Adding a New Disk Drive to a CentOS System

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Managing and Monitoring CentOS based KVM Guest SystemsAdding a New Disk to a CentOS Volume Group and Logical Volume

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One of the first problems encountered by users and system administrators these days is that desktop systems and servers tend to run out of disk space to store data. Fortunately disk space is now one of the cheapest IT commodities. In the next two chapters we will look at the steps necessary to configure CentOS to use the space provided via the installation of a new internal disk drive.


[edit] Mounted File Systems or Logical Volumes

There are two ways to configure a new disk drive into a CentOS system. One very simple method is to create one or more Linux partitions on the new drive, create Linux file systems on those partitions and then mount them at specific mount points so that they can be accessed. This is the approach that will be covered in this chapter.

Another approach is to add the new space to an existing volume group or create a new volume group. When CentOS is installed using the default disk configuration layout, a volume group is created and called VolGroup00. Within this volume group are two logical volumes named LogVol00 and LogVol01 that are used to store the / file system and swap partition respectively. By configuring the new disk as part of a volume group we are able to increase the disk space available to the existing logical volumes. Using this approach we are able, therefore, to increase the size of the / file system by allocating some or all of the space on the new disk to LogVol00. This topic will be discussed in detail in Adding a New Disk to a CentOS Volume Group and Logical Volume.

[edit] Getting Started

This tutorial assumes that the new physical hard drive has been installed on the system and is visible to the operating system. The best way to do this is to enter the system BIOS during the boot process and ensuring that the BIOS sees the disk drive. Sometimes the BIOS will provide a menu option to scan for new drives. If the BIOS does not see the disk drive double check the connectors and jumper settings (if any) on the drive.

[edit] Finding the New Hard Drive in CentOS

Assuming the drive is visible to the BIOS it should automatically be detected by the operating system. Typically, the disk drives in a system are assigned device names beginning hd or sd followed by a letter to indicate the device number. For example, the first device might be /dev/sda, the second /dev/sdb and so on.

The following is output from a system with only one physical disk drive:

ls /dev/sd*
/dev/hda  /dev/hda1  /dev/hda2  /dev/hdc

This shows that the disk drive represented by /dev/hda is itself divided into 2 partitions, represented by /dev/hda1 and /dev/hda2. In this instance, /dev/hdc is the CDROM. In all likelihood, when a second disk drive is detected by the system it will be assigned to /dev/hdb.

Another option is to install and run the Hardware Browser. If this is already installed it may be launched by selecting Hardware from the System -> Administration menu. If this option is not available it may be installed as follows:

su -
yum install hwbrowser

In the Hardware Browser scroll down the list of devices until the disk drives become visible. Select the new disk device and click on the Advanced tab to identify the device name. In the following figure the new hard disk has been assigned to /dev/hdb:

A new disk drive listed in the CentOS Hardware Manager tool

The following output is from the same system after a second hard disk drive has been installed:

ls /dev/hd*
/dev/hda  /dev/hda1  /dev/hda2  /dev/hdb  /dev/hdc

As shown above, the new hard drive has been assigned to the device file /dev/hdb. At this point the drive has no partitions shown (because we have yet to create any).

At this point we have a choice of creating partitions and file systems on the new drive and mounting them for access or adding the disk as a physical volume as part of a volume group. To perform the former continue with this chapter, otherwise read Adding a New Disk to a CentOS Volume Group and Logical Volume for details on configuring Logical Volumes.

[edit] Creating Linux Partitions

The next step is to create one or more Linux partitions on the new disk drive. This is achieved using the fdisk utility which takes as a command-line argument the device to be partitioned:

fdisk /dev/hdb
Device contains neither a valid DOS partition table, nor Sun, SGI or OSF disklabel
Building a new DOS disklabel. Changes will remain in memory only,
until you decide to write them. After that, of course, the previous
content won't be recoverable.

