FSCAN
Encyclopedia
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 subqueues. 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.
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 the waiting queue must wait longer to be fulfilled, but in exchange FSCAN is more fair to all requests.
I/O scheduling
Input/output scheduling is a term used to describe the method computer operating systems decide the order that block I/O operations will be submitted to storage volumes...
algorithm to determine the motion of the disk's arm and head in servicing read and write requests.
It uses two subqueues. During the scan, all of the requests are in the first queue and all new requests are put into the second queue
Queue
A queue is a particular kind of collection in which the entities in the collection are kept in order and the principal operations on the collection are the addition of entities to the rear terminal position and removal of entities from the front terminal position. This makes the queue a...
. 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-SCANN-Step-SCAN
N-Step-SCAN is a disk scheduling algorithm to determine the motion of the disk's arm and head in servicing read and write requests....
prevents "arm stickiness" unlike SSTF
Shortest seek first
Shortest seek first is a secondary storage scheduling algorithm to determine the motion of the disk's arm and head in servicing read and write requests.- Description :...
, SCAN
Elevator algorithm
The elevator algorithm is a disk scheduling algorithm to determine the motion of the disk's arm and head in servicing read and write requests....
, 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 the waiting queue must wait longer to be fulfilled, but in exchange FSCAN is more fair to all requests.
See also
Other variations include:- SCANElevator algorithmThe elevator algorithm is a disk scheduling algorithm to determine the motion of the disk's arm and head in servicing read and write requests....
- Elevator algorithm - LOOKLOOK algorithmLOOK is a disk scheduling algorithm used to determine the order in which new disk read and write requests are processed.-Description:LOOK is similar to SCAN in that the heads sweep across the disk surface in both directions performing reads and writes...
(and C-LOOK) - N-Step-SCANN-Step-SCANN-Step-SCAN is a disk scheduling algorithm to determine the motion of the disk's arm and head in servicing read and write requests....