How to create a Compressed DDRescue Image

As disk sizes explode, I've found myself having to mirror disks which I don't have enough storage for. My tool of choice is ddrescue. However, it doesn't support compression because it needs to be able to seek through the output as it rescues data. A solution I've found is to create a sparse file, format it btrfs, and mount it with the compression=lzo option. This allows ddrescue to operate normally, while giving me fast + decent compression.

  1. Create a sparse file which is larger than your source disk: dd if=/dev/zero bs=1 count=0 seek=4T of=image-repository.img
  2. Mount the file as a loop device: sudo losetup /dev/loop0 image-repository.img
  3. Partition the loop device: sudo gdisk /dev/loop0
  4. Create new GPT: o then y
  5. Create new partition, enter a few times to accept defaults: n
  6. Write the table, hit w then y
  7. Reread paritions from the loop device: sudo partprobe /dev/loop0
  8. Format the loop0p1 partition: sudo mkfs.btrfs /dev/loop0p1
  9. Mount the filesystem with compression enabled: sudo mount -o compress=lzo /dev/loop0p1 /mnt/img-repository
  10. Set the c attr on the mount directory: sudo chattr +c /mnt/img-repository

Now, when you create a ddrescue image inside of /mnt/img-repository, it will be transparently compressed!