Dell™ PowerEdge™ Expandable RAID Controller 4/SC and 4/DC User's Guide
A • C • D • F • G • H • I • L • M • O • P • R • S
A grouping of hard drives that combines the storage space on the hard drives into a single segment of contiguous storage space. The RAID controller can group hard drives on one or more channels into an array. A hot spare drive does not participate in an array.
Array spanning by a logical drive combines storage space in two arrays of hard drives into a single, contiguous storage space in a logical drive. The logical drive can span consecutively numbered arrays, each having the same number of hard drives. Array spanning promotes RAID level 1 to RAID level 10. See also Disk Spanning, and Spanning.
Operations that are not related to each other in time and can overlap. The concept of asynchronous I/O operations is central to independent access arrays in throughput-intensive applications.
A small amount of fast memory that holds recently accessed data. Caching speeds subsequent access to the same data. It is most often applied to processor-memory access, but can also be used to store a copy of data accessible over a network. When data is read from or written to main memory, a copy is also saved in cache memory with the associated main memory address. The cache memory software monitors the addresses of subsequent reads to see if the required data is already stored in cache memory. If it is already in cache memory (a cache hit), it is read from cache memory immediately and the main memory read is aborted (or not started.) If the data is not cached (a cache miss), it is fetched from main memory and saved in cache memory.
An electrical path for the transfer of data and control information between a disk and a disk controller.
An examination of the data in the hard drives in a logical drive to ensure that the data is redundant.
A cold swap requires that you power down the system before replacing a defective hard drive in a disk subsystem.
The amount of data per unit time moved through a channel. For disk I/O, bandwidth is expressed in megabytes per second (MB/s).
A logical drive that has become non-functional or has a hard drive that is non-functional.
A non-volatile, randomly addressable, rewritable mass storage device, including both rotating magnetic and optical disks and solid-state disks, or non-volatile electronic storage elements. It does not include specialized devices such as write-once-read-many (WORM) optical disks, nor does it include so-called RAM disks implemented using software to control a dedicated portion of a host computer's volatile random access memory.
A collection of disks from one or more disk subsystems combined using a configuration utility. The utility controls the disks and presents them to the array operating environment as one or more logical drives.
Disk mirroring is the process of duplicating the data onto another drive (RAID 1) or set of drives (in RAID 10), so that if a drive fails, the other drive has the same data and no data is lost.
Disk spanning allows multiple logical drives to function as one big logical drive. Spanning overcomes lack of disk space and simplifies storage management by combining existing resources or adding relatively inexpensive resources. See also Array Spanning and Spanning.
A type of disk array mapping. Consecutive stripes of data are mapped round-robin to consecutive array members. A striped array (RAID level 0) provides high I/O performance at low cost, but provides no data redundancy.
A collection of disks and the hardware that connects them to one or more host computers. The hardware can include an intelligent controller, or the disks can attach directly to a host computer.
A technique that achieves maximum data transfer bandwidth by constantly keeping two I/O requests for adjacent data outstanding. A software component begins a double-buffered I/O stream by issuing two requests in rapid sequence. Thereafter, each time an I/O request completes, another is immediately issued. If the disk subsystem is capable of processing requests fast enough, double buffering allows data to be transferred at the full-volume transfer rate.
A drive that has ceased to function or consistently functions improperly.
Software stored in read-only memory (ROM) or Programmable ROM (PROM). Firmware is often responsible for the startup routines and low-level I/O processes of a system when it is first turned on.
The FlexRAID Power Fail option allows a reconstruction to restart if a power failure occurs. This is the advantage of this option.
The process of writing zeros to all data fields in a physical drive (hard drive) to map out unreadable or bad sectors. Because most hard drives are factory formatted, formatting is usually only done if a hard disk generates many media errors.
(gigabyte) 1,073,741,824 bytes. It is the same as 1,024 MB (megabytes).
Any computer to which disks are directly attached. Mainframes, servers, workstations, and personal computers can all be considered host computers.
A stand-by drive ready for use if another drive fails. It does not contain any user data. Up to eight hard drives can be assigned as hot spares for an adapter.
The substitution of a replacement unit in a disk subsystem for a defective one, where the substitution can be performed while the subsystem is running (performing its normal functions). Hot swaps are manual.
(Integrated Device Electronics) Also known at ATA (Advanced Technology Attachment), this is a type of interface for the hard drive, in which the controller electronics are integrated onto the drive itself. With IDE, a separate adapter card is no longer needed; this reduces interface costs and makes it easier to implement firmware.
A host computer software component (usually part of the operating system) that controls the operation of peripheral controllers or adapters attached to the host computer. I/O drivers communicate between applications and I/O devices, and in some cases participates in data transfer.
The process of writing zeros to the data fields of a logical drive and generating the corresponding parity to bring the logical drive to a Ready state. Initializing erases previous data and generates parity so that the logical drive will pass a consistency check. Arrays can work without initializing, but they can fail a consistency check because the parity fields have not been generated.
A set of contiguous chunks on a physical disk. Logical disks are used in array implementations as constituents of logical volumes or partitions. Logical disks are normally transparent to the host environment, except when the array containing them is being configured.
A virtual drive within an array that can consist of more than one physical drive. Logical drives divide the contiguous storage space of an array of hard drives or a spanned group of arrays of drives. The storage space in a logical drive is spread across all the physical drives in the array or spanned arrays.
