## Disk tier 

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 . 

## 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 performance of the array. See also “Rebuild area” on page 810. 

## Easy Tier 

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. 

## Effective capacity 

See “Capacity” on page 797. 

## Encryption key 

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. 

## 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. 

## Enhanced Stretched Systems 

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. 

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. 

