disksraid-09 - Disks and RAID 50 Years Old! 13th September...

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Disks and RAID
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50 Years Old! 13th September 1956 The IBM RAMAC 350
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80000 times more data on the 8GB 1-inch drive in his right hand than on the 24-inch RAMAC one in his left…
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What does the disk look like?
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Some parameters 2-30 heads (platters * 2) – diameter 14’’ to 2.5’’ 700-20480 tracks per surface 16-1600 sectors per track sector size: 64-8k bytes 512 for most PCs note: inter-sector gaps capacity: 20M-100G main adjectives: BIG, slow
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Disk overheads To read from disk, we must specify: cylinder #, surface #, sector #, transfer size, memory address Transfer time includes: Seek time: to get to the track Latency time: to get to the sector and Transfer time: get bits off the disk Track Sector Seek Time Rotation Delay
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Modern disks Barracuda 180 Cheetah X15 36LP Capacity 181GB 36.7GB Disk/Heads 12/24 4/8 Cylinders 24,247 18,479 Sectors/track ~609 ~485 Speed 7200RPM 15000RPM Latency (ms) 4.17 2.0 Avg seek (ms) 7.4/8.2 3.6/4.2 Track-2- track(ms) 0.8/1.1 0.3/0.4
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Disks vs. Memory Smallest write: sector Atomic write = sector Random access: 5ms not on a good curve Sequential access: 200MB/s Cost $.002MB Crash: doesn’t matter (“non- volatile”) (usually) bytes byte, word 50 ns faster all the time 200-1000MB/s $.10MB contents gone (“volatile”)
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Disk Structure Disk drives addressed as 1-dim arrays of logical blocks the logical block is the smallest unit of transfer This array mapped sequentially onto disk sectors Address 0 is 1 st sector of 1 st track of the outermost cylinder Addresses incremented within track, then within tracks of the cylinder, then across cylinders, from innermost to outermost Translation is theoretically possible, but usually difficult Some sectors might be defective Number of sectors per track is not a constant
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Non-uniform #sectors / track Reduce bit density per track for outer layers (Constant Linear Velocity, typically HDDs) Have more sectors per track on the outer layers, and increase rotational speed when reading from outer tracks (Constant Angular Velcity, typically CDs, DVDs)
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Disk Scheduling The operating system tries to use hardware efficiently – for disk drives having fast access time, disk bandwidth Access time has two major components Seek time is time to move the heads to the cylinder containing the desired sector Rotational latency is additional time waiting to rotate the desired sector to the disk head. Minimize seek time Seek time seek distance Disk bandwidth is total number of bytes transferred, divided by the total time between the first request for service and the completion of the last transfer.
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Several scheduling algos exist service disk I/O requests. We illustrate them with a request queue (0-199).
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This note was uploaded on 02/19/2008 for the course CS 4410 taught by Professor Vollset during the Fall '07 term at Cornell University (Engineering School).

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disksraid-09 - Disks and RAID 50 Years Old! 13th September...

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