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opsschool-curriculum/filesystems_101.rst
Dennis Tsang eaf7342586 Copyediting filesystems
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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 EXT4.
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:
.. code-block:: console
user@opsschool ~$
By default you will be in the "current working directory" of 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:
.. code-block:: console
user@opsschool ~$ pwd
/home/opsschool
You can see the list of files and directories in this directory with the ``ls``
command:
.. code-block:: console
user@opsschool ~$ 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:
.. code-block:: console
user@opsschool ~$ 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:
.. code-block:: console
user@opsschool ~$ 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:
.. code-block:: console
user@opsschool ~$ cd /tmp
user@opsschool ~$ pwd
/tmp
There may be times you need to find a file on your filesystem, based on its
name, date, size, or other particulars. For this you can use the ``find``
command:
.. code-block:: console
user@opsschool ~$ 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:
.. code-block:: console
-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
======================================
The ``parted`` tool is for modifying and creating disk partitions and disk
labels. Disk labels describe the partitioning scheme. Legacy Linux
systems will have the msdos partitioning table, although newer systems with EFI
use the gpt partitioning table. Drives with ``msdos`` disk labels will have a
Master Boot Record, or MBR, at the beginning of the drive. This is where the
bootloader is installed. GPT-labeled drives will usually have a FAT-formatted
partition at the beginning of the disk for EFI programs and the bootloader.
``parted`` has a subshell interface. It takes as an argument a device name.
.. code-block:: console
root@opsschool ~# parted /dev/sda
GNU Parted 2.3
Using /dev/sda
Welcome to GNU Parted! Type 'help' to view a list of commands.
(parted) print
Model: ATA VBOX HARDDISK (scsi)
Disk /dev/sda: 42.9GB
Sector size (logical/physical): 512B/512B
Partition Table: msdos
Number Start End Size Type File system Flags
1 8225kB 42.9GB 42.9GB primary ext4 boot
(parted)
In this example ``parted`` ran against ``/dev/sda``. The user then used the
``print`` command to print out information about the disk and the current
partitioning scheme. The user found a 43 GB disk using the ``msdos`` partition
table format. The disk had one partition which was flagged as bootable.
Looking at a second example:
.. code-block:: console
root@opsschool ~# parted /dev/sdb
GNU Parted 2.3
Using /dev/sdb
Welcome to GNU Parted! Type 'help' to view a list of commands.
(parted) print
Error: /dev/sdb: unrecognised disk label
(parted) mklabel msdos
(parted) print
Model: ATA VBOX HARDDISK (scsi)
Disk /dev/sdb: 8590MB
Sector size (logical/physical): 512B/512B
Partition Table: msdos
Number Start End Size Type File system Flags
(parted) mkpart primary 1 1G
(parted) set 1 boot on
(parted) mkpart primary 1G 5G
(parted) mkpart primary 5G 7G
(parted) mkpart primary 7G 8G
(parted) p
Model: ATA VBOX HARDDISK (scsi)
Disk /dev/sdb: 8590MB
Sector size (logical/physical): 512B/512B
Partition Table: msdos
Number Start End Size Type File system Flags
1 1049kB 1000MB 999MB primary boot
2 1000MB 5000MB 3999MB primary
3 5000MB 7000MB 2001MB primary
4 7000MB 8590MB 1590MB primary
(parted)
``parted`` failed to read the label, so the user created a new ``msdos``
disk label on the disk. After that ``parted`` was able to see that the disk was
8GB. We created a primary 1GB partition at the beginning of the disk for
``/boot`` and set the bootable flag on that partition. We created 4GB, 2GB,
and 1GB partitions for root, var, and swap, respectively.
Formatting partitions with new file systems
===========================================
New filesystems are created with the ``mkfs`` family of commands. There are a
variety of file systems to choose from, ``man fs`` has a list of filesystems
with short descriptions of each. Choosing a filesystem involves characterizing
the workload of the filesystem and weighing engineering tradeoffs. On Linux
systems, ext4 is a good general purpose choice. Following from the example
above, we will create filesystems on each of the four partitions we created.
