Use case

Modify data on a read-only disk.

Describe the solution you'd like

A new disk type cow, is configured on top of two other disks, the read-only "source" disk and another disk with modifications.
The disk with modifications will contain patches, represented by two subdirectories - one with new files, that are replacing (shadow away) the original files, the other for deletion marks for the files that were deleted without creating a new file.