Chapter 8 File system

The purpose of a file system is to organize and store data. File systems typically support sharing of data among users and applications, as well as persistence so that data is still available after a reboot.

The xv6 file system provides Unix-like files, directories, and pathnames (see Chapter 1), and stores its data on a virtio disk for persistence. The file system addresses several challenges:

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  The file system needs on-disk data structures to represent the tree of named directories and files, to record the identities of the blocks that hold each file’s content, and to record which areas of the disk are free.

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  The file system must support crash recovery. That is, if a crash (e.g., power failure) occurs, the file system must still work correctly after a restart. The risk is that a crash might interrupt a sequence of updates and leave inconsistent on-disk data structures (e.g., a block that is both used in a file and marked free).

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  Different processes may operate on the file system at the same time, so the file-system code must coordinate to maintain invariants.

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  Accessing a disk is orders of magnitude slower than accessing memory, so the file system must maintain an in-memory cache of popular blocks.

The rest of this chapter explains how xv6 addresses these challenges.