## 4.3 Abstraction layers for drive virtualization 

Virtualization in the DS8900F refers to the process of preparing physical drives for storing data that belongs to a volume that is used by a host. This process allows the host to think that it is using a storage device that belongs to it, but it is really being implemented in the storage system. In open systems, this process is known as creating logical unit numbers (LUNs). In IBM Z, it refers to the creation of 3390 volumes. 

The basis for virtualization begins with the physical drives,  which are mounted in storage enclosures and connected to the internal storage servers. DS8900F only uses the High-Performance Flash Enclosure Gen2 (HPFE Gen2) storage enclosures. To learn more about the drive options and their connectivity to the internal storage servers , see 2.5, “Flash drive enclosures” on page 50. 

## 4.3.1 Array sites 

Virtualization builds upon the physical drives in a series of layers: 

GLYPH<SM590000> Array sites 

GLYPH<SM590000> Arrays 

GLYPH<SM590000> Ranks 

GLYPH<SM590000> Extent pools 

GLYPH<SM590000> Logical volumes 

GLYPH<SM590000> Logical subsystems 

An array site  is formed from a group of eight identical drives with the same capacity, speed, and drive class. The specific drives that are assigned to an array site are automatically chosen by the system at installation time to balance the array site capacity across both drive enclosures in a pair, and across the connections to both storage servers. The array site also determines the drives that are required to be reserved as spares. No predetermined storage server affinity exists for array sites. 

## 4.3.2 Arrays 

Array sites are the building blocks that are used to define arrays . 

An  array  is created from one array site.  When an array is created, its Redundant Array of Independent Disks (RAID) level, array type, and array configuration are defined. This process is also called defining  an array. In all DS8000 series implementations, one array is always defined as using one array site. 

The following RAID levels are supported in DS8900F: 

GLYPH<SM590000> RAID 6 (default) 

GLYPH<SM590000> RAID 10 

GLYPH<SM590000> RAID 5 (possible for flash drive sizes below 1 TB) 

An RPQ is required for flash drives greater than 1 TB (RPQ is not available for drive sizes of 4 TB and over). 

Each HPFE Gen2 pair can contain up to six array sites. The first set of 16 flash drives creates two 8-flash-drive array sites. RAID 6 arrays will be created by default on each array site. 

RAID 5 is optional for flash drives smaller than 1 TB, but is not advised. RAID 10 is optional for all flash drive sizes, except for the 7.68 TB and 15.36 TB Flash Tier 2 drives. 

