Creating Disk Image for Root Filesystem

To create a disk image:

1. Use dd command.
   A blank disk image is generated.

2. Create a filesystem.

Following command creates an ext3 filesystem on the above created disk image:

mkfs -t ext3 ImageFileName.img

Output of this command looks like:

mke2fs 1.39 (29-May-2006)

ImageFileName.img is not a block special device.

Proceed anyway? (y,n) y

Filesystem label=

OS type: Linux

Block size=4096 (log=2)

Fragment size=4096 (log=2)

786432 inodes, 1572864 blocks

78643 blocks (5.00%) reserved for the super user

First data block=0

Maximum filesystem blocks=1610612736

48 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

Writing inode tables: done                            

Creating journal (32768 blocks): done

Writing superblocks and filesystem accounting information: done

This filesystem is automatically checked after every 31 mounts or 180 days (whichever comes first).  Use tune2fs -c or -i to override. One of the easiest ways to install the necessary files on the guest OS root filesystem is to simply copy the files from the host operating system.

Suppose,you may need a minimal Linux installation for your guest OS. If the host system has graphical desktop environments,office productivity applications and other packages not needed by the guest OS then cloning the host environment may not be the best approach.

If,on the other hand,you are confident that everything on your host operating system is of use in the guest system,or are unconcerned about disk space then this approach is better.

Prior to copy any file onto the root filesystem disk image, you should mount it. Disk images are mounted using  the Linux loopback interface and require a pre-existing mount point. In the following example a mount point is created at /mnt:

mount -o loop ImageFileName.img /mnt

3. Copy the relevant files from the host operating system to guest root filesystem image
   
   cp -ax /{bin,dev,etc,lib,root,sbin,usr,var} /mnt

It takes some time. Once the files are copied, a number of additional directories need to be created.
 

mkdir /mnt/{home,proc,opt,sys,tmp}

The tmp directory created above must have read/write access for all users. To ensure this the chmod command needs to be run:

chmod 777 /mnt/tmp.

The /mnt/etc/fstab file currently present in the guest root filesystem is a direct replica of the file used by the host operating system. The following command shows a typical file from a CentOS, Red Hat Enterprise Linux or Fedora host operating system

cat /mnt/etc/fstab

Output of this command looks like:

The guest OS copy of the fstab file needs to be modified such as: