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Elevator Algorithm
The elevator algorithm, or SCAN, is a hard disk, disk-I/O scheduling, scheduling algorithm to determine the motion of the disk's arm and head in servicing read and write requests. This algorithm is named after the behavior of a building elevator, where the elevator continues to travel in its current direction (up or down) until empty, stopping only to let individuals off or to pick up new individuals heading in the same direction. From an implementation perspective, the disk drive, drive maintains a data buffer, buffer of pending read/write requests, along with the associated Cylinder (disk drive), cylinder number of the request, in which lower cylinder numbers generally indicate that the cylinder is closer to the spindle, and higher numbers indicate the cylinder is farther away. The algorithm is largely obsolete for data storage. With the current generation of magnetic disks it is not possible to know the location of specific data on the disk and solid state memory devices ha ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Hard Disk
A hard disk drive (HDD), hard disk, hard drive, or fixed disk is an electro-mechanical data storage device that stores and retrieves digital data using magnetic storage with one or more rigid rapidly rotating hard disk drive platter, platters coated with magnetic material. The platters are paired with disk read-and-write head, magnetic heads, usually arranged on a moving actuator arm, which read and write data to the platter surfaces. Data is accessed in a random-access manner, meaning that individual Block (data storage), blocks of data can be stored and retrieved in any order. HDDs are a type of non-volatile storage, retaining stored data when powered off. Modern HDDs are typically in the form of a small disk enclosure, rectangular box. Hard disk drives were introduced by IBM in 1956, and were the dominant secondary storage device for History of general-purpose CPUs, general-purpose computers beginning in the early 1960s. HDDs maintained this position into the modern er ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
I/O Scheduling
Input/output (I/O) scheduling is the method that computer operating systems use to decide in which order I/O operations will be submitted to storage volumes. I/O scheduling is sometimes called disk scheduling. Purpose I/O scheduling usually has to work with hard disk drives that have long access times for requests placed far away from the current position of the disk head (this operation is called a seek). To minimize the effect this has on system performance, most I/O schedulers implement a variant of the elevator algorithm that reorders the incoming randomly ordered requests so the associated data would be accessed with minimal head movement. I/O schedulers can have many purposes depending on the goals; common purposes include the following * To minimize time wasted by hard disk seeks * To prioritize a certain processes' I/O requests * To give a share of the disk bandwidth to each running process * To guarantee that certain requests will be issued before a particular d ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Elevator
An elevator (American English) or lift (Commonwealth English) is a machine that vertically transports people or freight between levels. They are typically powered by electric motors that drive traction cables and counterweight systems such as a hoist, although some pump hydraulic fluid to raise a cylindrical piston like a jack. Elevators are used in agriculture and manufacturing to lift materials. There are various types, like chain and bucket elevators, grain augers, and hay elevators. Modern buildings often have elevators to ensure accessibility, especially where ramps aren't feasible. High-speed elevators are common in skyscrapers. Some elevators can even move horizontally. History Pre-industrial era The earliest known reference to an elevator is in the works of the Roman architect Vitruvius, who reported that Archimedes ( – ) built his first elevator probably in 236 BC. Sources from later periods mention elevators as cabs on a hemp rope, powered by people o ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Disk Drive
Disc or disk may refer to: * Disk (mathematics), a two dimensional shape, the interior of a circle * Disk storage * Optical disc * Floppy disk Music * Disc (band), an American experimental music band * ''Disk'' (album), a 1995 EP by Moby Other uses * Disc harrow, a farm implement * Discus throw or disc throw, a track and field event involving a heavy disc * Intervertebral disc, a cartilage between vertebrae * Disk (functional analysis), a subset of a vector space * ''Disc'' (magazine), a British music magazine * Disk, a part of a flower * Disc number, numbers assigned to Inuit by the Government of Canada * Galactic disc, a disc-shaped group of stars Abbreviations * Death-inducing signaling complex * DISC assessment, a group of psychometric tests * Defence Intelligence and Security Centre or Joint Intelligence Training Group, the headquarters of the Defence College of Intelligence and the British Army Intelligence Corps * Delaware Independent School Conference, a high- ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Data Buffer
In computer science, a data buffer (or just buffer) is a region of memory used to store data temporarily while it is being moved from one place to another. Typically, the data is stored in a buffer as it is retrieved from an input device (such as a microphone) or just before it is sent to an output device (such as speakers); however, a buffer may be used when data is moved between processes within a computer, comparable to buffers in telecommunication. Buffers can be implemented in a fixed memory location in hardware or by using a virtual data buffer in software that points at a location in the physical memory. In all cases, the data stored in a data buffer is stored on a physical storage medium. The majority of buffers are implemented in software, which typically use RAM to store temporary data because of its much faster access time when compared with hard disk drives. Buffers are typically used when there is a difference between the rate at which data is received and the rate ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Cylinder (disk Drive)
