MBR, Primary, Extended & Logical partition in Red Hat

MBR, Primary, Extended & Logical partition in Red Hat

Hard disk and storage device are normally divided up into smaller chunks called partitions. So that different parts of it can be formatted with different file system or used for different purposes. For example, one partition could contain user home directories while another could contain system data and logs; by placing the data in two separate file system on two separate partitions, even if a user fills up the home directories partition with data, the system data partition may have space.
Most Red Hat Enterprise Linux system on the x86 and x86-64 processor architectures use the MBR partitioning format for their hard disk. This is the same format that is used by most Microsoft Windows systems, and dates back to the IBM PC.

                             Storage device: /dev/sda

In this format, the first sector of the disk is reserved for the Master Boot Record, or MBR. The first 446 bytes contain the first part of the bootloader that starts the system, followed by 64 bytes that contains the partition table. There is room for information about four primary partitions in the partition table; where they start, where they end, and a code that indicates what sort of information is stored in them. The partition must then be formatted with a file system before they can be used.

If more partitions are needed, one of the primary partitions can be converted into an extended partition which, rather than having a file system itself, is divided into multiple logical partitions. For various reasons, typically you are limited to no more than 15 partitions in total; fourteen are usable for file system, counting three primaries, the extended and eleven logical.

Storage devices are represented by device files in /dev. In Red Hat Enterprise Linux 6, the first SCSI, PATA/SATA, OR USB hard drive detected is /dev/sda, the second is /dev/sdb, and so on. This name represents the whole drive. The first primary partition on /dev/sda is /dev/sda1 the second partition is /dev/sda2, and so on. Partitions 1 through 4 are primary partitions; 4 is usually used as the extended partition if one is used; 5 and higher are always logical partitions.

Note:
One exception are paravirtualized hard drives in guest virtual machines, which instead show up as /dev/vda.


The first sector of each HD is called the MBR, or master boot record,  its structure is as follows:
 446 bytes are the master bootstrap.
  64 bytes are the four partition table entries (4 x 16)
   2 bytes are a signature ...
----
 512

List partitions

To list all partitions that are on your server, you would issue the fdisk command, with the list switch.

[root@rhel6 ~]# fdisk -l

Create new partitions

Step-1. In order to create new partitions you would first have to open the device in fdisk. I will be opening /dev/sdb and creating both a primary and extended partition. We use the n command to create a new partition.

[root@rhel6 ~]# fdisk  -cu /dev/sda

Command (m for help): n
Command action
   e   extended
   p   primary partition (1-4)
p
Partition number (1-4, default 1): 1
First sector (2048-8388607, default 2048): 
Using default value 2048
Last sector, +sectors or +size{K,M,G} (2048-8388607, default 8388607): +250M
 
Command (m for help): p 

Disk /dev/sdb: 4294 MB, 4294967296 bytes
255 heads, 63 sectors/track, 522 cylinders, total 8388608 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: 0xd26a7e50 

  Device Boot      Start         End      Blocks   Id  System
/dev/sda1            2048      514047      256000   83  Linux

Command (m for help): n
Command action
  e   extended
  p   primary partition (1-4)
e
Partition number (1-4, default 2): 
Using default value 2
First sector (514048-8388607, default 514048): 
Using default value 514048
Last sector, +sectors or +size{K,M,G} (514048-8388607, default 8388607): +500M

Command (m for help): w
The partition table has been altered!

Calling ioctl() to re-read partition table.
Syncing disks.
[root@rhel6 ~]# partprobe

[root@rhel6 ~]# init 6

Meaning of the commands:

1. fdisk -cu /dev/sda: This command is use for create new partition.

2. n: For create a new partition.

3. p: Print the all partition tables.

4. w: For save the entry.

5. init 6: For restart the pc.

Format Partitions

Step-2. In the second step we will format the partition:

[root@rhel6 ~]# mkfs.ext4 /dev/sda1

[root@rhel6 ~]# mkdir /anyname

[root@rhel6 ~]# gedit /etc/fstab 

Meaning of the commands:

1. mkfs.ext4 /dev/sda1: Create a ext4 type partition.

2. mkdir: Make a directory of anyname in root.

3. gedit: It is a graphical text editor which is used for text entry purpose only, and here we add a entry in /etc/fstab file.

Step-3. A file which only root can edit, /etc/fstab, lists what partitions should have their file systems mounted on what mount points with which options, one partition per line. A typical line might look like this:

/dev/sda1       /anyname    ext4     defaults 0 0

This indicates that the ext4 file system on the /dev/sda1 partition should be mounted on the directory /any name automatically using default options at boot time.

