Topic: Disks – file system devicesRotational MediaSlide 3What to do about Disks?Disk Scheduling (Seek Time)PoliciesRotational LatencySlide 8Slide 9Slide 10Build a Better Disk?RAIDRAID Level 5MEMS-based StoragePowerPoint PresentationData on Media SledDisk AnalogyLogical Blocks and LBNComparisonTopic: Disks – file system devicesRotational MediaSectorTrackCylinderHeadPlatterArm Access time = seek time + rotational delay + transfer timeseek time = 5-15 milliseconds to move the disk arm and settle on a cylinderrotational delay = 8 milliseconds for full rotation at 7200 RPM: average delay = 4 mstransfer time = 1 millisecond for an 8KB block at 8 MB/s Bandwidth utilization is less than 50% for any noncontiguous access at a block grain.Rotational MediaSectorTrackCylinderHeadPlatterArm Access time = seek time + rotational delay + transfer time + spinup timeseek time = 5-15 milliseconds to move the disk arm and settle on a cylinderrotational delay = 8 milliseconds for full rotation at 7200 RPM: average delay = 4 mstransfer time = 1 millisecond for an 8KB block at 8 MB/sSpinup/spindown time = ~1 second Bandwidth utilization is less than 50% for any noncontiguous access at a block grain.What to do about Disks?•Avoid them altogether! CachingDisk scheduling–Idea is to reorder outstanding requests to minimize seeks.•Layout on disk–Placement to minimize disk overheadBuild a better disk (or substitute)–Example: RAIDDisk Scheduling (Seek Time)•Assuming there are sufficient outstanding requests in request queue•Focus is on seek time - minimizing physical movement of head.•Simple model of seek performanceSeek Time = startup time (e.g. 3.0 ms) + N (number of cylinders ) * per-cylinder move (e.g. .04 ms/cyl)Policies•Generally use FCFS as baseline for comparison•Shortest Seek First (SSTF) -closest–danger of starvation•Elevator (SCAN) - sweep in one direction, turn around when no requests beyond–handle case of constant arrivals at same position•C-SCAN - sweep in only one direction, return to 0–less variation in response1, 3, 2, 4, 3, 5, 0FCFSSSTFSCANCSCANRotational Latency•Order requests according to sectors and service them as they rotate under read headRotational Latency•Order requests according to sectors and service them as they rotate under read headRotational Latency•Order requests according to sectors and service them as they rotate under read headRotational Latency•Order requests according to sectors and service them as they rotate under read headBuild a Better Disk?•“Better” has typically meant density to disk manufacturers - bigger disks are better.•I/O Bottleneck - a speed disparity caused by processors getting faster more quickly•One idea is to use parallelism of multiple disks–Striping data across disks–Reliability issues - introduce redundancyRAIDRedundant Array of Inexpensive DisksStriped Data Parity Disk(RAID Levels 2 and 3)RAID Level 50 1 2 3 P05 6 7 P1 410 11 P2 8 9block numbersDistribute parity info as well as data over all disksMEMS-based StorageComparing MEMS-based storage with disks–Request scheduling–Data layout–Fault tolerance–Power management• Settling time after X seek• Spring factor - non-uniform over sled positions• Turnaround timeData on Media SledDisk Analogy•16 tips•MxN = 3 x 280•Cylinder – same x offset•4 tracks of 1080 bits, 4 tips•Each track – 12 sectors of 80 bits (8 encoded bytes) •Logical blocks striped across 2 sectorsLogical Blocks and LBN•Sectors are smaller than disk•Multiple sectors can be accessed concurrently•Bidirectional accessComparisonMEMS•Positioning – X and Y seek (0.2-0.8 ms)•Settling time 0.2ms•Seeks near edges take longer due to springs, turnarounds depend on direction – it isn’t just distance to be moved.•More parts to break•Access parallelismDisk•Seek (1-15 ms) and rotational delay•Settling time 0.5ms•Seek times are relatively constant functions of distance•Constant velocity rotation occurring regardless of
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