``fdisk`` is another, older, tool to view and modify partitions on disk. It is
limited to the msdos disk label.
.. code-block:: console
root@opsschool ~# fdisk -l /dev/sdb
Disk /dev/sdb: 8589 MB, 8589934592 bytes
255 heads, 63 sectors/track, 1044 cylinders, total 16777216 sectors
Units = sectors of 1 * 512 = 512 bytes
Sector size (logical/physical): 512 bytes / 512 bytes
I/O size (minimum/optimal): 512 bytes / 512 bytes
Disk identifier: 0x0004815e
Device Boot Start End Blocks Id System
/dev/sdb1 * 2048 1953791 975872 83 Linux
/dev/sdb2 1953792 9764863 3905536 83 Linux
/dev/sdb3 9764864 13672447 1953792 83 Linux
/dev/sdb4 13672448 16777215 1552384 83 Linux
The first partition, to contain ``/boot``, will be ext2. Create this by running:
.. code-block:: console
root@opsschool ~# mkfs.ext2 /dev/sdb1
mke2fs 1.42 (29-Nov-2011)
Filesystem label=
OS type: Linux
Block size=4096 (log=2)
Fragment size=4096 (log=2)
Stride=0 blocks, Stripe width=0 blocks
61056 inodes, 243968 blocks
12198 blocks (5.00%) reserved for the super user
First data block=0
Maximum filesystem blocks=251658240
8 block groups
32768 blocks per group, 32768 fragments per group
7632 inodes per group
Superblock backups stored on blocks:
32768, 98304, 163840, 229376
Allocating group tables: done
Writing inode tables: done
Writing superblocks and filesystem accounting information: done
The second and third partitions, to contain ``/`` and ``/var``, will be ext4.
Create these by running:
.. code-block:: console
root@opsschool:~# mkfs.ext4 /dev/sdb2
root@opsschool:~# mkfs.ext4 /dev/sdb3
The output of ``mkfs.ext4`` is very close to the output of ``mkfs.ext2`` and so
it is omitted.
Finally, ``/dev/sdb4`` is set aside for swap space with the command:
.. code-block:: console
root@opsschool:~# mkswap /dev/sdb4
Setting up swapspace version 1, size = 1552380 KiB
no label, UUID=cc9ba6e5-372f-48f6-a4bf-83296e5c7ebe
Mounting a filesystem
=====================
Mounting a filesystem is the act of placing the root of one filesystem on
a directory, or mount point, of a currently mounted filesystem. The mount
command allows the user to do this manually. Typically, only the superuser
can perform mounts. The root filesystem, mounted on ``/``, is unique and
it is mounted at boot. See :doc:`boot_process_101`.
In the following example, the filesystem of ``/dev/sdb1`` will be mounted to
the folder ``/mnt``. That means anything inside ``/dev/sdb1`` will become
accessible under ``/mnt/...``. (Don't worry about the options ``-t`` and
``-o`` just yet; those will be described in more detail below).
.. code-block:: console
root@opsschool # mount -t ext4 -o noatime /dev/sdb1 /mnt
It is common to specify which filesystem type is present on ``/dev/sdb1`` and
which mounting options you would like to use, but if that information is not
specified then the Linux ``mount`` command is pretty good at picking sane
defaults. Most administrators would have typed the following instead of the
above:
.. code-block:: console
root@opsschool # mount /dev/sdb1 /mnt
The filesystem type refers to the format of the data structure that is used as
the filesystem on disk. Files (generally) do not care what kind of filesystem
they are on. You have to concern yourself with the filesystem type only in
the initial filesystem creation, automatic mounting, and performance tuning.
Example filesystem types are ``ext2``, ``ext3``, ``ext4``, ``FAT``, ``NTFS``,
``HFS``, ``JFS``, ``XFS``, ``ZFS``, ``Btrfs``. On Linux hosts, ``ext4`` is a
good default. For maximum compatibility with Windows and Macintosh, use ``FAT``.
