Restore image onto oversubscribed drive

gistfile1.txt

There's a good post over at [brashear.me][1] which creates a btrfs partition stored as a file. The partition supports compression and can be used by ddrescue transparently. The blog post is reproduced below, should it ever become unavailable.

> 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 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 by pressing `o` followed by `y`
> 
> 
> Create new partition by pressing `n` and pressing enter a few times to
> accept defaults.
> 
> Write the table, by pressing `w` then `y`
> 
> Reread partitions from the loop device:
> 
>     sudo partprobe /dev/loop0
> 
> Format the loop0p1 partition:
> 
>     sudo mkfs.btrfs /dev/loop0p1
> 
> Mount the filesystem with compression enabled. Valid compression options are
> `zlib, lzo, zstd`:
> 
>     mkdir /mnt/img-repo
>     sudo mount -o compress=zstd /dev/loop0p1 /mnt/img-repo
> 
> Set the c attr on the mount directory:
> 
>     sudo chattr +c /mnt/img-repo
> 
> Now, when you create a `ddrescue` image inside of the `/mnt/img-repo/`
> folder, it will be transparently compressed!

At a later date, you can access your ddrescue'd image using:

    losetup --find --show image-repository.img
    partprobe /dev/loopX     //where X is the output of the previous command
    mkdir /mnt/img-repo
    mount -o compress=zstd /dev/loopXp1 /mnt/img-repo
    chattr +c /mnt/img-repo
    cd /mnt/img-repo

And you can inspect the contents of the ddrescue'd image using:

    losetup --read-only --find --show /mnt/img-repo/disk.img
    mkdir /mnt/recovered_files/
    mount /dev/loopXp1 /mnt/recovered_files/     //where X is the output of the previous command

[1]: https://brashear.me/blog/2017/11/14/how-to-create-a-compressed-ddrescue-image/