File Systems / Partitions

The disk can be divided into smaller, separate pieces called Partitions

• particularly important in dual booting (windows will own some partitions and Linux others)
• partitions also can be used to separate file systems owned by the same OS
• partition refers to separated spaces

•  The partition table can define only 4 partitions
•  4 primary partitions or 3 primary partitions and one extended partition
• logical drives can be added to extended partitions
• extended partitions by themselves cannot be mounted

• provides a way to define more than 4 partitions on a single disk
• an extended partition is like a drive within a drive
• contains a linked list of partition table sectors for the logical drives within it
• Advanced Concept: the first table describes three primary partitions, the extended partition and the location of the next partition table sector

Partition Example: a machine with a SCSI drive

sfdisk options
• -l : list the partitions of a device
• -x : show the extended partitions

# sfdisk -l

Disk /dev/sda: 522 cylinders, 255 heads, 63 sectors/track
Units = cylinders of 8225280 bytes, blocks of 1024 bytes, counting from 0

   Device Boot Start     End   #cyls   #blocks   Id  System
/dev/sda1   *      0+    459     460-  3694918+  83  Linux
/dev/sda2        460     521      62    498015    5  Extended
/dev/sda3          0       -       0         0    0  Empty
/dev/sda4          0       -       0         0    0  Empty
/dev/sda5        460+    480      21-   168651   82  Linux swap
/dev/sda6        481+    521      41-   329301   83  Linux

Why is the first logical partition always hda5?

Here is an example of an IDE disk:

# sfdisk -l -x

Disk /dev/hda: 4111 cylinders, 255 heads, 63 sectors/track
Units = cylinders of 8225280 bytes, blocks of 1024 bytes, counting from 0

   Device Boot Start     End   #cyls   #blocks   Id  System
/dev/hda1   *      0+     12      13-   104391   83  Linux
/dev/hda2         13     535     523   4200997+  83  Linux
/dev/hda3        536     666     131   1052257+  82  Linux swap
/dev/hda4        667    4110    3444  27663930    5  Extended

/dev/hda5        667+   1189     523-  4200966   83  Linux
    -           1190+   1451     262-  2104484    5  Extended
    -            667     666       0         0    0  Empty
    -            667     666       0         0    0  Empty

/dev/hda6       1190+   1451     262-  2104483+  83  Linux
    -           1452+   1974     523-  4200966+   5  Extended
    -           1190+   1190-      0         0    0  Empty
    -           1190+   1190-      0         0    0  Empty

/dev/hda7       1452+   1974     523-  4200966   83  Linux
    -           1452+   1452-      0         0    0  Empty
    -           1452+   1452-      0         0    0  Empty
    -           1452+   1452-      0         0    0  Empty

Why Partition?

• Helps you organize your data
• Makes backups simpler
• Makes upgrades simpler
• Can improve performance
• put swap partition on a different disk
• Be careful, this is a can be a destructive process!
• make sure you plan first and understand thoroughly what you are doing

Disk Organization

Platters
• Several platters are stored one on top of another
• Data recorded on both sides
• One arm between every two platters with two read/write heads
Tracks
• Surface of platter divided into concentric circles (tracks)
• Each track divided into sectors
• A sector is the smallest addressable unit of a disk
Cylinders
• Stack of tracks (or rings) directly above and below each other is a cylinder
• Data on a cylinder accessed without moving the arm
Sectors
• A segment of a track
Seek
• Arm movement is a disk seek
• The slowest part of physically accessing data from disk

The Hard-Disk Drive


Like a miniature clean-room on the inside. Air is drawn into the hard drive through a filter and circulated. This creates a positive pressure inside the drive that helps prevent contaminants from entering the drive case.

The Platters

Inside a hard-disk drive you will see platters sitting on a spindle. Each platter has two read/write record heads-one for each side of the platter. These heads are responsible for aligning the magnetic particles in such a way that data can be recorded and read. The head does not actually come into direct contact with the platter to write or read the data.

A small electrical motor is needed to drive the platter. The fast spinning of the platter helps create a cushion of air keeping the record head aloft. The head usually rides about 0.5 microns above the platter-a distance smaller than the diameter of a human hair.

Format of the Platter

The tracks, or concentric magnetic grooves, are placed onto the platter with the read/write head. This occurs when your hard drive is formatted. Each track is then divided into sectors (see diagram). Each sector can hold 512 bytes of data.
 
