Disk tier Distributed RAID or DRAID Easy Tier Effective capacity Encryption key Encryption key server Encryption of data-at-rest Evaluation mode Event (error) Event code An event code is a value that is used to identify an event condition to a user. This value might  map to one or more event IDs or to values that are presented on the service window. This  value is used to report error conditions to IBM and to provide an entry point into the service  guide. An event is an occurrence of significance to a task or system. Events can include the  completion or failure of an operation, user action, or a change in the state of a process. The evaluation mode is an Easy Tier operating mode in which the host activity on all the  volume extents in a pool are “measured” only. No automatic extent migration is performed. 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 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 771. Easy Tier is a volume performance function within the IBM Storwize family 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 the overall performance of array. See also “Rebuild area” on page 783. MDisks (logical unit numbers (LUNs)) that are presented to the IBM Storwize V7000 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 . Appendix C. Terminology  775