occurs when a site fails in a standard stretched system configuration, an Enhanced Stretched  System provides a manual override that can be used to select which of two sites continues  operation.  Encryption of data-at-rest is the inactive encryption data that is stored physically on the  storage system. An internal or external system that receives and then serves existing encryption keys or  certificates to a storage system. The encryption key, also known as the master access key, is created and stored on USB flash  drives or on a key server when encryption is enabled. The master access key is used to  decrypt the data encryption key. See “Capacity” on page 797. Easy Tier is a volume performance function within the SVC that provides automatic data  placement of a volume’s extents  in a multitiered storage pool. The pool normally contains a  mix of Flash Disks and HDDs. Easy Tier measures host I/O activity on the volume’s extents  and migrates hot extents onto the Flash Disks to ensure the maximum performance. An alternative RAID scheme where the number of drives that are used to store the array can  be greater than the equivalent, typical RAID scheme. The same data stripes are distributed  across a greater number of drives, which increases the opportunity for parallel I/O and hence  improves performance of the array. See also “Rebuild area” on page 810. MDisks (logical unit numbers (LUNs)) that are presented to the SVC cluster likely have  different performance attributes because of the type of disk or RAID array on which they are  installed. The MDisks can be on 15,000 revolutions per minute (RPM) Fibre Channel (FC) or  serial-attached SCSI (SAS) disk, Nearline SAS, or Serial Advanced Technology Attachment  (SATA), or even Flash Disks. Therefore, a storage tier attribute is assigned to each MDisk and  the default is  generic_hdd . SVC 6.1 introduced a new disk tier attribute for Flash Disk, which  is known as  generic_ssd . Appendix C. Terminology  801