mjhjmtu

If I wish to clone my linux image using the dd utility, I can do for example dd if=/dev/SDX of=/dev/SDY , where SDX is the internal memory device where the linux image is stored, and SDY is the external memory device I would like to copy it to.

However if I do this without cloning to a different memory device and use the same internal memory, example : dd if=/dev/SDX of=/home/my_dir/my_image.img , this is creating an image file in the same internal memory device that the actual image resides. This seems to work fine for me, but how does it? /home/my_dir/ is inside /dev/SDX which I am cloning?

Indeed, this seems to work fine – with the emphasis on "seems".

To simplify the explanation, I assume a legacy filesystem without journaling (like ext2).

As soon as dd starts, the file my_image.img is created. The information that this file exists and where it physically resides on the disk, is stored in the filesystem's index (think of it like a "table of contents"). The file's final size is not yet known, so it takes up a minimal default size (this is usually not shown to the user application).

As data is written to the file, it grows and the index is updated on regular intervals. However, the index resides on the disk, too. Once dd copied the index, the index in the copy is no longer updated. If you actually tried using the copy, you would notice that it is incomplete.

All this is academical of course, as dd cannot fit the full image file inside itself (unless you employ some kind of compression).

With commands like these, you can try it for yourself:

dd if=/dev/zero of=demo.raw bs=1M count=256 # create a dummy "partition" for demonstration
mkfs.ext4 demo.raw # create ext4 filesystem
target=$(mktemp -d) # create a temporary mountpoint
sudo mount demo.raw $target # mount the filesystem
sudo chown $USER $target # make filesystem root writable for current user
dd if=demo.raw of=$target/inner.raw bs=1M # copy fileystem into itself – error as expected (no space left on device)
rm $target/inner.raw
dd if=demo.raw bs=1M | gzip -c > $target/inner.raw.gz # copy fileystem into itself, using compression
gzip -c $target/inner.raw.gz > inner.raw # decompress image
sudo mount inner.raw /mnt # mount inner copy – error as expected (copy is an incomplete file)
sudo umount $target
rm demo.raw inner.raw

this shouldnt work.
you cannot put an 8G RAW image inside the same filesystem with 6G of free space.
possible explanation: /dev/SDX has read errors (bad sector, or alike), and dd stopped before reaching the end of the disk, resulting in the output image smaller than the disk (so it fits). try again the dd with:

# dd if=/dev/SDX oflag=direct conv=noerror,notrunc of=/path/image.img

and also check the size of disk:

# df -h

and the size of the generated image:

# ls -lh /path/image.img