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Tag: imaging

Disk and Partition Imaging using dd

by on Jan.26, 2010, under Hardware/Disk Management, Server Administration

Linux provides an abundance of advanced command line tools to manage and modify just about anything on your system.  Today we will explore the use of dd, the primary tool on linux for creating and restoring disk images, among other things.

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:

  1. The disk in question can not be in use by an operating system
  2. A destination medium or network resource must be present that is large enough to hold the image.

To backup a disk using dd, the following procedure can be used.

  1. Boot the computer with the disk in question from a Linux Live CD, such as Ubuntu or Knoppix
  2. Mount a destination disk (such as a usb disk drive or nfs mount)
  3. Run dd command to backup disk
  4. Note the size of the disk partition if partitioning a new device is necessary when restoring the image

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:

As a root user, do the following:

Mount USB disk drive

# mount -t ext3 /dev/sdb1 /mnt/sdb1

Run dd command (piping output through gzip to save space):

# dd if=/dev/sda1 conv=sync,noerror bs=64k | gzip -c > /mnt/sdb1/windowsxp-c.img.gz

Definition of the dd command parameters:

“if=/dev/sda1” is the input file for the dd command, in this case, its linux device sda1
“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.

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.

“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.

Noting Partition configuration:

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:

# fdisk -l /dev/sda


Disk /dev/sda: 959.9 GB, 959966085120 bytes
255 heads, 63 sectors/track, 116709 cylinders
Units = cylinders of 16065 * 512 = 8225280 bytes
Disk identifier: 0x2e2d2e2d


Device Boot Start  End   Blocks     Id System
/dev/sda1*      1  15298 122881153+ 7  HPFS/NTFS
/dev/sda2   15299  51771 292969372+ 83 Linux
/dev/sda3   51772  52767   8000370  82 Linux swap / Solaris
/dev/sda4   52768 116709 513614115  83 Linux

The destination disk should have a partition defined identical to the source partition, total number of blocks is the important parameter here.

The partition geometry information can be backed up the the USB hard disk with the following command:

# fdisk -l /dev/sda > /mnt/sdb1/sda_fdisk.txt

Restoring a dd image to a disk/partition

The steps are similar to the backup process:

  1. Boot computer with destination disk from a Linux Live CD
  2. Partition the destination disk, if needed
  3. Mount the source media (usb disk or nfs mount)
  4. Use gunzip and dd to restore image to disk or partition

Here is an example session, to restore the image taken with the above steps:

As a root user, do the following:

Mount image source (USB hard disk at /dev/sdb1)

# mount -t ext3 /dev/sdb1 /mnt/sdb1

Partition destination disk:

# fdisk /dev/<device node in question>; in our case, sda.

<create partition if needed>

Restore image (destination partition is /dev/sda1)

# gunzip -c /mnt/sdb1/windowsxp-c.img.gz | dd of=/dev/sda1 conv=sync,noerror bs=64k

Note: On a fast machine, ie C2Q 6600, and 3ware RAID disk array, a 120GB image takes 25 minutes to create.

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.

A excellent guide on using fdisk for disk partitioning can be found here.

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