How do I create a compressed backup image of a Red Hat Enterprise Linux machine to an external drive using dd?

Environment

• Red Hat Enterprise Linux 4
• Red Hat Enterprise Linux 5
• Red Hat Enterprise Linux 6
• Red Hat Enterprise Linux 7

Issue

• How do I create a compressed backup image of a Red Hat Enterprise Linux machine to an external drive using dd?
• Is it possible to create an image of installed system and use it to boot the system?

Solution

To create a compressed disk image with "dd" :

• Boot into rescue mode with the installation media
• Create a mount point for your external drive:

# mkdir /mnt/external

• Mount your external drive:

# mount /dev/sdb1 /mnt/external

• Create the compressed disk image:

# dd if=/dev/sda | gzip -9 > /mnt/external/filename.img.gz

• To recover this in future, run the following command :

# zcat /mnt/external/filename.img.gz | dd of=/dev/sda

Note: Do not use dd to backup block devices that are currently in use. Doing so will likely cause inconsistency on the filesystem(s).

Create an image of installed system

• To create an image of installed system, get USB of exact same size.
• Recreate the initrd with USB support

# vi /etc/dracut.conf
        add_drivers+="ehci-hcd uhci-hcd xhci-hcd usb-storage scsi-mod sd-mod sd hid-microsoft hid-generic usbhid"     
# dracut -f -v

• Force a file check on boot

#  touch /forcefsck

• Create image using dd command:

#  dd if=/dev/sda  of=/dev/<usb device> bs=8192

• Add any subsequent partitions to the usb drive in 4gb chunks.
• Insert usb and boot machine.