DiskManagementtrack'tspindleBadBlockManagementarmassemsectorsread-writecylindercheadplatter小arMaintaina list of bad blocks (initialized duringlow-level formatting)andpreserveanamountofsparesectorsSector sparing/forwarding: replace a bad sector logically with one sparesectorProblem:invalidatedisk scheduling algorithmSolution:spare sectors in eachcylinder+sparecylinderSector slipping: remap to the next sector (data movement is needed)
Disk Management Bad Block Management Maintain a list of bad blocks (initialized during low-level formatting) and preserve an amount of spare sectors Sector sparing/forwarding: replace a bad sector logically with one spare sector • Problem: invalidate disk scheduling algorithm • Solution: spare sectors in each cylinder + spare cylinder Sector slipping: remap to the next sector (data movement is needed)
TopicsDiskstructureDisk schedulingSolid-state drives (SSDs)RAID & Erasure coding12
12 Topics - Disk structure - Disk scheduling - Solid-state drives (SSDs) - RAID & Erasure coding
Whyneeded? Requests are placed in the queue of pendingrequests for that drive if the drive/controller is busyPendingR1R2RnqueueRead/write,diskaddress,memoryaddress,numberofsectorstobetransferred13
Why needed? • Requests are placed in the queue of pending requests for that drive if the drive/controller is busy 13 R1 R2 . Rn Pending queue Read/write, disk address, memory address, number of sectors to be transferred
What is disk schedulingtrack tspindleI/O access procedurearmassemb-Seeksectors.move thehead to thedesired cylinderread-writecylinder c-- RotateheadspintothetargetsectoronthetrackplatterarmrotationRequest ordering significantlyaffectsTrack/theaccess performance (seek + rotate)Cylinderso scheduling is neededDisk scheduling:Choose the nextSectorrequestinthe pendingqueue toserviceso as to minimize the seek timeHeads8Heads,(schedulingalgorithms)4Platters14
What is disk scheduling • I/O access procedure – Seek • move the head to the desired cylinder – Rotate • spin to the target sector on the track 14 Request ordering significantly affects the access performance (seek + rotate), so scheduling is needed Disk scheduling：Choose the next request in the pending queue to service so as to minimize the seek time (scheduling algorithms)
FCFSScheduling· First-come, first-served (FCFS)-Intrinsicallyfair,but does not provide the fastest service15
FCFS Scheduling • First-come, first-served (FCFS) – Intrinsically fair, but does not provide the fastest service 15