Backup is important part of the Linux system administration. Without backups a system is not safe and also the system administrator position too 🙂 . There are different ways to take backup. Some tools provides user level backup operation like copying or sync files. Other backup tools provides more file system level backup operation. One part of the Linux operating system is low level. Simply it has a lot of low level applications those helps system administrators. In this tutorial we will look dd command or tool which can do low level raw disk operations.

Warning

First of all dd command is low level command and use cautiously. While providing disk and partitions parameters use fdisk command to get disks path. Check twice before running command. You can find fdisk command tutorial below.

Linux fdisk Tutorial With Examples

Copy/Backup Entire Hard Disk

As we know Linux operating system provides raw access to the disk and partitions. These disks and partitions are provided in /dev/ path. So in order to use dd we will provide exact disk and partition file names. This operations also called as clone because a single is copied as a whole into another disk without a modification.

In this example we will create backup of vda to the disk sda . Before starting backup or copy operations be sure that sda size is bigger than vda .

$ sudo dd if=/dev/vda of=/dev/sda

• if provides the source hard disk
• of provides the destination hard disk

Copying operation duration can be changed according to write performance of the destination disk and read performance of source disk.

Set Byte Size

dd copy operations reads some bytes at a time and write to new disk. If we want to make operation faster and know that our disks are performative. We can change the a time copy size with bs parameter. bs size is provided by byte but following suffixes can be used too.

• kB=1000 byte
• K = 1024 byte
• MB = 1000 * 1000 byte
• M = 1024*1024 byte
• GB = 1000*1000*1000 byte
• G = 1024*1024*1024 byte

In this example we will set the byte size as 5M for copy operation.

$ sudo dd bs=5M if=/dev/vda of=/dev/sda

Create Hard Disk Image

In previous examples we have copied the whole disk image into another disk. In most situations we generally use disk images for backup creation. We can write copied disk image as a file into another disk.

In this example we will create disk image as a file named sda.img into folder /mnt/backup/ which is mounted from other disk. Other or destination disk can be an NFS, DAS, NAS, SAN etc.

$ sudo dd if=/dev/vda  of=/mnt/backup/sda.img

Create Floppy Image

Once a time floppy images were very popular in IT. Event operation system was installed from Floppies and backing up them were important. Actually creating a floppy image is the same as disk image because all of the sources are same type.

In this example we assume the floppy device is named as fd0 and we will copy into ~/ home directory as floppy.img with the following command.

$ sudo dd if=/dev/fd0  of=~/floppy.img

Copy/Backup CDROM

Another device popular used in Linux servers is CDROM. CDROM can provide data about Linux installation or other critical data. We can copy CDROM device too with dd command. But here we need to check one point. To make things without error we should unmount the CDROM if it is mounted.

In this example the cdrom device name is cdrom and the backup name is linux.iso .

$ sudo dd if=/dev/cdrom  of=~/linux.iso

Copy/Backup A Partition

Up to now we have copied who disks or devices. But generally we need to copy only a single partition of a hard disk. Actually there is no difference for copying disk partitions. Disk partitions generally named by numbering the partitions index and disk index. For example first partition of the disk named vda will vda0 and second partition will be named vda1 etc.

In this example we will copy or backup the second partition of the disk vda to a file named back.img

$ sudo dd if=/dev/vda1  of=/mnt/backup/back.img

Redirect and Compress DD Backups

While taking backups about the disk there will be a lot of data. This will make the size very big. There are different ways to lower the size of the backup image taken by dd command. What the dd command do is it reads from given source device and then we can compress the image before saving to a file.

In this example we will backup the device vda1 and save as back.img.gz . We will redirect dd output with | pipe to the gzip command.

$ sudo dd if=/dev/vda1  | gzip /mnt/backup/back.img.gz

In this example we have used gzip but also use other compression formats and applications 7z , xz  , … as is.

Fill Hard Disk with Zeros

dd provides low level operations. We can also write specified hard disk with a given data source. If we want to delete all data resides on the disk we can use dd too. In Linux operation systems devices named zero exists which is psedeu devices. This devices creates zeros as stream. In order to fill a hard disk device with zero we will read this devices and write the destination devices like below.

$ sudo dd if=/dev/zero  of=/dev/vda

Fill Hard Disk with Random Data

Filling whole disk with zero is an option. But there is also other option which is more secure. We can use random device of Linux operating system. This device will generate random numbers and we will write these into the destination hard disk or device.

In this example we will write random data device named vda.

$ sudo dd if=/dev/random  of=/dev/vda