A volume with a backing store of its own is not in a snapshot of the docker data root: it appears there as an existing empty directory, so the copy succeeds, the generation is stamped complete, and the volume is empty in it. The existing check only asked whether the path was inside the snapshot, which that empty directory answers with yes.
The driver, its options and the filesystem the mountpoint sits on now decide, per volume. An uncaptured volume is copied live - correct data without the point in time - while every other volume of the same run keeps its snapshot. One NFS volume no longer costs the whole host its consistent backup.
A volume resolving outside the subject degrades the same way instead of aborting the run.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
volume_is_fully_ignored can only skip a volume when every container using it is ignored, so a container holding a derived tree next to state that must be kept cannot express the exclusion at all. The matrix docker-in-docker runner is exactly that: matrix_mdad_docker, matrix_mdad_matrix and matrix_mdad_state all hang off one container, and the derived one is an inner overlay2 store that no rsync in the chain can restore faithfully (none carries -X, so trusted.overlay.* is stripped in both directions).
--volumes-no-backup-required names volumes directly. The check runs before containers_using_volume, so an excluded volume costs no docker call and the decision no longer depends on which containers happen to exist at backup time.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
