1.介绍
磁盘调度算法用于确定磁盘I/O请求的顺序。这些算法在优化磁盘性能和减少访问时间方面起着至关重要的作用。本博文将重点介绍两种流行的磁盘调度算法：SCAN和C-SCAN。
2.理解磁盘调度
磁盘调度是将磁盘I/O请求安排得尽可能减少寻道时间和最大化吞吐量的过程。其目标是减少访问磁盘数据所需的时间，从而提高系统性能。
3.磁盘调度算法的类型
3.1 SCAN
SCAN算法将磁盘臂向一个方向移动，处理其路径上的请求。当磁盘臂到达磁盘末端时，它会改变方向，继续处理请求。
解释：
- 电梯算法： SCAN算法使臂部沿单一方向移动，类似于电梯在楼层之间移动。
- 减少寻道时间： 通过沿单一方向移动，SCAN算法减少了方向变更的次数，从而减少了寻道时间。
3.2 C-SCAN
C-SCAN 是 SCAN 算法的变体，它沿一个方向移动磁盘臂，处理请求，然后迅速返回到另一端，不处理相反方向的请求。
解释：
- 环形扫描：在到达一端后，机械臂立即返回另一端，不处理任何请求，形成环形路径。
- 更公平的分配：C-SCAN确保所有请求最终都会得到处理，防止饥饿现象。
4. 在C语言中实现磁盘调度
以下是一个简单的C程序，模拟SCAN和C-SCAN算法：
c
#include <stdio.h>
#include <stdlib.h>
void scan(int *requests, int n, int head, int direction) {
int i, j, temp;
int seek_sequence[n + 1];
int seek_operations = 0;
// Create a copy of requests
int *request_copy = malloc(n * sizeof(int));
for (i = 0; i < n; i++) {
request_copy[i] = requests[i];
}
// Sort the requests
for (i = 0; i < n; i++) {
for (j = i + 1; j < n; j++) {
if (request_copy[i] > request_copy[j]) {
temp = request_copy[i];
request_copy[i] = request_copy[j];
request_copy[j] = temp;
}
}
}
// Find the index of the head in the sorted array
int index;
for (i = 0; i < n; i++) {
if (request_copy[i] == head) {
index = i;
break;
}
}
// Service in the current direction
for (i = index; (direction == 1) ? (i < n) : (i >= 0); i += direction) {
seek_sequence[seek_operations] = request_copy[i];
seek_operations++;
}
// Reverse direction
direction = -direction;
// Move to the other end and service in the new direction
if (direction == -1) {
for (i = index - 1; i >= 0; i--) {
seek_sequence[seek_operations] = request_copy[i];
seek_operations++;
}
} else {
for (i = index + 1; i < n; i++) {
seek_sequence[seek_operations] = request_copy[i];
seek_operations++;
}
}
// Print the seek sequence
printf("SCAN Seek Sequence:\n");
for (i = 0; i < seek_operations; i++) {
printf("%d ", seek_sequence[i]);
}
printf("\n");
free(request_copy);
}
void c_scan(int *requests, int n, int head, int direction) {
int i, j, temp;
int seek_sequence[n + 1];
int seek_operations = 0;
// Create a copy of requests
int *request_copy = malloc(n * sizeof(int));
for (i = 0; i < n; i++) {
request_copy[i] = requests[i];
}
// Sort the requests
for (i = 0; i < n; i++) {
for (j = i + 1; j < n; j++) {
if (request_copy[i] > request_copy[j]) {
temp = request_copy[i];
request_copy[i] = request_copy[j];
request_copy[j] = temp;
}
}
}
// Find the index of the head in the sorted array
int index;
for (i = 0; i < n; i++) {
if (request_copy[i] == head) {
index = i;
break;
}
}
// Service in the current direction
for (i = index; (direction == 1) ? (i < n) : (i >= 0); i += direction) {
seek_sequence[seek_operations] = request_copy[i];
seek_operations++;
}
// Move to the other end without servicing
if (direction == 1) {
seek_sequence[seek_operations] = 199; // Assume disk size is 200
seek_operations++;
} else {
seek_sequence[seek_operations] = 0;
seek_operations++;
}
// Service in the new direction
if (direction == 1) {
for (i = 0; i < index; i++) {
seek_sequence[seek_operations] = request_copy[i];
seek_operations++;
}
} else {
for (i = index + 1; i < n; i++) {
seek_sequence[seek_operations] = request_copy[i];
seek_operations++;
}
}
// Print the seek sequence
printf("C-SCAN Seek Sequence:\n");
for (i = 0; i < seek_operations; i++) {
printf("%d ", seek_sequence[i]);
}
printf("\n");
free(request_copy);
}
int main() {
int requests[] = {55, 58, 39, 18, 90, 160, 150, 38, 184};
int n = sizeof(requests) / sizeof(requests[0]);
int head = 50;
int direction = 1; // 1 for moving towards higher cylinders, -1 for lower
scan(requests, n, head, direction);
c_scan(requests, n, head, direction);
return 0;
}解释：
- scan(): 实现SCAN算法。
- c_scan(): 实现C-SCAN算法。
- requests[]: 柱面请求数组。
- head: 磁头当前所在位置。
- direction: 磁头移动的方向。
5. 结论
磁盘调度算法对于优化磁盘性能至关重要。通过理解和实施这些算法，开发人员可以创建更高效的存储系统。