Data Caching
Temporary storage of new write data or high-demand read data in solid state memory in order to accelerate performance. The cached data is later overwritten with newly cached data once it is either written to disk or deemed to be of low demand.
Demand caching
A performance caching technique in which the currently requested data is read in anticipation of another request before its allocated blocks are recycled. Reassignment of the blocks is done on the basis of least recently used (LRU).
Degraded Mode
A RAID mode used when a component drive has failed.
Disk
A non-volatile, randomly addressable, re-writable data storage device, including rotating magnetic and optical disks as well as solid-state disks or other electronic storage elements.
Disk Array
A collection of disks from one or more commonly accessible disk systems. Disk arrays, also known as RAID, allow disk drives to be used together to improve fault tolerance, performance, or both. Disk arrays are commonly used on servers and are becoming more popular on desktops and workstations. See also Array.
Disk Drive
A device for the electronic digital storage of information.
Disk Failure Detection
A RAID controller automatically detects SCSI disk failures. A monitoring process running on the controller checks, among other things, elapsed time on all commands issued to disks. A time-out causes the disk to be “reset” and the command to be retried. If the command times out again, the controller could take the disk “offline.” Mylex DAC960 controllers also monitor SCSI bus parity errors and other potential problems. Any disk with too many errors will also be taken “offline.” See also Offline.
Disk Traveling, Drive Traveling
A process that occurs when the drives are placed in a different order than the original order. Disk traveling can occur whether or not a drive has failed.
Disk System
A storage system capable of supporting only disks.
Drive Groups, Drive Packs
A group of individual disk drives (preferably identical) that are logically tied to each other and are addressed as a single unit. In some cases this may be called a drive “pack” when referring to just the physical devices. All the physical devices in a drive group should have the same size; otherwise, each of the disks in the group will effectively have the capacity of the smallest member. The total size of the drive group will be the size of the smallest disk in the group multiplied by the number of disks in the group. For example, if you have 4 disks of 400MB each and 1 disk of 200MB in a pack, the effective capacity available for use is only 1000MB (5×200), not 1800MB.
Disk/data striping
Spreading data evenly over multiple disks to enhance performance. Sometimes referred to as RAID 0, data striping actually has no redundancy scheme and, therefore, does not provide any fault tolerance (data protection).
Drive
Synonym for disk, hard drive, hard disk, disk drive.
Dual Active
A pair of components, such as storage controllers in a failure tolerant storage system, that share a task or set of tasks when both are functioning normally. When one component of the pair fails, the other takes the entire load. Dual active controllers (also called Active/Active controllers) are connected to the same set of devices and provide a combination of higher I/O performance and greater failure tolerance than a single controller.
Duplexing
Mirroring across two host adapters. Used only with software-based RAID storage systems (usually the embedded network operating system RAID software such as NetWare and Windows NT).