ECE344H1 Lecture 31: Ece344 Lec 31 Disks and RAID
20 Dec 2018
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Document Summary
Redundancy in storage systems, raid: disks. Each platter has an arm and a head for accessing data. The arms for the different platters move together. The different heads can access data in parallel. A sector typically has a preamble, data and 16 bytes of ecc. A cylinder is the same track across different platters. Time to access a disk sector is determined by 3 delays. Time to move head to correct track. Time for disk to rotate to correct sector. Time to read/write the bits of sector. 40% per year (2x every two years) 8% per year (1/2 every 10 years) Seek and rotation time are typically 4-8 ms today. No seek time, no rotational delay, only transfer time. Seek time due to head movement, rotational delay, transfer time. Disk scheduling aims to minimize seek time and rotational delay. Older disks required os to specify all parameters for transferring data. E. g. , cylinder #, track #, sector #, transfer size, etc.
