File System Interface

Why Do We Need File System?¶

File system vs. Disk
- File system presents abstraction of disk
- File → Track/sector
To user process
- File system provides coherent view of a group of files
- File: a contiguous block of bytes (Unix)
File system provides protection

简单来说：

File is a contiguous logical space for storing information

There are different types of file:

data: character (e.g. markdown, txt), binary (e.g. memory dump), and application-specific (e.g. ppt, doc)
program (e.g. a.out)
special one: proc file system - use file-system interface to retrieve system information
- 如果你执行 df /proc，就能看到这个“隐藏”的 proc file system

文件的 metadata 如下：

Name - only information kept in human-readable form
Identifier - unique tag (number) identifies file within file system
Type - needed for systems that support different types
Location - pointer to file location on device
Size - current file size
Protection - controls who can do reading, writing, executing
Time, date, and user identification - data for protection, security, and usage monitoring
Information about files are kept in the directory structure, which is maintained on the disk
Many variations, including extended file attributes such as file checksum

Several data are needed to manage open files:

Open-file table: tracks open files
File pointer: pointer to last read/write location, per process that has the file open
File-open count: counter of number of times a file is open - to allow removal of data from open-file table when last processes closes it
Disk location of the file: cache of data access information
Access rights: per-process access mode information