lec1b - 5. Hard Disks Hard Disks read-write head (contd)...

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5. Hard Disks Hard Disks (cont’d) read-write head positioned very close to the platter surface (almost touching it) reads or writes magnetically encoded information. surface of platter divided into circular tracks over 16,000 tracks per platter on typical hard disks each track is divided into sectors. a sector is the smallest unit of data that can be read or written. sector size typically 512 bytes typical sectors per track: 200 (on inner tracks) to 400 (on outer tracks) Hard Disks (cont’d) To read/write a sector disk arm swings to position head on right track platter spins continually; data is read/written as sector passes under head Head-disk assemblies multiple disk platters on a single spindle (typically 2 to 4) one head per platter, mounted on a common arm But often no concurrent access Performance Measures of Hard Disks access time – the time it takes from when a read or write request is issued to when data transfer begins. Consists of: seek time – time it takes to reposition the arm over the correct track. Average seek time is 1/2 the worst case seek time. 4 to 10 milliseconds on typical disks rotational latency – time it takes for the sector to be accessed to appear under the head. Average latency is 1/2 of the worst case latency. 4 to 11 milliseconds on typical disks (5400 to 15000 r.p.m.) data-transfer rate – the rate at which data can be retrieved from or stored to the disk. 20 to 60 MB per second is typical Disk Access Cost Model Average seek time: 4.8ms Average rotational latency: 4.2ms (7200 RPM) Data-transfer rate: 20MB per second The time to read/write a 4k data block on this hard disk is: 4.8ms + 4.2ms + 4KB / (20000KB/s) = 9ms + 0.2ms = 9.2ms The time to read/write 4MB of data that is separately stored on the hard disk as 1,000 blocks of 4k each, is: 9.2*1000 = 9200ms The time to read/write 4MB of data that is stored in consecutive form on this hard disk is: 4.8 + 4.2 + 4000KB / (20000KB/s) = 9ms + 200ms = 209ms Random accesses for small pieces of data on hard disks are
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lec1b - 5. Hard Disks Hard Disks read-write head (contd)...

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