Alice and Bob in I/O Land

Alice and Bob go picking an elevator

Once there was a single type of elevator in the Linux "market".
Then several types of elevators were developed - but you had to choose only one for the entire system - as a kernel boot parameter.
And later on, a possibility was added to change the elevator per block device, during normal operation.
Linux elevators differ mostly in:
- how they prioritize I/O requests
- how aggressive they are in attempts to merge requests
Let us send Alice and Bob to the market, to pick an elevator.

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