15 UNIX File System

Single root directory, not necessarily a single device Need to designate a disk to be the root file system For external devices’ file system, need to mount it to some place at that file system

• mount -l shows all mount points

  /dev/sda1 on / type ext4
  students251 on /mnt/disks/aspstudents251 type zfs
  

• dev: a region for devices
• sda1: hard disk device
• type ext4: Metadata structure to use this device

Mounting types

/proc

An entire fake file system maintained by the kernel, where entries are PIDs, with all info about it

• This directory is mounted to a file system that is not on the disk, but on the memory
• When kernel reads, it will not go to the disk

• With root permissions, you can write to these /proc files

• /proc/self tells about the current process
• /proc/self/maps shows all mmaps

  proc on /proc type proc (rw,nosuid,nodev,noexec,relatime)
  

Bind mount

Bind-mounts allow you to mount a directory within an existing filesystem onto somewhere else in the hierarchy, effectively creating a new entry point.

• sudo mount --bind <existing-dir> <mount-point>
• Similar to a sym link
• Appears in the mount table

Berkley File System (FFS)

inode structure

• Super block: FS metadata (file system size, inode count, offset, …)
• Cylinder: further partition
  • Metadata
  • i-node map: which i-node is free/used
  • Block map: which block is free/used
  • i-nodes: array of i-nodes. Each stores a file or directory’s
    • Metadata
    • Pointers to data blocks
  • Data blocks

Directory

A directory is a special file with

• i-node number
• Filename Two directories may point to the same i-node. There can be multiple names for a single file. i-node number is the only unique identifier. Filename is just what the directory refers to the i-node by

Directory of “2549” is referred by “testdir” in 1267, and “.” in 2549 (itself)

Hard link

echo howdy > f1
ln f1 f2            # hard link a new f2 to f1

Once hard links are created, can’t distinguish. They refer to the same i-node number in ll

• f2 is not a copy or a shortcut
• ll also shows the link count: how many filenames are linked with the i-node Hard links are not allowed for directories
• Too dangerous, unless you’re the system that creates . and ..

Symlink

ln -s f1 s1         # sym link s1 to f1

A separate file (i-node) that is marked with a special type

• Content: just characters f1
• greptile can ignore symlink via lstat, but can’t ignore hard link
• If a the file is removed, it will be a broken symlink

Symlink can point to a path, but you can’t create a hard link that’s from another file system, since i-node doesn’t have any meaning in a different file system