Elevator algorithm
Encyclopedia
The elevator algorithm (also SCAN) is a disk
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 drive maintains a buffer of pending read/write requests, along with the associated cylinder
number of the request. Lower cylinder numbers indicate that the cylinder is closest to the spindle, and higher numbers indicate the cylinder is further away.
Other variations include:
Both SCAN and C-SCAN behave in the same manner until they reach the last track queued. For the sake of this example let us assume that the SCAN algorithm is currently going from a lower track number to a higher track number (like the C-SCAN is doing). For both methods, one takes the difference in magnitude (i.e. absolute value) between the next track request and the current track.
At this point both have reached the highest (end) track request. SCAN will just reverse direction and service the next closest disk request (in this example, 20) and C-SCAN will always go back to track 0 and start going to higher track requests.
Note: Even though six seeks were performed using the C-SCAN algorithm, only five I/Os were actually done.
However, the elevator algorithm is not always better than Shortest seek first
, which is slightly closer to optimal, but can result in high variance in response time and even in starvation
when new requests continually get serviced prior to existing requests.
Anti-starvation techniques can be applied to the shortest seek time first algorithm to guarantee an optimum response time.
Hard disk
A hard disk drive is a non-volatile, random access digital magnetic data storage device. It features rotating rigid platters on a motor-driven spindle within a protective enclosure. Data is magnetically read from and written to the platter by read/write heads that float on a film of air above the...
scheduling
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.
This algorithm is named after the behavior of a building elevator
Elevator
An elevator is a type of vertical transport equipment that efficiently moves people or goods between floors of a building, vessel or other structures...
, 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 drive maintains a buffer of pending read/write requests, along with the associated cylinder
Cylinder (disk drive)
A disk drive cylinder is a division of data in a disk drive, as used in the CHS addressing mode of a Fixed Block Architecture disk or the cylinder–head–record addressing mode of a CKD disk. The concept is concentric, hollow, cylindrical slices through the physical disks , collecting the respective...
number of the request. Lower cylinder numbers indicate that the cylinder is closest to the spindle, and higher numbers indicate the cylinder is further 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 additional requests arrive, requests are serviced only in the current direction of arm movement until the arm reaches the edge of the disk. When this happens, the direction of the arm reverses, and the requests that were remaining in the opposite direction are serviced, and so on.Variations
One variation of this method ensures all requests are serviced in only one direction, that is, once the head has arrived at the outer edge of the disk, it returns to the beginning and services the new requests in this one direction only (or vice versa). This is known as the "Circular Elevator Algorithm" or C-SCAN. This results in more equal performance for all head positions, as the expected distance from the head is always half the maximum distance, unlike in the standard elevator algorithm where cylinders in the middle will be serviced as much as twice as often as the innermost or outermost cylinders.Other variations include:
- FSCANFSCANFScan 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...
- 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....
Example
The following is an example of how to calculate average disk seek times for both the SCAN and C-SCAN algorithms.- Example list of pending disk requests (listed by track number): 100, 50, 10, 20, 75.
- The starting track number for the examples will be 35.
- The list will need to be sorted in ascending order: 10, 20, 50, 75, 100.
Both SCAN and C-SCAN behave in the same manner until they reach the last track queued. For the sake of this example let us assume that the SCAN algorithm is currently going from a lower track number to a higher track number (like the C-SCAN is doing). For both methods, one takes the difference in magnitude (i.e. absolute value) between the next track request and the current track.
- Seek 1 : 50 - 35 = 15
- Seek 2 : 75 - 50 = 25
- Seek 3 : 100 - 75 = 25
At this point both have reached the highest (end) track request. SCAN will just reverse direction and service the next closest disk request (in this example, 20) and C-SCAN will always go back to track 0 and start going to higher track requests.
- Seek 4 (SCAN) : 20 - 100 = 80
- Seek 5 (SCAN) : 10 - 20 = 10
- Total (SCAN) : 155
- Average (SCAN) : 155 / 5 = 31
- Seek 4 (C-SCAN) : 0 - 100 = 100 // c-scan always goes back to the first track
- Seek 5 (C-SCAN) : 10 - 0 = 10
- Seek 6 (C-SCAN) : 20 - 10 = 10
- Total (C-SCAN) : 185
- Average (C-SCAN) : 185 / 5 = 37
Note: Even though six seeks were performed using the C-SCAN algorithm, only five I/Os were actually done.
Analysis
The arm movement is thus always less than twice the number of total cylinders then, for both versions of the elevator algorithm. The variation has the advantage to have a smaller variance in response time. The algorithm is also relatively simple.However, the elevator algorithm is not always better than Shortest seek first
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 :...
, which is slightly closer to optimal, but can result in high variance in response time and even in starvation
Resource starvation
In computer science, starvation is a multitasking-related problem, where a process is perpetually denied necessary resources. Without those resources, the program can never finish its task....
when new requests continually get serviced prior to existing requests.
Anti-starvation techniques can be applied to the shortest seek time first algorithm to guarantee an optimum response time.
See also
- FCFSFIFOFIFO is an acronym for First In, First Out, an abstraction related to ways of organizing and manipulation of data relative to time and prioritization...
- FSCANFSCANFScan 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...
- 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...
- 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....
- Shortest seek time firstShortest seek firstShortest 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 :...