Step-4. Mount the all drive partitions:

[root@rhel6 ~]# mount -a

Note: If you want to check what partition is mounted or not, use this command:

[root@rhel6 ~]# df -h

Delete partitions

Deleting partitions is even easier. You would just type d at the fdisk prompt, tell it which partition number you are deleting, and the write the changes with the w flag.

[root@rhel6 ~]# fdisk /dev/sda

Command (m for help): p

Disk /dev/sdb: 4294 MB, 4294967296 bytes
255 heads, 63 sectors/track, 522 cylinders, total 8388608 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: 0x08bafe2e

  Device Boot      Start         End      Blocks   Id  System
/dev/sda1            2048     1026047      512000   83  Linux
/dev/sda2         1026048     1538047      256000    5  Extended

Command (m for help): d
Partition number (1-5): 2

Command (m for help): p

Disk /dev/sdb: 4294 MB, 4294967296 bytes
255 heads, 63 sectors/track, 522 cylinders, total 8388608 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: 0x08bafe2e

  Device Boot      Start         End      Blocks   Id  System
/dev/sda1            2048     1026047      512000   83  Linux 

Command (m for help): w
The partition table has been altered!
 
Calling ioctl() to re-read partition table.
Syncing disks.
[root@rhel6 ~]# partprobe

Set partition type

In order to use the disk we have to set a partition type. In this case we just make it a general Linux format #83. If it were going to be part of a raid array, we would make it Linux raid auto, #fd.

[root@rhel6 ~]# fdisk /dev/sda 

Command (m for help): t
Selected partition 1
Hex code (type L to list codes): L 

 0  Empty           24  NEC DOS         81  Minix / old Lin bf  Solaris        
 1  FAT12           27  Hidden NTFS Win 82  Linux swap / So c1  DRDOS/sec (FAT-
 2  XENIX root      39  Plan 9          83  Linux           c4  DRDOS/sec (FAT-
 3  XENIX usr       3c  PartitionMagic  84  OS/2 hidden C:  c6  DRDOS/sec (FAT-
 4  FAT16 <32M      40  Venix 80286     85  Linux extended  c7  Syrinx         
 5  Extended        41  PPC PReP Boot   86  NTFS volume set da  Non-FS data    
 6  FAT16           42  SFS             87  NTFS volume set db  CP/M / CTOS / .
 7  HPFS/NTFS/exFAT 4d  QNX4.x          88  Linux plaintext de  Dell Utility   
 8  AIX             4e  QNX4.x 2nd part 8e  Linux LVM       df  BootIt         
 9  AIX bootable    4f  QNX4.x 3rd part 93  Amoeba          e1  DOS access     
 a  OS/2 Boot Manag 50  OnTrack DM      94  Amoeba BBT      e3  DOS R/O        
 b  W95 FAT32       51  OnTrack DM6 Aux 9f  BSD/OS          e4  SpeedStor      
 c  W95 FAT32 (LBA) 52  CP/M            a0  IBM Thinkpad hi eb  BeOS fs        
 e  W95 FAT16 (LBA) 53  OnTrack DM6 Aux a5  FreeBSD         ee  GPT            
 f  W95 Ext'd (LBA) 54  OnTrackDM6      a6  OpenBSD         ef  EFI (FAT-12/16/
10  OPUS            55  EZ-Drive        a7  NeXTSTEP        f0  Linux/PA-RISC b
11  Hidden FAT12    56  Golden Bow      a8  Darwin UFS      f1  SpeedStor      
12  Compaq diagnost 5c  Priam Edisk     a9  NetBSD          f4  SpeedStor      
14  Hidden FAT16 <3 61  SpeedStor       ab  Darwin boot     f2  DOS secondary  
16  Hidden FAT16    63  GNU HURD or Sys af  HFS / HFS+      fb  VMware VMFS    
17  Hidden HPFS/NTF 64  Novell Netware  b7  BSDI fs         fc  VMware VMKCORE 
18  AST SmartSleep  65  Novell Netware  b8  BSDI swap       fd  Linux raid auto
1b  Hidden W95 FAT3 70  DiskSecure Mult bb  Boot Wizard hid fe  LANstep        
1c  Hidden W95 FAT3 75  PC/IX           be  Solaris boot    ff  BBT            
1e  Hidden W95 FAT1 80  Old Minix      
Hex code (type L to list codes): 83

Command (m for help): w
The partition table has been altered! 

Calling ioctl() to re-read partition table.
Syncing disks.