A block layer introduction part 1: the bio layer

One of the key values provided by an operating system like Linux is that it provides abstract interfaces to concrete devices. Though the original "character device" and "block device" abstractions have been supplemented with various others including "network device" and "bitmap display", the original two have not lost their importance. The block device interface, in particular, is still central to managing persistent storage and, even with the growth of persistent memory, this central role is likely to remain for some time. Unpacking and explaining some of that role is the goal of this pair of articles.

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