 



Disk Controller

The disk controller is software that knows how to position the heads in the right location to access or write data. For example, the kernel needs to access cylinder 10, sector 5  on the disk. The controller knows to move the heads 2.398cm from the edge of the platter in order to access data.

Partitions and Mount Points

• The top of the structure is the root (/) directory
• Under the root are directories critical to system operation

$ ls /
bin   dev  lost+found  proc  sbin  usr
boot  etc  home   lib   mnt   opt   root  tmp   var

Below I highlighted the filesystems which I recommend occupying their own partition.

swap partition

• Swap partitions are used to support virtual memory.
• The minimum size of your swap partition should be twice the amount of RAM.
• If you have a separate disk, you could configure swap space on that disk as well
• spreads the disk activity over two disks and improve the performance
• set the priority of each swap area to the same value for this to work.
root partition (/)
• The root filesystem is at the top of the directory structure.
• Underneath / are directories used by your system to boot the system, tools to manage the system, log files, user home directories config files, etc.
• The size of the root partition depends completely on how you slice the rest of your disk. It should be about 150 MB if you intend to create a separate /usr, /var, and /home partition.
/etc filesystem
• Contains system configuratoin files used by system administrators.
• Usually lives on root partition.
• Small in size.
/usr filesytem
• The /usr partition is where much of the software on a Linux system resides.
• This partition should be no smaller then 300MB for minimum installation
• The entire distribution requires nearly 4 GB of space!
• If possible, be generous with the /usr partition.
• Most RPM-based packages are installed under /usr
/sbin filesystem
• typically mounted on the root partition
• Contains system utilities needed for system startup
/home partition
• This is where users' home directories go.
• On a multiuser system it is good to create a separate partition for your users.
/usr/local filesystem
• Typically this filesystem contains software that is not available as an RPM package.
• The size depends on the amount of additional software you anticipate putting on your system.
• You can make the /usr partition large to accommodate /usr/local or create a separate partition for /usr/local.
/usr/src filesystem
• Linux Kernel Sources -- The complete sources for the Linux kernel are stored here
• Sources For RPM-Based Packages
• If a source package file (aka SRPM) is installed, the files are stored here and often packages are built here.
• Keep this in mind when sizing /usr.
/tmp filesystem
• The /tmp filesystem is for temporary files.
• Creating a partition dedicated to /tmp is a good idea for larger, multiuser systems or network server machines.
• The reason is that many active users can fill the root partition (/), if /tmp is located there.
• Reserving 150 MB for /tmp is a good place to start. Bigger is better
/var partition
• You system will write to log files in /var/log
• Files queued for printing will normally be written to /var/spool
• User email is also stored in /var/mail
• Web pages and FTP file are normally stored under /var
• It is best to make /var its own partition.
• 300 MB, if you can spare it, is a good starting point.
/opt filesytem
• Most often found on Solaris machines
• Some third-party software is designed to install itself under /opt.
• Software that attempts to install itself under /opt will end up on the root partition if you do not make a separate partition.
/boot partition
• All the files LILO or Grub need to access (at boot time) are in the /boot directory.
• including the Linux kernel which is about 2 megabytes.
• You may need to locate the physical partition ahead of the 1023rd cylinder boundary on your disk
• 30 MB should be plenty for the /boot partition

File Systems Mounted in the Directory Structure

• All the directories that live directly under / live on the root partition.
• such as /etc, /sbin, /usr, /boot, /home, /export, etc.
• Some of these directories may be mount points and have partitions mounted on them
• these partitions contain other filesystems

|---------------------------------------------------------|
|           |_____ /export     -------------> /dev/hdb1   |
|           |----- swap        -------------> /dev/hdb2   |--> hdb
|---------------------------------------------------------|

• Remember that that a partition is a physical separation of space on a disk.
• In the above example, /home is a mount point for /dev/hda2.
• The directory /home must first be created under /
• which provides a mount point for the device.

The /etc/fstab file for this machine would look something like:

Device          Mount Point  FS Type    Options     Dump Fsck

Back to you disk:

Information that defines the "physical disk geometry" for your hard disk

• bytes per sector (typically 512 bytes)
• sectors per track (typically 63)
• number of cylinders (variable -- lots on the newer big disks
• number of heads (variable)

(# of cylinders) × (# heads) × (63 sectors/track) × (512 bytes/sector)

For my old IDE disk at home:

(3148 cyl) × (16 heads) × (63 sectors/track) × (512 bytes/sect)

1,624,670,208 bytes -->  1.6 GB