File Systems

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14.1 Device Special Files (Devices)

The udev program relies on the sysfs file system, which exports information about devices and other kernel objects into user space via a pseudo-file system mounted under /sys.

Each device file has a major ID number and a minor ID number. The major ID identifies the general class of device, and is used by the kernel to look up the appropriate driver for this type of device. The minor ID uniquely identifies a particular device within a general class.

Each device driver registers its association with a specific major device ID, and this association provides the connection between the device special file and the device driver.

Each disk is divided into one or more (nonoverlapping) partitions. Each partition is treated by the kernel as a separate device residing under the /dev directory.

14.2 Disks and Partitions

The system administrator determines the number, type, and size of partitions on a disk using the fdisk command. The command fdisk –l lists all partitions on a disk. The Linux-specific /proc/partitions file lists the major and minor device numbers, size, and name of each disk partition on the system.

A swap area is created using the mkswap(8) command. A privileged (CAP_SYS_ADMIN) process can use the swapon() system call to notify the kernel that a disk partition is to be used as a swap area. The swapoff() system call performs the converse function, telling the kernel to cease using a disk partition as a swap area.

14.3 File Systems

A file system contains the following parts:

  1. boot block This is always the first block in a file system. The boot block is not used by the file system; rather, it contains information used to boot the operating system. Although only one boot block is needed by the operating system, all file systems have a boot block (most of which are unused).
  2. super block
  3. I-node table
  4. Data blocks

14.6 Journaling File Systems

Journaling file systems eliminate the need for lengthy file-system consistency checks after a system crash.

The most notable disadvantage of journaling is that it adds time to file updates, though good design can make this overhead low.

With Linux 2.4.19 and later, things became more complicated. The kernel now supports per-process mount namespaces. This means that each process potentially has its own set of file-system mount points, and thus may see a different single directory hierarchy from other processes

14.8 Mounting and Unmounting File Systems

Before looking at these system calls, it is useful to know about three files that contain information about the file systems that are currently mounted or can be mounted:

  1. A list of the currently mounted file systems can be read from the Linux-specific /proc/mounts virtual file. /proc/mounts is an interface to kernel data structures, so it always contains accurate information about mounted file systems.
  2. The mount(8) and umount(8) commands automatically maintain the file /etc/mtab, which contains information that is similar to that in /proc/mounts, but slightly more detailed.
  3. The /etc/fstab file, maintained manually by the system administrator, contains descriptions of all of the available file systems on a system, and is used by the mount(8), umount(8), and fsck(8) commands

主要介绍了 mount 和 unmount 两个函数的使用

14.9 Advanced Mount Features

@todo mount 的一些骚操作

14.10 A Virtual Memory File System: tmpfs

By default, a tmpfs file system is permitted to grow to half the size of RAM, but the size=nbytes mount option can be used to set a different ceiling for the file-system size, either when the file system is created or during a later remount. (A tmpfs file system consumes only as much memory and swap space as is currently required for the files it holds.)

14.11 Obtaining Information About a File System: statvfs()

Aside from use by user applications, tmpfs file systems also serve two special purposes:

  1. An invisible tmpfs file system, mounted internally by the kernel, is used for implementing System V shared memory (Chapter 48) and shared anonymous memory mappings (Chapter 49).
  2. A tmpfs file system mounted at /dev/shm is used for the glibc implementation of POSIX shared memory and POSIX semaphores. 
    struct ustatvfs {

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