A linked group of one or more physically independent hard disk drives is known as a Disk Array, and is typically used to replace larger single disk drive systems. The most common Disk Arrays are configured in a “daisy chain” or implement Redundant Arrays of Independent Disks (or RAID) technology. These Disk Array structures improve speed while increasing protection against the loss of vital data. A Disk Array may contain several disk drive trays in order to do so.
Disk Arrays arrange stored data into what are known as Logical Units (LUs). A small Disk Array, with a few disks, might support up to eight LUs; a large Disk Array, with hundreds of disk drives, can support thousands of LUs.
High-performance storage systems utilize Disk Arrays as an integral part of their structure. The importance and scale of Disk Arrays grows exponentially as continuous access to information becomes ever-more critical to the day-to-day operation of modern businesses.
Put simply, a Disk Drive is a device that reads data from, and writes data onto, a disk. Rotating the disk at high speeds, a Disk Drive has one or more “heads” that read and write this data.
There are two types of Disk Drives: Internal, which is housed within the computer, and External, which is housed in a separate box or device that connects to the computer.
Different types of disks require different types of Disk Drives. A Magnetic Disk Drive reads magnetic disks, an Optical Drive reads optical disks; a Hard Disk Drive reads and writes hard disks, and a Floppy Drive reads and writes data to floppy disks.
RAID is an acronym for “redundant array of independent disks.” In its original incarnation, it stood for “redundant array of inexpensive disks.”
RAID is a method utilized to store the same data in different places – i.e. redundantly -- on multiple hard disks. It improves performance via storing data on multiple disks so that input/output operations overlap in a balanced way. The use of multiple disks increases the Mean Time Between Failures (MTBF), and the redundancy also increases fault tolerance.
To the operating system, a RAID appears as a single logical hard disk. RAID utilizes Disk Striping, which involves partitioning each drive's storage space into discrete units, ranging from a sector (512 bytes) up to several megabytes. All of the disks’ stripes are interleaved and addressed in order.
RAID is often employed in a single-user environment in which large record files, such as medical or other scientific images, are stored. Under this scenario, the stripes usually are small (such as 512 bytes) so a single record easily spans the RAID and are accessed quickly by reading all disks at the same time.
In a multi-user system, better performance requires establishing a stripe wide enough to hold the typical- to maximum-size record, allowing overlapping disk I/O across drives.
Solid State Disk
A Solid State Disk (SSD) is a high-performance, plug-and-play storage device that contains no moving parts. The components of an SSD include either DRAM or EEPROM memory boards, a memory bus board, a CPU, and a battery card.
SSDs are considerably faster that conventional rotating hard disks because they contain their own CPUs to manage data storage (18MBps for SCSI-II and 35 MBps for UltraWide SCSI interfaces). For that reason, SSDs offer the highest possible I/O rates.
When I/O response time is crucial, SSDs are most efficient for server applications and server systems. They are the best storage medium for anything that creates bottlenecks, such as swap files, authorization and login information, library and index files, and databases.