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Wednesday, January 27, 2010

Secondary Storage Structure - Disk Structure

Secondary storage structure helps to :
- 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 and HSM.

Disk provide the bulk of secondary storage for modern computer systems. Disk drives are addressed as large 1-dimensional arrays of logical blocks, where the logical block is the smallest unit of transfer. The 1-dimensional array of logical blocks is mapped onto the sectors of the disk sequentially.
- Sector 0 is the first sector of the first track on the outermost cylinder.
- Mapping proceeds in order through that track, then the rest of the tracks in that cylinder, and then through the rest of the cylinders from outermost to innermost.
- The operating system is responsible for using hardware efficiently — for the disk drives, this means having a fast access time and disk bandwidth.
- Access time has two major components.
* Seek time is the time for the disk arm to move the heads to the cylinder containing the desired sector.
* Rotational latency is the additional time waiting for the disk to rotate the desired sector to the disk head.
- Minimize seek time.
- Seek time seek distance.
- Disk bandwidth is the total number of bytes transferred, divided by the total time between the first request for service and the completion of the last transfer.
- Several algorithms exist to schedule the servicing of disk I/O requests.

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