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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.

Navigating the filesystem

When you log into a Unix system, you will be given a command line by the :doc:`shell <shells_101>` which may look something like this:

bash-4.0$

By default you will be in the "current working directory" if the process that spawned the shell. Normally this is the home directory of your user. It can be different in some edge cases, such as if you manually change the current working directory, but these cases are rare until you start doing more advanced things.

You can find the name of the current directory with the pwd command:

bash-4.0$ pwd
/home/opsschool

You can see the list of files and directories in this directory with the ls command:

bash-4.0$ ls
file1.txt file2.txt tmpdir

The ls command also accepts the -l argument to provide a long-listing, which will show you permissions, dates, ownership and other information:

bash-4.0$ ls -l
-rw-r--r--  1 opsschool opsgroup   2444 Mar 29  2012 file1.txt
-rw-r--r--  1 opsschool opsgroup  32423 Jun 03  2011 file2.txt
drwxr-xr-x 15 opsschool opsgroup   4096 Apr 22  2012 tmpdir

You can see the contents of other directories, by giving the name of the directory:

bash-4.0$ ls -l /
dr-xr-xr-x    2 root root  4096 Apr 26  2012 bin
dr-xr-xr-x    6 root root  1024 Sep 18 14:09 boot
drwxr-xr-x   19 root root  8660 Jan  8 16:57 dev
drwxr-xr-x  112 root root 12288 Feb  8 06:56 etc
drwxr-xr-x   67 root root  4096 Feb  7 19:43 home
dr-xr-xr-x   13 root root  4096 Mar  6  2012 lib
drwx------    2 root root 16384 Sep 18  2011 lost+found
drwxr-xr-x    5 root root  4096 Nov 19 18:53 mnt
drwxr-xr-x    4 root root  4096 Sep  4 15:15 opt
dr-xr-xr-x 1011 root root     0 Sep 23  2011 proc
dr-xr-x---   10 root root  4096 Jan 23 23:14 root
dr-xr-xr-x    2 root root 12288 Oct 16 22:23 sbin
drwxr-xr-x   13 root root     0 Sep 23  2011 sys
drwxrwxrwt   65 root root 16384 Feb 11 04:37 tmp
drwxr-xr-x   16 root root  4096 Feb  8  2012 usr
drwxr-xr-x   27 root root  4096 Nov  4 03:47 var

You may have noticed that the names of directories follow a pattern. / is also called the root directory. All directories and files are contained under it. From the first example, the / directory contains the /home directory, which in turn contains the /home/opsschool directory.

To change directories, use the cd command:

bash-4.0$ cd /tmp
bash-4.0$ pwd
/tmp

There may be times you need to find a file on your filesystem, based on its name, date, size, or other patriculars. For this you can use the find command:

bash-4.0$ find /home/opsschool -type f -name file3.txt
/home/opsschool/tmpdir/file3.txt

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

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.