Cylinder-head-sector (CHS) is an early method for giving addresses to each physical block of data on a hard disk drive. It is a 3D-coordinate system made out of a vertical coordinate ''head'', a horizontal (or radial) coordinate ''cylinder'', and an angular coordinate ''sector''. Head selects a circular surface: a platter in the disk (and one of its two sides). Cylinder is a cylindrical intersection through the stack of platters in a disk, centered around the disk's spindle. Combined, cylinder and head intersect to a circular line, or more precisely: a circular strip of physical data blocks called ''track''. Sector finally selects which data block in this track is to be addressed, as the track is subdivided into several equally-sized portions, each of which is an arc of (360/n) degrees, where n is the number of sectors in the track. CHS addresses were exposed, instead of simple linear addresses (going from ''0'' to the ''total block count on disk - 1''), because early hard drive ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
FSCAN
FSCAN is a disk scheduling algorithm to determine the motion of the disk's arm and head in servicing read and write requests. It uses two sub-queues. During the scan, all of the requests are in the first queue and all new requests are put into the second queue. Thus, service of new requests is deferred until all of the old requests have been processed. When the scan ends, the arm is taken to the first queue entries and is started all over again. Analysis FSCAN along with N-Step-SCAN prevents "arm stickiness" unlike SSTF, SCAN, and C-SCAN. Arm stickiness in those other algorithms occurs when a stream of requests for the same track causes the disk arm to stop progressing at that track, preferring to satisfy the no-seek requests for the track it is on. Because FSCAN separates requests into two queues, with new requests going into a waiting queue, the arm continues its sweep to the outer track and is therefore not "sticky." There is an obvious trade-off in that the requests in ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
LOOK Algorithm
LOOK is a hard disk scheduling algorithm used to determine the order in which new disk read and write requests are processed. Description The LOOK algorithm, similar to the SCAN algorithm, honors requests on both sweep directions of the disk head, however, it additionally "looks" ahead to see if there are any requests pending in the direction of head movement. If no requests are pending in the direction of head movement, then the disk head traversal will be reversed to the opposite direction and requests on the other direction can be served. In LOOK scheduling, the arm goes only as far as final requests in each direction and then reverses direction without going all the way to the end. Consider an example, Given a disk with 200 cylinders (0-199), suppose we have 8 pending requests: 98, 183, 37, 122, 14, 124, 65, 67 and that the read/write head is currently at cylinder 53. In order to complete these requests, the arm will move in the increasing order first and then will move in d ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
C-LOOK
LOOK is a hard disk scheduling algorithm used to determine the order in which new disk read and write requests are processed. Description The LOOK algorithm, similar to the SCAN algorithm, honors requests on both sweep directions of the disk head, however, it additionally "looks" ahead to see if there are any requests pending in the direction of head movement. If no requests are pending in the direction of head movement, then the disk head traversal will be reversed to the opposite direction and requests on the other direction can be served. In LOOK scheduling, the arm goes only as far as final requests in each direction and then reverses direction without going all the way to the end. Consider an example, Given a disk with 200 cylinders (0-199), suppose we have 8 pending requests: 98, 183, 37, 122, 14, 124, 65, 67 and that the read/write head is currently at cylinder 53. In order to complete these requests, the arm will move in the increasing order first and then will move in de ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
N-Step-SCAN
N-Step-SCAN (also referred to as N-Step LOOK) is a disk scheduling algorithm to determine the motion of the disk's arm and head in servicing read and write requests. It segments the request queue into subqueues of length ''N''. Breaking the queue into segments of ''N'' requests makes service guarantees possible. Subsequent requests entering the request queue will not get pushed into ''N'' sized subqueues which are already full by the elevator algorithm. As such, starvation is eliminated and guarantees of service within ''N'' requests is possible. Another way to look at N-step SCAN is this: A buffer for ''N'' requests is kept. All the requests in this buffer are serviced in any particular sweep. All the incoming requests in this period are not added to this buffer but are kept up in a separate buffer. When these top ''N'' requests are serviced, the IO scheduler chooses the next ''N'' requests and this process continues. This allows for better throughput and avoids starvation. ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Shortest Seek First
Shortest seek first (or shortest seek time first) is a secondary storage scheduling algorithm to determine the motion of the disk read-and-write head in servicing read and write requests. Description This is an alternative to the first-come first-served (FCFS) algorithm. The drive maintains an incoming buffer of requests, and tied with each request is a cylinder number of the request. Lower cylinder numbers indicate the cylinder is closer to the spindle, while higher numbers indicate the cylinder is further away. The shortest seek first algorithm determines which request is closest to the current position of the head, and services that request next. Analysis The shortest seek first algorithm has the benefit of simplicity, in that overall arm movement is reduced, resulting in a lower average response time. However, since the buffer is always getting new requests, these can skew the service time of requests that may be furthest away from the disk head's current location, i ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Resource Starvation
In computer science, resource starvation is a problem encountered in concurrent computing where a process is perpetually denied necessary resources ''Resource'' refers to all the materials available in our environment which are Technology, technologically accessible, Economics, economically feasible and Culture, culturally Sustainability, sustainable and help us to satisfy our needs and want ... to process its work. Starvation may be caused by errors in a scheduling or mutual exclusion algorithm, but can also be caused by resource leaks, and can be intentionally caused via a denial-of-service attack such as a fork bomb. When starvation is impossible in a concurrent algorithm, the algorithm is called starvation-free, lockout-freed or said to have finite bypass. This property is an instance of liveness, and is one of the two requirements for any mutual exclusion algorithm; the other being correctness. The name "finite bypass" means that any process (concurrent part) of th ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
