Internals

Dboxed volumes must be locked and mounted before they can be used. These are actions done internally by dboxed, but it is useful to understand these concepts to make debugging issues easier.

Locking

Before a volume can be mounted, it must be locked. Locking prevents that other machines mount the same volume, which would cause multiple versions of the volume to diverge.

Locking happens when you run dboxed volume-mount create <volume-name>. This is done automatically for attached volumes.

Mounting

Mounting happens when you run the mentioned volume-mount create command and also when you run dboxed volume-mount mount <volume-name>. The mount point is always /var/lib/dboxed/volumes/<mount-name>/mount.

Mounting requires multiple steps, including setup of a Linux loop device and lvm2 volumes. Dboxed uses lvm2 snapshots to ensure crash-level consistency of incremental backups. Without this, backups would basically always be inconsistent when workloads are actively writing to them while dboxed is backing them up.

Serving

Serving a volume means that the volume is periodically and incrementally backed up. To do this, dboxed will always first create a snapshot of the current volume filesystem and then back up the snapshot. After each backup, the snapshot is released again.