Before you use fdisk to create or revise partitions, you should check your free space and the partitions that are currently mounted. You can do this with df and mount commands. The following example illustrates how the df command displays the total, used, and available free space on all currently mounted filesystems.
Note the numbers under the 1k-blocks column. In this case (except for the mounted DVD), they add up to about 35GB of allocated space. If your hard drive is larger, you may have unallocated space that you can use for another partition. Just remember to leave room for expansion in appropriate directories, such as /home, /tmp, and /var.
[root@Enterprise root]# df
Filesystem 1k-blocks Used Available Use% Mounted on
/dev/mapper/VolGroup00-LogVol00
9903432 2333948 7058304 25% /
/dev/hda3 101105 19821 76063 21% /boot
tmpfs 451200 0 451200 0% /dev/shm
/dev/mapper/VolGroup00-LogVol02
4951688 149164 4546936 4% /home
/dev/md0 302684 10326 276730 4% /tmp
/dev/mapper/VolGroup00-LogVol03
4951688 194308 4501792 5% /var
/dev/hdc 7384 7384 0 100%
/media/Red Hat Enterprise Linux Server
/dev/hda2 11715984 9509432 2206552 82% /DosD
[root@Enterprise root]#
The second command, mount, includes the filesystem type. In this case, examine the partition represented by device /dev/hda2 mounted with the VFAT file type on the /DosD directory. It provides direct access to the D: drive of the Windows operating system. For the following example, I've set up the data shown from the mount command in columns for clarity.
[root@Enterprise root]# mount
/dev/mapper/VolGroup00-LogVol00 on / type ext3 (rw)
proc on /proc type proc (rw)
sysfs on /sys type sysfs (rw)
devpts on /dev/pts type devpts (rw,gid=5,mode=620)
/dev/hda3 on /boot type ext3 (rw)
tmpfs on /dev/shm type tmpfs (rw)
/dev/mapper/VolGroup00-LogVol02 on /home type ext3 (rw)
/dev/md0 on /tmp type ext3 (rw)
/dev/mapper/VolGroup00-LogVol01 on /var type ext3 (rw)
none on /proc/sys/fs/binfmt_misc type binfmt_misc (rw)
sunrpc on /var/lib/nfs/rpc_pipefs type rpc_pipefs (rw)
/dev/hdc on /media/RHEL-5 i386 Disc 1type iso9660 (ro,noexec,nosuid,nodev,uid=500)
/dev/hda2 on DosD type vfat (rw)
[root@Enterprise root]#
One of the benefits is that you can move and copy files between the Linux and the DOS partitions using standard Linux commands. You cannot, however, run any Windows applications within Linux unless you run a DOS or Windows emulation package such as Wine (www.winehq.org) or CrossOver Office (www.codeweavers.com).
The fdisk Utility
The fdisk tility is a universally available tool that you should know well. There are many commands within fdisk, more in expert mode, but you need to know only the few discussed here.
Though you can modify the physical disk partition layout using many programs, this section explores the Linux implementation of fdisk. FDISK.EXE from DOS has the same name and is also used for creating partitions, but it doesn't incorporate any Linux-compatible features. It also uses a different interface.
Using fdisk: Starting, Getting Help, and Quitting
The following screen output lists commands that show how to start the fdisk program, how to get help, and how to quit the program. The /dev/hda drive is associated with the first PATA/IDE drive on a regular PC. Your computer may have a different hard drive; you can check the output from the df and mount commands for clues.
As you can see, once you start fdisk, it opens its own command line prompt:
# fdisk /dev/hda
Command (m for help): m
Command action
a toggle a bootable flag
b edit bsd disklabel
c toggle the dos compatibility flag
d delete a partition
l list known partition types
m print this menu
n add a new partition
o create a new empty DOS partition table
p print the partition table
q quit without saving changes
s create a new empty Sun disklabel
t change a partition's system id
u change display/entry units
v verify the partition table
w write table to disk and exit
x extra functionality (experts only)
Command (m for help): q
There are a wide variety of commands associated with fdisk and more when you run the x command to access fdisk's extra functionality.
Using fdisk: In a Nutshell
At the fdisk command line prompt, start with the print command (p) to print the partition table. This allows you to review the current entries in the partition table. Assuming you have free space, you then create a new (n) partition, either primary (p) or logical (l). If it doesn't already exist, you can also create an extended partition (e) to contain your logical partitions. Remember that you can have up to four primary partitions, which would correspond to numbers 1 through 4. One of the primary partitions can be redesignated as an extended partition. The remaining partitions are logical partitions, numbered 5 and above. The Linux fdisk utility won't allow you to create more than 16 partitions on the drive.
When you assign space to a partition, you're assigning a block of cylinders on that hard disk. If you have free space, the fdisk default starts the new partition at the first available cylinder. The actual size of the partition depends on disk geometry; do not worry about exact size here.
