Difference between revisions of "Disk cloning"
Revision as of 00:23, 1 January 2010
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.
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.
Disk cloning using dd
One of the disadvantages of the dd method over software specifically designed for the job is that dd will store the entire partition, including blocks not currently used. The overhead is not important as long as you can compress the image. To make sure that the emtpy blocks do not contain random junk that hare difficult to compress, it is recommended to blank all unused blocks before making the image. After doing that, the unallocated blocks will contain mostly zeros and will therefore compress down to almost nothing.
Mount the partition, then create a file of zeros which fills the entire disk, then delete it again.
# dd if=/dev/zero of=/tmp/zero.crap bs=8M; rm -f /tmp/zero.crap; sync
Backing up the MBR
The MBR is stored in the the first 512 bytes of the disk. It consist of 3 parts 1. The first 446 bytes contain the boot loader. 2. The next 64 bytes contain the partition table (4 entries of 16 bytes each, one entry for each primary partition). 3. 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".
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
To restore your system:
# gunzip -c /mnt/sda1/hda.img.gz | dd of=/dev/hda