See the following Wikipedia page for a more thorough `comparison of file systems.
<http://en.wikipedia.org/wiki/Comparison_of_file_systems>`_
Automatic mounting using ``fstab``
----------------------------------
The fstab, or file system table, is the file that configures automatic mounting
at boot. It tabulates block devices, mount points, type and options for each
mount. The dump and pass fields control booting behavior. Dumping is the act
of creating a backup of the filesystem (often to tape), and is not in common use.
Pass is much more important. When the pass value is nonzero, the filesystem is
analyzed early in the boot process by ``fsck``, the file system checker, for errors.
The number, ``fs_passno``, indicates priority. The root filesystem should always be
1, other filesystems should be 2 or more. A zero value causes checks to be
skipped, an option often used to accelerate the boot process. In ``/etc/fstab``,
there are a number of ways to specify the block device containing the filesystem
. ``UUID``, or universally unique identifier, is one common way in modern Linux
based systems to specify a filesystem.
.. code-block:: console
root@opsschool # cat /etc/fstab
# <file system> <mount point> <type> <options> <dump> <pass>
/dev/sda5 / ext4 errors=remount-ro 0 1
/dev/sda6 none swap sw 0 0
/dev/sda1 /boot/efi auto auto 0 0
This ``/etc/fstab`` file mounts ``/dev/sda5`` on ``/`` using the ext4 filesystem
. If it encounters a filesystem corruption it will use the ``fsck`` utility
early in the boot process to try to clear the problem. If the physical disk
reports errors in writing while the filesystem is mounted, the OS will remount
``/`` readonly. The ``/dev/sda6`` partition will be used as swap. The
``/dev/sda1`` partition will be mounted on ``/boot/efi`` using autodetection, the
partition will not be scanned for filesystem errors.
Automatic mounting using ``autofs``
-----------------------------------
``autofs`` is another way to tabulate filesystems for mounting. It is different from
the ``/etc/fstab`` because the filesystems listed in ``auto.master`` are not mounted
at boot. The automounter allows the system to mount filesystems on demand, then clean
up those filesystems when they are no longer being used.
The ``auto.master`` file controls the ``autofs`` service:
.. code-block:: console
root@opsschool # cat /etc/auto.master
/home -rw,hard,intr,nosuid,nobrowse bigserver:/exports/home/&
/stash ldap:ou=auto_stash,ou=Autofs,dc=example,dc=com -rw,hard,intr,nobrowse
In the above example, the system mounts home directories for each user from a remote NFS
server. The filesystem remains unmounted until the user logs in, and is unmounted
a short time after the user logs out. The automounter is triggered by an attempt to
``cd`` into ``/home/<key>``. It will then attempt to find an NFS share on
``/exports/home/<key>`` and mount it on ``/home/key``, then allow the ``cd`` command
to return successfully. The ``/home`` example above is using the ``&`` expansion syntax.
The second line is using the LDAP syntax to look up a key under ``/stash/<key>`` in
LDAP. LDAP will be covered later in the curriculum.
The ``auto.master`` file is known as ``auto_master`` on FreeBSD, Solaris, and Mac OS X.
Filesystem options
==================
Passing options to the ``mount`` command, or inserting them into the ``/etc/fstab`` file, control how the filesystem behaves.
Different filesystems at different versions support different options, but some options are ubiquitous.
async
-----
The ``async`` option sets writing operations to the filesystem to be asynchronous.
This means that the ``cp`` command will exit normally before the entire copy is done, and that the system will persist the files to the disk at some point later on.
You don't have a lot of guarantees here about when that will happen, though for a generally unloaded system you won't have to wait long.
It is also hard to tell when the filesystem is actually done with the copy.
The ``sync`` utility can be used to force a filesystem sync to immediately persist to disk.
