12_secondary storage

12_secondary storage - Chapter 12: Chapter...

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Chapter 12: Chapter 12: Secondary Secondary - - Storage Structure Storage Structure
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12.2 Modified by Bo Li ©2009 Operating System Concepts Chapter 12: Secondary Chapter 12: Secondary - - Storage Structure Storage Structure Overview of Mass Storage Structure Disk Structure Disk Attachment Disk Scheduling Disk Management RAID Structure Operating System Issues Performance Issues
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12.3 Modified by Bo Li ©2009 Operating System Concepts Objectives Objectives Describe the physical structure of secondary and tertiary storage devices and the resulting effects on the uses of the devices Explain the performance characteristics of mass-storage devices Discuss operating-system services provided for mass storage, including RAID
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12.4 Modified by Bo Li ©2009 Operating System Concepts Overview of Mass Storage Structure Overview of Mass Storage Structure Magnetic disks provide bulk of secondary storage of modern computers Drives rotate at 60 to 200 times per second The surface of a platter is logically divided into circular tracks , which is subdivided into sectors (hundreds of). The set of tracks (thousands of) that at one arm position makes up a cylinder Transfer rate is rate at which data flow between drive and computer, typically several megabytes of data per second Positioning time ( random-access time ) is time to move disk arm to desired cylinder ( seek time ) and time for desired sector to rotate under the disk head ( rotational latency ), usually several milliseconds. Drive attached to computer via I/O bus Busses vary, including EIDE, ATA, SATA, USB, Fibre Channel, SCSI Host controller in computer uses communicates with disk controller built into drive or storage array through bus
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12.5 Modified by Bo Li ©2009 Operating System Concepts Moving Moving - - head Disk Mechanism head Disk Mechanism
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12.6 Modified by Bo Li ©2009 Operating System Concepts Disk Performance Examples Disk Performance Examples Assumptions: Ignoring queuing and controller times for now Avg seek time of 5ms, 7200RPM Time for one rotation: 8ms Transfer rate of 4MByte/s, sector size of 1 KByte Read sector from random place on disk: Seek (5ms) + Rot. Delay (4ms) + Transfer (0.25ms) Approx 10ms to fetch/put data: 100 KByte/sec Read sector from random place in same cylinder: Rot. Delay (4ms) + Transfer (0.25ms) Approx 5ms to fetch/put data: 200 KByte/sec Read next sector on same track: Transfer (0.25ms): 4 MByte/sec Key to using disk effectively (esp. for file systems) is to minimize seek and rotational delays
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12.7 Modified by Bo Li ©2009 Operating System Concepts Hard Disk Drives Hard Disk Drives IBM/Hitachi Microdrive Western Digital Drive http://www.storagereview.com/guide/ Read/Write Head Side View
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12.8 Modified by Bo Li ©2009 Operating System Concepts Disk Structure Disk Structure Disk drives are addressed as large 1-dimensional arrays of logical blocks
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12_secondary storage - Chapter 12: Chapter...

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