The number of cylinders for this disk is set to 47536.
There is nothing wrong with that, but this is larger than 1024,
and could in certain setups cause problems with:
1) software that runs at boot time (e.g., old versions of LILO)
2) booting and partitioning software from other OSs
   (e.g., DOS FDISK, OS/2 FDISK)
Warning: invalid flag 0x0000 of partition table 4 will be corrected by w(rite)

Command (m for help):

In order to view the current partitions on the disk enter the p command:

Command (m for help): p

Disk /dev/hdb: 24.5 GB, 24533532672 bytes
16 heads, 63 sectors/track, 47536 cylinders
Units = cylinders of 1008 * 512 = 516096 bytes

   Device Boot      Start         End      Blocks   Id  System

As we can see from the above fdisk output the disk currently has no partitions because it is a previously unused disk. The next step is to create a new partition on the disk, a task which is performed by entering n (for new partition) and p (for primary partition):

Command (m for help): n
Command action
   e   extended
   p   primary partition (1-4)
Partition number (1-4): 

In this example we only plan to create one partition which will be partition 1. Next we need to specify where the partition will begin and end. Since this is the first partition we need it to start at cylinder 1 and since we want to use the entire disk we specify the last cylinder as the end. Note that if you wish to create multiple partitions you can specify the size of each partition by cylinders, bytes, kilobytes or megabytes.

Partition number (1-4): 1
First cylinder (1-47536, default 1):
Using default value 1
Last cylinder or +size or +sizeM or +sizeK (1-47536, default 47536):
Using default value 47536

Now that we have specified the partition we need to write it to the disk using the w command:

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

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

If we now look at the devices again we will see that the new partition is visible as /dev/hdb1:

ls /dev/hd*
/dev/hda  /dev/hda1  /dev/hda2  /dev/hdb  /dev/hdb1  /dev/hdc

The new partition will similarly be visible in the Hardware browser. The next step is to create a file system on our new partition.

[edit] Creating a File System on a CentOS Disk Partition

We now have a new disk installed, it is visible to CentOS and we have configured a Linux partition on the disk. The next step is to create a Linux file system on the partition so that the operating system can use it to store files and data. The easiest way to create a file system on a partition is to use the mkfs.ext3 utility which takes as arguments the label and the partition device:

/sbin/mkfs.ext3 -L /userdata /dev/hdb1

mke2fs 1.39 (29-May-2006)
Filesystem label=/userdata
OS type: Linux
Block size=4096 (log=2)
Fragment size=4096 (log=2)
2998272 inodes, 5989528 blocks
299476 blocks (5.00%) reserved for the super user
First data block=0
Maximum filesystem blocks=0
183 block groups
32768 blocks per group, 32768 fragments per group
16384 inodes per group
Superblock backups stored on blocks:
        32768, 98304, 163840, 229376, 294912, 819200, 884736, 1605632, 2654208,

Writing inode tables: done
Creating journal (32768 blocks): done
Writing superblocks and filesystem accounting information: done

This filesystem will be automatically checked every 32 mounts or
180 days, whichever comes first.  Use tune2fs -c or -i to override.

[edit] Mounting a File System

Now that we have created a new file system on the Linux partition of our new disk drive we need to mount it so that it is accessible to the CentOS system and its users. In order to do this we need to create a mount point. A mount point is simply a directory or folder into which the file system will be mounted. For the purposes of this example we will create a /userdata directory to match our file system label (although it is not necessary that these values match):

mkdir /userdata

The file system may then be manually mounted using the mount command:

mount /dev/hdb1 /userdata

Running the mount command with no arguments shows us all currently mounted file systems (including our new file system):

/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/hda1 on /boot type ext3 (rw)
tmpfs on /dev/shm type tmpfs (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/CentOS_5.4_Final type iso9660 (ro,noexec,nosuid,nodev,uid=500)
/dev/hdb1 on /userdata type ext3 (rw)

[edit] Configuring CentOS to Automatically Mount a File System

In order to set up the system so that the new file system is automatically mounted at boot time an entry needs to be added to the /etc/fstab file.

The following example shows an fstab file configured to automount our /userdata partition:

/dev/VolGroup00/LogVol00 /                      ext3    defaults        1 1
LABEL=/boot             /boot                   ext3    defaults        1 2
tmpfs                   /dev/shm                tmpfs   defaults        0 0
devpts                  /dev/pts                devpts  gid=5,mode=620  0 0
sysfs                   /sys                    sysfs   defaults        0 0
proc                    /proc                   proc    defaults        0 0
/dev/VolGroup00/LogVol01 swap                   swap    defaults        0 0
LABEL=/userdata         /userdata               ext3    defaults        1 2

With the appropriate configuration line added to the fstab file, the file system will automatically mount on the next system restart. 

Purchase and download the fully updated CentOS 6 Edition of this eBook in PDF, ePub & Kindle formats for only $9.99
Kindle/ePub/PDF edition contains 39 chapters and over 240 pages.

Add to Cart

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Managing and Monitoring CentOS based KVM Guest SystemsAdding a New Disk to a CentOS Volume Group and Logical Volume
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