Section 5.8. Summary

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5.8. Summary

When seen from the outside, a file system is a collection of files and directories, plus operations on them. Files can be read and written, directories can be created and destroyed, and files can be moved from directory to directory. Most modern file systems support a hierarchical directory system, in which directories may have subdirectories ad infinitum.

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When seen from the inside, a file system looks quite different. The file system-designers have to be concerned with how storage is allocated, and how the system keeps track of which block goes with which file. We have also seen how different systems have different directory structures. File system reliability and performance are also important issues.

Security and protection are of vital concern to both the system users and system designers. We discussed some security flaws in older systems, and generic problems that many systems have. We also looked at authentication, with and without passwords, access control lists, and capabilities, as well as a matrix model for thinking about protection.

Finally, we studied the MINIX 3 file system in detail. It is large but not very complicated. It accepts requests for work from user processes, indexes into a table of procedure pointers, and calls that procedure to carry out the requested system call. Due to its modular structure and position outside the kernel, it can be removed from MINIX 3 and used as a free-standing network file server with only minor modifications.

Internally, MINIX 3 buffers data in a block cache and attempts to read ahead when making sequential access to file. If the cache is made large enough, most program text will be found to be already in memory during operations that repeatedly access a particular set of programs, such as a compilation.