Using fdisk: Deleting Partitions
The following example removes the only configured partition. The sample output screen first starts fdisk. Then you print (p) the current partition table, delete (d) the partition by number (1 in this case), write (w) the changes to the disk, and quit (q) from the program. Needless to say, do not perform this action on any partition where you need the data.
# fdisk /dev/hdb
Command (m for help): p
Disk /dev/hdb: 255 heads, 63 sectors, 525 cylinders
Units = cylinders of 16065 * 512 bytes
Device Boot Start End Blocks Id System
/dev/hdb1 * 1 525 4217031 6 FAT16
Command (m for help): d
Partition number (1-1): 1
This is the last chance to change your mind before deleting the current partition. If you want to change your mind, exit from fdisk with the q command. If you're pleased with the changes that you've made and want to make them permanent, proceed with the w command:
Command (m for help): w
You did it! Now you have an empty hard disk or hard disk area where you can create the partitions you need.
You no longer have to reboot to get Linux to read the new partition table. Now, the partprobe command rereads the partition table without a reboot.
Using fdisk: Creating Partitions
The following screen output sample shows the steps used to create (n) the first (/boot) partition, make it bootable (a), and then finally write (w) the partition information to the disk. (Note that although you may ask for a 100MB partition, the geometry of the disk may not allow that precise size, as shown in the example.)
# fdisk /dev/hdb
Command (m for help): n
Command action
e extended
p primary partition (1-4)
p
Partition number (1-4):
First cylinder (1-256, default 1): 1
Last cylinder or +size or +sizeM or +sizeK (2-256,def 256): +100M
Command (m for help): a
Partition number (1-4): 1
Command (m for help): p
Disk /dev/hdb: 255 heads, 63 sectors, 256 cylinders
Units = cylinders of 16065 * 512 bytes
Device Boot Start End Blocks Id System
/dev/hdb1 * 1 12 98163 83 Linux
Command (m for help):
Repeat the commands to create any other partitions that you might need. One possible group of partitions is illustrated here:
Command (m for help): p
Disk /dev/hdb: 255 heads, 63 sectors, 256 cylinders
Units = cylinders of 16065 * 512 bytes
Device Boot Start End Blocks Id System
/dev/hdb1 * 1 2 16044 83 Linux
/dev/hdb2 3 18 64176 82 Linux swap / Solaris
/dev/hdb3 19 169 1203300 83 Linux
/dev/hdb4 170 250 649782 5 Extended
/dev/hdb5 170 201 248682 83 Linux
/dev/hdb6 202 257 449232 83 Linux
Command (m for help): w
| On the Job | The number of blocks that you see may vary slightly depending on the size of your hard disk; the number of heads, sectors, and cylinders on that disk; as well as the version of fdisk that you're using. |
Using fdisk: A New PC with No Partitions
After installing Linux on a new PC, you'll want to use fdisk to configure additional physical disks attached to the system. For example, if the additional disk is the first disk attached to the secondary IDE controller, run the fdisk /dev/hdc command. Remember the limitations on partitions. If you need more than four partitions on the new physical disk, configure type Primary for the first three partitions, and then Extended for the rest of the disk as partition 4. You can then creating logical partitions 5–16 within the extended partition.
Using fdisk: Creating a Swap Partition
You need to create a partition before you can reassign it as a swap partition. At the fdisk prompt, run the l command. You'll see a large number of file types, listed as hex codes. When you create a partition, fdisk creates a Linux Native type partition by default. As you can see from the output of the l command, the associated hex code is (83).
It's easy to reassign a partition as a swap partition. Run the p command. Remember the number of the partition you want to change. Make sure that partition doesn't contain data that you want to save.
Now run the t command. Type in the number associated with the partition that you want to change. Type in the hex code for the type you want-in this case, 82 for a Linux swap partition. For example, I could run the following sequence of commands to set up a new swap partition on the second IDE hard drive. The commands that I type are in boldface. The details of what you see depend on the partitions that you may have created. It'll be a 1GB swap space on the first primary partition (/dev/hdb1).
# fdisk /dev/hdb
Command (m for help): n
Command action
e extended
p primary partition (1-4)
p
Partition number (1-4): 1
First cylinder (1-10402, default 1):
Using default value 1
Last cylinder or +size or +sizeM or +sizeK (1-10402, default 10402): +1000M
Command (m for help): p
Disk /dev/hdb: 5368 MB, 5368709120 bytes
16 heads, 63 sectors/track, 10402 cylinders
Units = cylinders of 1008 * 512 = 516096 bytes
Device Boot Start End Blocks Id System
/dev/hdb1 1 1939 977224+ 83 Linux
Command (m for help): t
Selected partition 1
Hex code (type L to list codes): 82
Changed system type of partition 1 to 82 (Linux swap / Solaris)
Command (m for help): w
The partition table has been altered!
Calling ioctl() to re-read partition table.
Syncing disks.
#
The fdisk utility doesn't actually write the changes to your hard disk until you run the write (w) command. You have a chance to cancel your changes with the quit (q) command. To make sure Linux rereads the partition table after fdisk writes it, run the partprobe command
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