Linux provides an abundance of advanced command line tools to manage and modify just about anything on your system.\u00a0 Today we will explore the use of dd, the primary tool on linux for creating and restoring disk images, among other things.<\/p>\n
The dd (diskdump) on Linux can be used to backup an entire disk or partition to an image file. Several caveats apply to this method:<\/p>\n
To backup a disk using dd, the following procedure can be used.<\/p>\n
Here is an example session, to back up a single partition (sda1) containing a Windows XP installation to a USB hard disk mounted at \/mnt\/sdb1:<\/p>\n
As a root user, do the following:<\/p>\n
Mount USB disk drive “if=\/dev\/sda1” is the input file for the dd command, in this case, its linux device sda1 Even if your Hard disk exhibits no errors, dd will read every single block, including any which the OS avoids because it has marked them as bad.<\/p>\n “bs=64k” is the block size of 64 kilobytes. Using a large block size speeds up the copy process. The output of this is then passed to gzip for compression and storage in a file on the destination device.<\/p>\n Noting Partition configuration:<\/p>\n Using the command fdisk -l \/dev\/<device> where <device> is the device node of the disk being backed up, make note of the number of blocks used to create the partition: The partition geometry information can be backed up the the USB hard disk with the following command: The steps are similar to the backup process:<\/p>\n Here is an example session, to restore the image taken with the above steps:<\/p>\n As a root user, do the following:<\/p>\n Mount image source (USB hard disk at \/dev\/sdb1) <create partition if needed><\/p>\n Restore image (destination partition is \/dev\/sda1) In order to have a bootable system, some other configuration may be needed such as restoring a boot block. Check out the next post for details on boot sector management with dd.<\/p>\n
\n
\n# mount -t ext3 \/dev\/sdb1 \/mnt\/sdb1
\n<\/code>
\nRun dd command (piping output through gzip to save space):
\n
\n# dd if=\/dev\/sda1 conv=sync,noerror bs=64k | gzip -c > \/mnt\/sdb1\/windowsxp-c.img.gz
\n<\/code>
\nDefinition of the dd command parameters:<\/p>\n
\n“conv=sync,noerror instructs dd that if it can’t read a block due to an error then it should at least write something to its output of the correct length.<\/p>\n
\n
\n# fdisk -l \/dev\/sda
\n<\/code>
\n
\nDisk \/dev\/sda: 959.9 GB, 959966085120 bytes
\n255 heads, 63 sectors\/track, 116709 cylinders
\nUnits = cylinders of 16065 * 512 = 8225280 bytes
\nDisk identifier: 0x2e2d2e2d
\n<\/code>
\n
\nDevice Boot Start\u00a0 End\u00a0\u00a0 Blocks\u00a0\u00a0\u00a0\u00a0 Id System
\n\/dev\/sda1*\u00a0\u00a0\u00a0\u00a0\u00a0 1\u00a0 15298 122881153+ 7\u00a0 HPFS\/NTFS
\n\/dev\/sda2\u00a0\u00a0 15299\u00a0 51771 292969372+ 83 Linux
\n\/dev\/sda3\u00a0\u00a0 51772\u00a0 52767 \u00a0 8000370 \u00a082 Linux swap \/ Solaris
\n\/dev\/sda4\u00a0\u00a0 52768 116709 513614115 \u00a083 Linux
\n<\/code>
\nThe destination disk should have a partition defined identical to the source partition, total number of blocks is the important parameter here.<\/p>\n
\n
\n# fdisk -l \/dev\/sda > \/mnt\/sdb1\/sda_fdisk.txt
\n<\/code>
\nRestoring a dd image to a disk\/partition<\/p>\n\n
\n
\n# mount -t ext3 \/dev\/sdb1 \/mnt\/sdb1
\n<\/code>
\nPartition destination disk:
\n
\n# fdisk \/dev\/<device node in question>; in our case, sda.<\/code><\/p>\n
\n
\n# gunzip -c \/mnt\/sdb1\/windowsxp-c.img.gz | dd of=\/dev\/sda1 conv=sync,noerror bs=64k
\n<\/code>
\nNote: On a fast machine, ie C2Q 6600, and 3ware RAID disk array, a 120GB image takes 25 minutes to create.<\/p>\n