2 files changed, 3 insertions, 5 deletions

diff --git a/modules/zfssnap/default.nix b/modules/zfssnap/default.nix
index 1dbd7e17..5093f365 100644
--- a/modules/zfssnap/default.nix
+++ b/modules/zfssnap/default.nix
@@ -77,7 +77,7 @@ in {
         ExecStart = let
           mkSectionName = name: strings.escape [ "[" "]" ] (strings.toUpper name);
           zfssnapConfig = generators.toINI { inherit mkSectionName; } cfg.config;
-        in "${zfssnap}/bin/zfssnap -v prune --config=${pkgs.writeText "zfssnap.ini" zfssnapConfig}";
+        in "${zfssnap}/bin/zfssnap -v prune --dry-run --config=${pkgs.writeText "zfssnap.ini" zfssnapConfig}";
       };
     };
 
diff --git a/modules/zfssnap/zfssnap.py b/modules/zfssnap/zfssnap.py
index 32bc4464..96202cf4 100644
--- a/modules/zfssnap/zfssnap.py
+++ b/modules/zfssnap/zfssnap.py
@@ -71,17 +71,15 @@ def prune(config, dry_run, keep_newest):
                 continue
             items[base_name].append(Snap(name=row.name, creation=creation))
 
-    keep = set()
     kept_count = defaultdict(lambda: defaultdict(lambda: 0))
     KeptBecause = namedtuple('KeptBecause', ['rule', 'ix', 'base', 'period'])
     kept_because = OrderedDict()
     def keep_because(base, snap, rule, period=None):
-        nonlocal KeptBecause, keep, kept_count, kept_because
+        nonlocal KeptBecause, kept_count, kept_because
         kept_count[rule][base] += 1
         if snap not in kept_because:
             kept_because[snap] = deque()
         kept_because[snap].append(KeptBecause(rule=rule, ix=kept_count[rule][base], base=base, period=period))
-        keep.add(snap)
 
     within = config.gettimedelta('KEEP', 'within')
     if within > timedelta(seconds=0):
@@ -143,7 +141,7 @@ def prune(config, dry_run, keep_newest):
         reasons_str = ', '.join(map(str, reasons))
         logger.info(f'Keeping ‘{snap}’ because: {reasons_str}')
     all_snaps = {snap.name for _, snaps in items.items() for snap in snaps}
-    to_destroy = all_snaps - keep
+    to_destroy = all_snaps - {*kept_because}
     if not to_destroy:
         logger.info('Nothing to prune')