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File systems

Background

A filesystem is the street grid of your hard drive. It's a map of addresses to where data is located on your drive. Your operating system uses the filesystem to store data on the drive.

There are a number of different types of filesystems. Some are better at handling many small files (ReiserFS), some are much better at large files and deleting files quickly (XFS, EXT4).

The version of Unix you use will have picked a filesystem which is used by default, on Linux this is often EXT3.

Understanding the way filesystems work is important when you have to fix issues related to disk space, performance issues with reading and writing to disk, and a host of other issues.

In this section we will discuss creating partitions, file systems on those partitions, and then mounting those file systems so your operating system can use them.

Working with disks in Linux

Disks in Linux are normally named /dev/sda, /dev/sdb, etc. If you are in a VM, they may be named /dev/xvda, /dev/xvdb, etc. The last letter ("a", "b", "c"..) relates to the physical hard drive in your computer. "a" is the first drive, "b" is the second.

If you have an already configured system, you will likely see entries like this:

-bash-4.1$ ls -la /dev/sd*
brw-rw---- 1 root disk 8, 0 Jul  6 16:51 /dev/sda
brw-rw---- 1 root disk 8, 1 Sep 18  2011 /dev/sda1
brw-rw---- 1 root disk 8, 2 Sep 18  2011 /dev/sda2
brw-rw---- 1 root disk 8, 3 Sep 18  2011 /dev/sda3

The number at the end of each drive maps to the partition on the drive. A partition refers to a fixed amount of space on the physical drive. Drives must have at least one partition. Depending on your specific needs, you might want more than one partition, but to start with, we'll assume you just need one big partition.

Configuring your drive with partitions

man parted

Formatting partitions with new file systems

man mkfs

Mounting a filesystem

.. todo:: explain different kinds of mounts, autofs, /etc/fstab

Filesystem options

noatime nobarriers

How filesystems work

Files, directories, inodes

Navigating the filesystem

cd, ls, rm, find

Inodes

What the contain, how they work

The POSIX standard dictates files must have the following attributes:

  • File size in bytes.
  • A device id.
  • User ID of file's owner.
  • Group ID of file.
  • The file's mode (permissions).
  • Additional system and user flags (e.g. append only or ACLs).
  • Timestamps when the inode was last modified (ctime), file content last modified/accessed (mtime/atime).
  • Link count of how many hard links point to the inode.
  • Pointers to the file's contents.

http://en.wikipedia.org/wiki/Inode

File system layout

File system hierarchy standard is a reference on managing a Unix filesystem or directory structure.

http://www.pathname.com/fhs/

Fragmentation in unix filesystems

Filesystem objects

Filesystem contain more than just files and directories. Talk about devices (mknod), pipes (mkfifo), sockets, etc.