98, 183, 37, 122, 14, 124, 65, 67 Head pointer 53ĥ Illustration shows total head movement of 640 cylinders.įCFS Illustration shows total head movement of 640 cylinders.Ħ SSTF Selects the request with the minimum seek time from the current head position. We illustrate them with a request queue (0-199). Several algorithms exist to schedule the servicing of disk I/O requests. Minimize seek time Seek time seek distance Disk bandwidth 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. Rotational latency is the additional time waiting for the disk to rotate the desired sector to the disk head. Access time has two major components Seek time is the time for the disk are to move the heads to the cylinder containing the desired sector. 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.ģ Disk Scheduling The operating system is responsible for using hardware efficiently - for the disk drives, this means having a fast access time and disk bandwidth. Sector 0 is the first sector of the first track on the outermost cylinder. The 1-dimensional array of logical blocks is mapped into the sectors of the disk sequentially. Disk Structure Disk Scheduling Disk Management Swap-Space Management Disk Reliability Stable-Storage Implementation Tertiary Storage Devices Operating System Issues Performance IssuesĢ Disk Structure Disk drives are addressed as large 1-dimensional arrays of logical blocks, where the logical block is the smallest unit of transfer.
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