Cloning with DD command

How can I backup a partition (not the whole disk) to another partition on an external hard-disk using DD?

I achieved this with the following command:

sudo dd if=/dev/sda6 of=/dev/sdb3 bs=1M 

However my requirements are:

1. I need to backup /dev/sda6 and store that as an image (.img) file in /dev/sdb3

2. Preferably as a compressed (.gz) image file (this is actually what I am trying to achieve)

(Here are my foolish attempts that, obviously, didn't work:

sudo dd if=/dev/sda6 of=/dev/sdb3/backup.img bs=4096 conv=notrunc,noerror

sudo dd bs=1M if=/dev/sda6 | gzip -c > /dev/sdb3/backup.gz

Solution 1:

Unless /dev/sdb3 is actually mounted on /dev/sdb3 ( I doubt it, please research a bit on devices and mount points), you'd need to:

1. Find out where /dev/sdb3 is mounted. Use the mount command for this. Assume that /dev/sdb3 is mounted in /home.

2. Point your file writing to that place:

   sudo dd if=/dev/sda6 of=/home/backup.img
   

3. Once done, verify the img file contains what you expect.

Also, is there any need to back up the entire partition? I usually find it easier and more useful to back up files using rsync or something similar, then if needed compressing the resulting backup directory. But this is really up to you.

Solution 2:

What you want to do involves 3 simple steps:

1. Creating a disk file
2. Copying data from partition into the disk file
3. Compress the disk file

Creating a disk file

1. Mount and CD to the partition where you want to keep a disk file

2. Use fallocate to create and preallocate blocks to a disk file. It's much faster than creating a file by filling it with zeros using dd or other tools. To specify the disk file size you can use MB/MiB/GB/GiB prefixes. For example if you want a disk image with size 50 GiB, you do

   $ fallocate -l 50GiB part_backup.img

3. Format the disk file. In this example, I use ext4 and disable journaling feature because there's no need for it in this case

   $ mkfs.ext4 -O '^has_journal' part_backup.img

4. Now that we have our disk file ready, mount the disk file with losetup. Pass --show option to it so it prints the loopback device that your disk file is mounted to, eg /dev/loop0

   $ sudo losetup --show -f part_backup.img

5. Create a mount folder in /mnt and mount the loopback device to the folder

   $ sudo mount /dev/loop0 /mnt/my_disk_file

Copying data from partition into the disk file

1. Mount the partition that contains files you want to backup and identify the mount point

2. Use rsync to copy the files into the disk file you just mounted.

   $ sudo rsync -avzPSX /mnt/media_partition/ /mnt/my_disk_file

3. Please look up what each of those rsync opts does in rsync man if you're unusure. Also read the note about trailing slash in backup source

Compress the disk file

1. Unmount the disk file as per normal

   $ sudo umount /mnt/my_disk_file
$ sudo losetup -d /dev/loop0

2. Use gzip to compress the disk file