kevinveenbirkenbach/docker-volume-backup
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fix(backup): carve the btrfs snapshot inside its subject, not beside it

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The kernel refuses a snapshot whose destination sits on another
filesystem. Placing it at <parent>/.baudolo-<tag> hit that on the most
sensible layout there is: a dedicated disk mounted straight onto
/var/lib/docker, where the parent directory belongs to a different
filesystem and every run failed with EXDEV.

Placing it at <subject>/.baudolo-<tag> makes source and destination the
same filesystem by construction, so the failure cannot occur on any
layout. It also aligns the two backends: the zfs path already resolves
its snapshot inside the subject, at <subject>/.zfs/snapshot/<tag>.

btrfs does not include nested subvolumes in a snapshot, so a leftover
from an interrupted run appears in the next snapshot as an empty
directory rather than recursing, and the copy only ever reads the volume
tree below it.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
This commit is contained in:
Kevin Veen-Birkenbach
2026-07-31 19:19:42 +02:00
parent 4e2b3641f9
commit eeaa838d02
3 changed files with 11 additions and 8 deletions
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2
tests/unit/backup/test_app_snapshot.py
View File

@@ -64,7 +64,7 @@ class TestSnapshotBranch(unittest.TestCase):
def test_it_reads_from_the_snapshot_and_not_from_the_live_tree(self) -> None:
source = drive(present=True)[0]["source"]
self.assertTrue(source.startswith("/var/lib/.baudolo-"), source)
self.assertTrue(source.startswith("/var/lib/docker/.baudolo-"), source)
def test_it_compares_by_content_against_the_previous_generation(self) -> None:
self.assertTrue(drive(present=True)[0]["authoritative"])

12
tests/unit/backup/test_snapshot.py
View File

@@ -22,27 +22,27 @@ class Runner:
class TestBtrfs(unittest.TestCase):
def test_it_creates_a_read_only_snapshot_beside_the_subject(self) -> None:
def test_it_creates_a_read_only_snapshot_inside_the_subject(self) -> None:
run = Runner()
with volume_snapshot("btrfs", "/var/lib/docker", "20260731", run=run):
pass
self.assertEqual(
run.calls[0],
"btrfs subvolume snapshot -r /var/lib/docker /var/lib/.baudolo-20260731",
"btrfs subvolume snapshot -r /var/lib/docker /var/lib/docker/.baudolo-20260731",
)
def test_it_removes_the_snapshot_afterwards(self) -> None:
run = Runner()
with volume_snapshot("btrfs", "/var/lib/docker", "20260731", run=run):
pass
self.assertEqual(run.calls[-1], "btrfs subvolume delete /var/lib/.baudolo-20260731")
self.assertEqual(run.calls[-1], "btrfs subvolume delete /var/lib/docker/.baudolo-20260731")
def test_it_maps_a_volume_path_into_the_snapshot(self) -> None:
run = Runner()
with volume_snapshot("btrfs", "/var/lib/docker", "20260731", run=run) as resolve:
self.assertEqual(
resolve("/var/lib/docker/volumes/postgres_data/_data"),
"/var/lib/.baudolo-20260731/volumes/postgres_data/_data",
"/var/lib/docker/.baudolo-20260731/volumes/postgres_data/_data",
)
def test_it_keeps_the_trailing_slash_rsync_reads_as_contents(self) -> None:
@@ -50,7 +50,7 @@ class TestBtrfs(unittest.TestCase):
with volume_snapshot("btrfs", "/var/lib/docker", "20260731", run=run) as resolve:
self.assertEqual(
resolve("/var/lib/docker/volumes/postgres_data/_data/"),
"/var/lib/.baudolo-20260731/volumes/postgres_data/_data/",
"/var/lib/docker/.baudolo-20260731/volumes/postgres_data/_data/",
)
def test_it_removes_the_snapshot_even_when_the_body_raises(self) -> None:
@@ -109,7 +109,7 @@ class TestRejections(unittest.TestCase):
def test_the_subject_itself_resolves_to_the_snapshot_root(self) -> None:
run = Runner()
with volume_snapshot("btrfs", "/var/lib/docker", "20260731", run=run) as resolve:
self.assertEqual(resolve("/var/lib/docker"), "/var/lib/.baudolo-20260731")
self.assertEqual(resolve("/var/lib/docker"), "/var/lib/docker/.baudolo-20260731")
class Busy(Runner):