Outline Disk Structure “口4814在：4生”主QC
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Outline Disk Structure
Disk Structure Disk drives are addressed as large 1-D arrays of logical blocks, The logical block is the smallest unit of transfer. Usually,512B The 1-D array of logical blocks is mapped into the sectors of the disk sequentially. Cylinder:track:sector Sector 0 is the first sector of the first track on the outermost cylinder. Mapping proceeds in order through that track,then the rest of the tracks in that cylinder,and then through the rest of the cylinders from outermost to innermost. However,in practise,the mapping is difficult,because 1.Defective sectors 2.Sectors/track constant →zones of cylinder 4口”484在4色，主月QC
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Disk Structure ▶ Disk drives are addressed as large 1-D arrays of logical blocks, ▶ The logical block is the smallest unit of transfer. ▶ Usually, 512B ▶ The 1-D array of logical blocks is mapped into the sectors of the disk sequentially. ▶ Cylinder: track: sector ▶ Sector 0 is the first sector of the first track on the outermost cylinder. ▶ Mapping proceeds in order through that track, then the rest of the tracks in that cylinder, and then through the rest of the cylinders from outermost to innermost. ▶ However, in practise, the mapping is difficult, because 1. Defective sectors 2. Sectors/track ̸= constant ⇒ zones of cylinder
Outline Disk Scheduling(磁盘调度) 4口”484在4色，主月QC
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Outline Disk Scheduling (磁盘调度)
Disk Scheduling(磁盘调度) The OS is responsible for using hardware efficiently For the disk drives,this means having a fast access time and disk bandwidth. Access time has two major components 1.Seek time is the time for the disk to move the heads to the cylinder containing the desired sector. Minimize seek time Seek time seek distance 2.Rotational latency is the additional time waiting for the disk to rotate the desired sector to the disk head. Disk bandwidth(磁盘带宽)is the total number of bytes transferred,divided by the total time between the first request for service and the completion of the last transfer. 口”484在年生”主)QC
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Disk Scheduling (磁盘调度) ▶ The OS is responsible for using hardware efficiently. For the disk drives, this means having a fast access time and disk bandwidth. ▶ Access time has two major components 1. Seek time is the time for the disk to move the heads to the cylinder containing the desired sector. ▶ Minimize seek time ▶ Seek time ≈ seek distance 2. Rotational latency is the additional time waiting for the disk to rotate the desired sector to the disk head. ▶ Disk bandwidth (磁盘带宽) is the total number of bytes transferred, divided by the total time between the first request for service and the completion of the last transfer
Disk Scheduling(磁盘调度) Request queue(请求队列) empty or not How? Several algorithms exist to schedule the servicing of disk I/O requests. 1.FCFS 2.SSTF (shortest-seek-time-first) 3.SCAN(elevator algorithm) 4.C-SCAN 5.C-L00K We illustrate them with a request queue(0-199). 98,183,37,122,14,124,65,67 Head points to 53 initially 口”4814在年生”主)QC
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Disk Scheduling (磁盘调度) ▶ Request queue (请求队列) ▶ empty or not ▶ How? Several algorithms exist to schedule the servicing of disk I/O requests. 1. FCFS 2. SSTF (shortest-seek-time-first) 3. SCAN (elevator algorithm) 4. C-SCAN 5. C-LOOK ▶ We illustrate them with a request queue (0-199). 98, 183, 37, 122, 14, 124, 65, 67 Head points to 53 initially