Appendix C. Terminology 787
Throttles
Throttling is a mechanism to control the amount of resources that are used when the system 
is processing I/Os on supported objects. The system supports throttles on hosts, host 
clusters, volumes, copy offload operations, and storage pools. If a throttle limit is defined, the 
system either processes the I/O for that object, or delays the processing of the I/O to free 
resources for more critical I/O operations.
Transparent Cloud Tiering
Transparent Cloud Tiering is a separately installable feature of IBM Spectrum Scale™ that 
provides a native cloud storage tier.
Total capacity savings
See “Capacity” on page 771.
T10 DIF
T10 DIF is a Data Integrity Field (DIF) extension to SCSI to enable end-to-end protection of 
data from host application to physical media.
Unique identifier
A unique identifier (UID) is an identifier that is assigned to storage-system logical units when 
they are created. It is used to identify the logical unit regardless of the logical unit number 
(LUN), the status of the logical unit, or whether alternative paths exist to the same device. 
Typically, a UID is used only once.
Unmanaged mode
An access mode in which an external storage MDisk is not configured in the system, so no 
operations can be performed. See also “Image mode” on page 778 and “Managed mode” on 
page 780.
Virtualization
In the storage industry, virtualization is a concept in which a pool of storage is created that 
contains several storage systems. Storage systems from various vendors can be used. The 
pool can be split into volumes that are visible to the host systems that use them. See also 
“Capacity licensing” on page 772.
Virtualized storage
Virtualized storage is physical storage that has virtualization techniques applied to it by a 
virtualization engine.
Virtual local area network
Virtual local area network (VLAN) tagging separates network traffic at the layer 2 level for 
Ethernet transport. The system supports VLAN configuration on both IPv4 and IPv6 
connections.
Virtual storage area network
A virtual storage area network (VSAN) is a logical fabric entity defined within the storage area 
network (SAN). It can be defined on a single physical SAN switch or across multiple physical 
switched or directors. In VMware terminology, the VSAN is defined as a logical layer of 
storage capacity built from physical disk drives attached directly into the ESXi hosts. This 
solution is not considered for the scope of our publication.