Disk Subsystem (visualised as  Hard disk server): A disk subsystem is a storage subsystem that supports only disk devices. A disk subsystem is a type of disk array, or a type of disk storage, that contains an integrated collection of disk drives, plus any required control software. As with other sorts of disk array, a disk subsystem provides storage services to a set of computers.

Disk Subsystems provides block-oriented storage.

Standard I/O Techniques: SCSI, Fibre Channel, Increasingly Serial ATA(SATA) and Serial attached SCSI(SAS)

I/O Channels: I/O channels can be designed with built-in redundancy in order to increase the fault-tolerance of a disk subsystem.

1. Active
2. Active/Passive
3. Active/Active (No load sharing)
4. Active/Active (Load Sharing)

Disk Subsystems that support RAID (Redundant Array of Inexpensive Disks/Redundant Array of Independent Disks) are called RAID Arrays.

RAID has 2 main goals: 1. Increase performance by stripping 2)  Increase fault-tolerance by redundancy

NAS( Network attached storage) is the name for preconfigured file servers. They consist of one or more internal servers, preconfigured disk capacity and a stripped-down or special OS.

Storage Virtualisation: The objectives of the storage virtualisation are,

1. Simplification of the administration and access of storage resources.
2. Full utilization and possibilities of storage network
3. Realization of advanced storage functions.