The conversion between multiple data addressing schemes, especially conversions between member disk block addresses and block addresses of the virtual disks presented to the operating environment.
(Megabyte) An abbreviation for 1,048,576 (10²) bytes. It is the same as 1,000 KB (kilobytes).
Having multiple concurrent or pseudo-concurrent execution sequences. Used to describe processes in computer systems. Multi-threaded processes allow throughput-intensive applications to efficiently use a disk array to increase I/O performance.
The operating environment includes the host computer where the group of hard drives is attached, any I/O buses and controllers, the host operating system, and any additional software required to operate the array. For host-based arrays, the operating environment includes I/O driver software for the member disks.
Parity is an extra bit added to a byte or word to reveal errors in storage (in RAM or disk) or transmission. Parity is used to generate a set of redundancy data from two or more parent data sets. The redundancy data can be used to reconstruct one of the parent data sets; however, parity data does not fully duplicate the parent data sets. In RAID, this method is applied to entire drives or stripes across all hard drives in an array. Parity consists of dedicated parity, in which the parity of the data on two or more drives is stored on an additional drive, and distributed parity, in which the parity data are distributed among all the drives in the system. If a single drive fails, it can be rebuilt from the parity of the respective data on the remaining drives.
A separate logical area of memory or a storage device that acts as though it were a physically separate area.
A hard drive that stores data. A hard drive consists of one or more rigid magnetic discs rotating about a central axle with associated read/write heads and electronics.
The ability of some adapters to detect when hard drives have been moved to a different slots in the computer, for example, after a hot swap.
(Redundant Array of Independent Disks) An array of multiple independent hard disk drives that yields better performance than a Single Large Expensive Disk (SLED). A RAID disk subsystem improves I/O performance on a server using only a single drive. The RAID array appears to the host server as a single storage unit. I/O is expedited because several disks can be accessed simultaneously.
A style of redundancy applied to a logical drive. It can increase the performance of the logical drive and can decrease usable capacity. Each logical drive must have a RAID level assigned to it. The RAID level drive requirements are: RAID 0 requires at least one physical drive, RAID 1 requires two physical drives, RAID 5 requires at least three physical drives and RAID 10 requires at least four physical drives. RAID 10 results when a RAID 1 logical drive spans arrays.
RAID migration is used to move between optimal RAID levels or to change from a degraded redundant logical drive to an optimal RAID 0. In Novell, the utility used for RAID migration is MEGAMGR.
A memory caching capability in some adapters that allows them to read sequentially ahead of requested data and store the additional data in cache memory, anticipating that the additional data will be needed soon. Read-Ahead supplies sequential data faster, but is not as effective when accessing random data.
A condition in which a workable hard drive is neither online nor a hot spare and is available to add to an array or to designate as a hot spare.
The regeneration of all data from a failed disk in a RAID level 1, 5, 10, or 5 array to a replacement disk. A disk rebuild normally occurs without interruption of application access to data stored on the array virtual disk.
The percentage of CPU resources devoted to rebuilding.
The act of remaking a logical drive after changing RAID levels or adding a physical drive to an existing array.
The provision of multiple interchangeable components to perform a single function to cope with failures or errors. Redundancy normally applies to hardware; a common form of hardware redundancy is disk mirroring.
A disk available to replace a failed member disk in a RAID array.
A component or collection of components in a disk subsystem that are always replaced as a unit when any part of the collection fails. Typical replacement units in a disk subsystem includes disks, controller logic boards, power supplies, and cables. Also called a hot spare.
(small computer system interface) A processor-independent standard for system-level interfacing between a computer and intelligent devices, including hard disks, diskettes, CD drives, printers, scanners, etc. SCSI can connect up to seven devices to a single adapter (or host adapter) on the computer's bus. SCSI transfers eight or 16 bits in parallel and can operate in either asynchronous or synchronous modes. The synchronous transfer rate is up to 320 MB/s. SCSI connections normally use single ended drivers, as opposed to differential drivers.
The original standard is now called SCSI-1 to distinguish it from SCSI-2 and SCSI-3, which include specifications of Wide SCSI (a 16-bit bus) and Fast SCSI (10 MB/s transfer.) Ultra 160M SCSI is a subset of Ultra3 SCSI and allows a maximum throughput of 160 MB/s, which is more than twice as fast as Wide Ultra2 SCSI. Ultra320 SCSI allows a maximum throughput of 320 MB/s.
Array spanning by a logical drive combines storage space in two arrays of hard drives into a single, contiguous storage space in a logical drive. Logical drives can span consecutively numbered arrays that each consist of the same number of hard drives. Array spanning promotes RAID level 1 to RAID levels 10. See also Array Spanning, and Disk Spanning.
A hard drive available to back up the data of other drives.
The amount of data contiguously written to each disk. You can specify stripe sizes of 4 KB, 8 KB, 16 KB, 32 KB, 64 KB, and 128 KB for each logical drive. For best performance, choose a stripe size equal to or smaller than the block size used by the host computer.
The number of hard drives across which the data are striped.
Segmentation of logically sequential data, such as a single file, so that segments can be written to multiple physical devices in a round-robin fashion. This technique is useful if the processor can read or write data faster than a single disk can supply or accept it. While data is being transferred from the first disk, the second disk can locate the next segment. Data striping is used in some modern databases and in certain RAID devices.