The Need for Change
The research editions of UNIX had a single filesystem type, as described in Chapter 6. The tight coupling between the kernel and the filesystem worked well at this stage because there was only one filesystem type and the kernel was single threaded, which means that only one process could be running in the kernel at the same time.
Before long, the need to add new filesystem types—including non-UNIX filesystems—resulted in a shift away from the old style filesystem implementation to a newer, cleaner architecture that clearly separated the different physical filesystem implementations from those parts of the kernel that dealt with file and filesystem access.
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