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.
- 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
- Mount the file as a loop device:
sudo losetup /dev/loop0 image-repository.img
- Partition the loop device:
sudo gdisk /dev/loop0
- Create new GPT:
o then y
- Create new partition, enter a few times to accept defaults:
n
- Write the table, hit
w then y
- Reread paritions from the loop device:
sudo partprobe /dev/loop0
- Format the loop0p1 partition:
sudo mkfs.btrfs /dev/loop0p1
- Mount the filesystem with compression enabled:
sudo mount -o compress=lzo /dev/loop0p1 /mnt/img-repository
- 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!