Disk array
A disk array (German: disk subsystem or disk subsystem ) is a device with several hard disks that are made available to servers . Functionalities such as RAID , storage virtualization, copies or mirrors within the same or between disk arrays are carried out directly on these without the involvement of the server.
history
The first disk arrays were simple external hard disk enclosures without RAID functionality ( JBOD ). Modern disk arrays have several independent controllers to enable high availability through redundancy, high data transfer performance through load distribution and parallel access from several servers at the same time (often via a storage area network ). The use of cache subsystems is also common. This means that a write process is reported to the operating system as finished when the data has been cached in the disk array but has not yet been physically saved on the hard disks. This has a positive effect on the writing performance of the overall system.
Initially, the connection was made via SCSI . In the late 1990s, Fiber Channel became the common interface . Depending on the application, a connection via iSCSI is also conceivable. In the mainframe environment, ESCON and FICON are common protocols.
Trivia
Often, individual hard disks in disk arrays can be inserted or exchanged during operation; this is known as hot-plug . Each hard disk requires a special installation frame for this. Hot spare hard disks can be configured to take over the data from a defective hard disk in the event of an error.
Disk arrays from the early 2000s can manage hundreds of hard drives and capacities up to several hundred terabytes and have caches of several gigabytes. Examples from 2006 are the EMC Symmetrix DMX-3 with 525 TB of usable capacity on 2400 hard drives with 256 GB cache and the HP XP 12000 with 332 TB of usable capacity on 1152 hard drives with 128 GB cache (each with the theoretical maximum configuration).
The name “sub-system” is derived from the interaction between the respective disk system and the actual main computer on which the applications run.