The launcher declined on every btrfs host for a reason no operator could
guess: `/var/lib/docker` is created by mkdir, so it is an ordinary
directory inside the enclosing subvolume, and `btrfs subvolume snapshot`
needs a subvolume root as its source. Documenting that as a manual
prerequisite would have left the feature switched on and never firing.
ensure_subject_subvolume.sh converts the data root on the deploy that
introduces the mode. The copy uses `cp --reflink=always`: both trees live
on the same btrfs, so extents are shared instead of duplicated and the
conversion costs seconds and no extra space no matter how large the data
root is - where an rsync copy would have meant hours of downtime and a
free-space requirement of 110%. The original tree is kept until the daemon
has answered on the converted root, so a failed copy, a mismatched
verification pass or a daemon that will not start all put it back and
start docker on it again.
The mode itself moves from group_vars into meta/services.yml as
`snapshot_mode`, read through `lookup('config', ...)`, so a single host can
override it from the inventory. Falsey spellings normalise to `never` and
anything else fails a deploy-time assert, because YAML reads a bare
off/no/false as a boolean and `--mode False` would kill the unit nightly.
btrfs-progs is installed wherever a btrfs mount exists, with centos opted
out: centos stream ships neither the kernel module nor the package, and
demanding it there would fail the package-availability test for the whole
RedHat family.
The pin moves to 3.2.2, which is not cosmetic: that release carves the
snapshot inside its subject, and the reaper here looks for leftovers in
exactly that place. Against 3.2.1 it would search beside the data root and
never find one.
Two drills run before anything else in test.sh: subvolume_probe.sh proves
the conversion carries every path, payload and symlink across and that the
result can actually be snapshotted; snapshot_probe.sh pins the launcher's
decision, including the refusals that keep a host on the live copy.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
kevinveenbirkenbachandClaude Opus 5