Disk cloning is the process of making an image of a partition or an entire hard drive. This can be useful both for copying the drive to other computers and for backup/recovery purposes.
The dd command is a simple, yet versatile and powerful tool. It can be used to copy from source to destination, block-by-block, regardless of their filesystem types or operating systems. A convenient method is to use dd from a live environment, as in a livecd.
Cloning a partition
From physical disk /dev/sda, partition 1, to physical disk /dev/sdb, partition 1.
dd if=/dev/sda1 of=/dev/sdb1 bs=4096 conv=notrunc,noerror
If output file of (sdb1 in the example) does not exist, dd will start at the beginning of the disk and create it.
Cloning an entire hard disk
From physical disk /dev/sda to physical disk /dev/sdb
dd if=/dev/sda of=/dev/sdb bs=4096 conv=notrunc,noerror
This will clone the entire drive, including MBR (and therefore bootloader), all partitions and data.
- notrunc or 'do not truncate' maintains data integrity by instructing dd not to truncate any data.
- noerror instructs dd to continue operation, ignoring all input errors. Default behavior for dd is to halt at any error.
- bs=4096 sets the block size to 4k, an optimal size for hard disk read/write efficiency and therefore, cloning speed.
Backing up the MBR
The MBR is stored in the the first 512 bytes of the disk. It consist of 3 parts:
- The first 446 bytes contain the boot loader.
- The next 64 bytes contain the partition table (4 entries of 16 bytes each, one entry for each primary partition).
- The last 2 bytes contain an identifier
To save the MBR into the file "mbr.img":
# dd if=/dev/hda of=/mnt/sda1/mbr.img bs=512 count=1
To restore (be careful : this could destroy your existing partition table and with it access to all data on the disk):
# dd if=/mnt/sda1/mbr.img of=/dev/hda
If you only want to restore the boot loader, but not the primary partition table entries, just restore the first 446 bytes of the MBR:
# dd if=/mnt/sda1/mbr.img of=/dev/hda bs=446 count=1.
To restore only the partition table, one must use
# dd if=/mnt/sda1/mbr.img of=/dev/hda bs=1 skip=446 count=64".
You can also get the MBR from a full dd disk image.
#dd if=/path/to/disk.img of=/mnt/sda1/mbr.img bs=512 count=1
Create disk image
1. Boot from a liveCD or liveUSB.
2. Make sure no partitions are mounted from the source hard drive.
3. Mount the external HD
4. Backup the drive.
# dd if=/dev/hda conv=sync,noerror bs=64K | gzip -c > /mnt/sda1/hda.img.gz
5. Save extra information about the drive geometry necessary in order to interpret the partition table stored within the image. The most important of which is the cylinder size.
# fdisk -l /dev/hda > /mnt/sda1/hda_fdisk.info
NOTE: You may wish to use a block size (bs=) that is equal to the amount of cache on the HD you are backing up. For example, bs=8192K works for an 8MB cache. The 64K mentioned in this article is better than the default bs=512 bytes, but it will run faster with a larger bs=.
To restore your system:
# gunzip -c /mnt/sda1/hda.img.gz | dd of=/dev/hda
The cp program can be used to clone a disk, one partition at a time. An advantage to using cp is that the filesystem type of the destination partition(s) may be the same or different than the source. For safety, perform the process from a live environment.
The basic procedure from a live environment will be:
- Create the new destination partition(s) (using fdisk, cfdisk or other tools available in the live environment.
- Create a filesystem on each of the newly created partitions. Example:
mkfs -t ext3 /dev/sdb1
- Mount the source and destination partitions. Example:
mount -t ext3 /dev/sda1 /mnt/source mount -t ext3 /dev/sdb1 /mnt/destination
- Copy the files from the source partition to the destination
cp -a /mnt/source /mnt/destination
-a: preserve permissions, never follow symbolic links and copy recursively
- Change the mount points of the newly cloned partitions in /etc/fstab accordingly
- Finally, install the GRUB bootloader if necessary. (See GRUB)
Disk cloning software
Disk cloning in Arch
The ncurses program PartImage is in the community repos. The interface is not exceptionally intuitive but it works. There are currently no GTK/QT based disk cloners for Linux. Another option is to use dd, a small CLI image/file creation utility. The wikipedia has a list of various version of dd, specifically oriented to this purpose . dd_rescue works efficiently with corrupt disks copying error free areas first and later retrying error areas.
Disk cloning outside of Arch
If you wish to backup or propagate your Arch install root, you are probably better off booting into something else and clone the partition from there. Some suggestions:
- PartedMagic has a very nice live cd/usb with PartImage and other recovery tools.
- Mindi is a linux distribution specifically for disk clone backup. It comes with its own cloning program, Mondo Rescue.
- Acronis True Image is a commercial disk cloner for Windows. It allows you to create a live cd (from within Windows), so you do not need a working Windows install on the actual machine to use it.
- FSArchiver allows you to save the contents of a file system to a compressed archive file. Can be found on the System Rescue CD.
- Clonezilla is an enhanced partition imager which can also restore entire disks as well as partitions.