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LOOK Algorithm
LOOK is a disk scheduling algorithm used to determine the order in which new disk read and write requests are processed. Description The LOOK algorithm is the same as the SCAN algorithm in that it also honors requests on both sweep direction of the disk head, however, this algorithm "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 ... [...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 arm/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 ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Shortest Seek Time 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 a direct improvement upon a 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 that the cylinder is closer to the spindle, while higher numbers indicate the cylinder is farther away. The shortest seek first algorithm determines which request is closest to the current position of the head, and then services that request next. Analysis The shortest seek first algorithm has the direct benefit of simplicity and is clearly advantageous in comparison to the FIFO method, 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 t ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Elevator Algorithm
The elevator algorithm (also 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. Description When a new request arrives while the drive is idle, the initial arm/head movement will be in the direction of the cylinder where the data is stored, either ''in'' or ''out''. As addi ... [...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 th ... [...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 won't 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. An ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
