CPU 和 I/O 设备交互的方式有：polling, Interrupt

快的设备用：polling

慢的设备用：Interrupt

Thus, if a device is fast, it may be best to poll; if it is slow, interrupts, which allow overlap, are best. If the speed of the device is not known, or sometimes fast and sometimes slow, it may be best to use a hybrid that polls for a little while and then, if the device is not yet finished, uses interrupts. This two-phased approach may achieve the best of both worlds.

DMA (Direct Memory Access)

Cause DMA

怎么去除 c: CPU copy data from memory to device

Ans: Use an DMA Engine to do this tedius job

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iShot_2022-12-28_17.29.43

Hard Disk Drive

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image-20221228181440974 $$ T_{I/O} = T_{seek} + T_{rotation}+T_{transfer} $$

RAID

RAID Level 0: 没有备份

✅ Performance

✅ Capacity

❎ Reliability

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RAID 1: Mirroring

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每块盘增加一块备份盘

RAID 4: Parity 增加奇偶校验盘

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问题：比如同时修改block 4 和block 13时，需要更新P1和P3

，但是P1和P3在同一盘上，不能sitmultaneously地进行

RAID 5: Rotating Parityimage-20221228200400335