ECS 154A Lecture Notes - Lecture 23: Constant Angular Velocity, Defragmentation, Parity Bit
2 Aug 2016
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Lectures 23: external memory: magnetic read and write mechanisms. Write head is has coils of wire around a gapped rectangular doughnut. Disadvantage: outer sectors have lower density than possible: multiple zoned layout has the number of sectors based on the circumference of the tracks. Zero on fixed head drives: rotational delay = average time it takes a sector to reach the head. Raid = redundant array of independent disks: three common characteristics, viewed by the os as a single logical drive, data is distributed across the physical drives of an array in a scheme known as striping. A logical drive is divided into strips that may be physical blocks, sectors, or some other unit. Transfers can be up to ~n times faster than a single disk if the request is for contiguous data of at least n * sizeof(strip) bytes.
