Disk arm positions over the radial position tracks where data is stored It

# Disk arm positions over the radial position tracks

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Disk arm positions over the radial position (tracks) where data is stored It swings across tracks (but do not extend/shrink) Data is read/written by a read/write head as platter spins 21
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Internal of hard disk 23 Actuator Spindle Disk head Platter
Disk arm and platter 24
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Disk head movement Hard disk head movement while copying files between two folders (e.g., partition c to d) 28
2GB Storage in 1980s (\$250,000) 29
Physical characteristics 3.5" (diameter, common in desktops) 2.5" (common in laptops) Rotational speed 5,400 RPM 7,200 RPM 4,800 RPM 10,000 RPM (6ms/rotation) Between 5-7 platters Current capacity up to 10TB (Western Digital) 30
Disk organization Disk has multiple tracks Each track is divided into N fixed size sectors Typical sector size is 512 bytes (old) or 4KB (new) Sectors can be numbered from 0 to N-1 Entire sector is written “atomically” All or nothing 31
CHS (cylinder-head-sector) Early way to address a sector Now LBA ( Logical Block Addressing ) more common 32
A simple disk drive (one track only) 33 track sector
Rotational latency Waiting for the right sector to rotate under the head On average: ½ of time for a full rotation Worst case? Best case? 34
Rotation time Assume 10,000 RPM (rotations per minute) 𝑇𝑇𝑇𝑇 𝑇𝑚 1 𝑟𝑟𝑟𝑟𝑟𝑇𝑟𝑟 = 60000 𝑇𝑚 1000 𝑟𝑟𝑟𝑟𝑟𝑇𝑟𝑟 = 6 𝑇𝑚 𝑟𝑟𝑟𝑟𝑟𝑇𝑟𝑟 35
Multiple tracks: add seek times 36 Average seek time is about 1/3 max seek time
Transmission time Assume that we transfer 512KB Assume 128 MB/sec transmission bandwidth 512KB/128MB * 1000ms = 4ms 37
Completion time T = T seek + T rotation + T transfer T seek : Time to get the disk head on right track T rotation :Time to wait for the right sector to rotate under the head T transfer : Time to actually transfer the data 38
Example Capacity 4TB # platters: 4 # heads: 8 Bytes per sector: 4096 Transmission bandwidth: 100MB/sec Maximum seek time: 12ms RPM: 10,000 39
Time to transfer a file The file occupies 100 sectors (sequentially) Avg. seek time =? Avg. rotational latency =? Transfer time = ? 40

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