The opposite of this option, the ``sync`` option is the default on most filesystems.
noatime
-------
The ``noatime`` option tells the filesystem not to keep track of ``atime`` or access time.
If you recall your ``inode`` lessons, you'll remember that the ``inode`` keeps track of three dates: ``ctime`` (change time), ``mtime`` (modification time), and ``atime`` (access time).
Under normal circumstances, whenever a user reads from a file, the operating system will write a new ``atime`` to the ``inode``.
For large groups of small files, read by a number of people, or by automated processes, this final write operation can hurt disk performance.
As a result, many admins will turn off ``atime`` on filesystems to increase performance.
Note that ``atime`` is not really a security/auditing feature. Any regular user can use the ``touch`` utility on a file to set the ``atime`` to some point in the past, if they have the appropriate permissions for that file.
Also note that contrary to popular belief, ``ctime`` does *not* store a file's creation time.
The change time stored in ``ctime`` is when the file's attributes or contents were changed, whereas the modification time stored in ``mtime`` only changes if the file's contents are modified.
ro
--
The ``ro`` option, called 'read-only', tells the filesystem to not allow writes to any of the files on the filesystem.
This is useful for a legitimately read-only backing store such as a CD-ROM.
It is also useful for protecting the filesystem from yourself or others, such as when you are mounting a drive you pulled from a different machine.
If the filesystem is mounted read-only, you can't accidentally delete any data off of it.
It is common for network shares to be mounted read-only, this way client machines can copy files from the network share, but can't corrupt the share with write locks or deletes.
Sometimes, when the operating system detects that the block device (such as a hard drive) beneath the filesystem is beginning to fail, the operating system will remount the filesystem read-only.
This will cause lots of problems for your running system, but it is intended to give you the maximum amount of time to copy your important data off of the filesystem before the disks beneath it fail completely.
If this happens you can usually see it by checking ``mount`` for filesystems that should be read-write mounted as read-only.
You can also check ``dmesg`` for messages like:
.. code-block:: console
root@opsschool # dmesg
[26729.124569] Write(10): 2a 00 03 96 5a b0 00 00 08 00
[26729.124576] end_request: I/O error, dev sda, sector 60185264
[26729.125298] Buffer I/O error on device sda2, logical block 4593494
[26729.125986] lost page write due to I/O error on sda2
These messages strongly indicate that the disk ``/dev/sda`` is dying.
In some cases you can recover the filesystem with the file system checker ``fsck``.
You may also be able to force remount the filesystem read-write, as shown in the ``remount`` section below.
rw
--
The ``rw`` option, called 'read-write', tells the filesystem to allow reads and writes to all files on the filesystem.
Mounting a filesystem read-write is usually the default.
There are times where you will not be able to make a read-write mount, such as when the backing physical media is fundamentally read-only, like a CD-ROM.
remount
-------
Sometimes a filesystem will be mounted read-only, either by default, or because the operating system has remounted it ``ro`` because of disk failures.
It is possible to remount the filesystem read-write with the following command:
.. code-block:: console
root@opsschool ~# mount | grep boot
/dev/sda1 on /boot type ext3 (ro)
root@opsschool ~# mount -o remount,rw /boot
root@opsschool ~# mount | grep boot
/dev/sda1 on /boot type ext3 (rw)
The syntax of the remount option is ``-o remount,<option>``.
noexec
------
The ``noexec`` option tells the filesystem to ignore the execute bit on a filesystem.
This would never work for the root filesystem, but it is often used as a security measure on world-writeable filesystems such as :file:`/tmp` . Note that there are many ways to get around this restriction.
nosuid
------
Like the ``noexec`` option, the ``nosuid`` option ignores any setuserid bits set on files in the filesystem.
This is also used for security purposes.
It is generally recommended for removable devices such as CD-ROMs, USB sticks, and network filesystems.
nobarrier
---------
rbind
-----
How filesystems work
====================
Files, directories, inodes
Inodes
======
What they 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.
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