Appendix C. Terminology 775
Disk tier
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.
Distributed RAID or DRAID
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.
Easy Tier
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.
Effective capacity
See “Capacity” on page 771.
Encryption key
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.
Encryption key server
An internal or external system that receives and then serves existing encryption keys or 
certificates to a storage system.
Encryption of data-at-rest
Encryption of data at rest is the inactive encryption data that is stored physically on the 
storage system.
Evaluation mode
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.
Event (error)
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.
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.