backup: enforce a minimum compaction window size of 3

Previously, the minimum compaction window size was set to 2. With a
compaction window of 2, if a chain is at length `n` prior to a backup
that triggers a compaction, the length of the chain after compaction is
also `n`. Under these conditions, if compactions fail/are blocked for
any reason and the length of the chain exceeds the threshold set by
`backup.compaction.threshold`, the chain will never recover and drop
below the threshold.

This patch enforces that the minimum compaction window size is at least
3. That way, if the length of the chain exceeds the threshold, each
successful compaction afterwards will always bring the chain closer to
the threshold. This allows compactions to self-correct the chain to be
under the threshold.

Epic: None

Release note: `backup.compaction.window_size` must be at least 3 instead
of 2.

Author: kev-cao

pkg/backup/compaction_job.go

@@ -52,7 +52,7 @@ var (
 		"the required backup chain length for compaction to be triggered (0 to disable compactions)",
 		0,
 		settings.WithVisibility(settings.Reserved),
-		settings.IntInRangeOrZeroDisable(3, math.MaxInt64),
+		settings.IntInRangeOrZeroDisable(4, math.MaxInt64),
 	)
 )
 

pkg/backup/compaction_policy.go

@@ -20,10 +20,10 @@ var (
 	backupCompactionWindow = settings.RegisterIntSetting(
 		settings.ApplicationLevel,
 		"backup.compaction.window_size",
-		"the number of backups to compact per compaction (must be greater than one and less than threshold)",
+		"the number of backups to compact per compaction (must be greater than two and less than threshold)",
 		3,
 		settings.WithVisibility(settings.Reserved),
-		settings.IntWithMinimum(2),
+		settings.IntWithMinimum(3),
 	)
 )
 

pkg/backup/compaction_test.go

@@ -458,8 +458,8 @@ func TestScheduledBackupCompaction(t *testing.T) {
 	// impacted by when the test runs.
 	th.env.SetTime(time.Date(2025, 3, 27, 1, 0, 0, 0, time.UTC))
 
-	th.sqlDB.Exec(t, "SET CLUSTER SETTING backup.compaction.threshold = 3")
-	th.sqlDB.Exec(t, "SET CLUSTER SETTING backup.compaction.window_size = 2")
+	th.sqlDB.Exec(t, "SET CLUSTER SETTING backup.compaction.threshold = 4")
+	th.sqlDB.Exec(t, "SET CLUSTER SETTING backup.compaction.window_size = 3")
 	schedules, err := th.createBackupSchedule(
 		t, "CREATE SCHEDULE FOR BACKUP INTO $1 RECURRING '@hourly'", "nodelocal://1/backup",
 	)
@@ -477,21 +477,15 @@ func TestScheduledBackupCompaction(t *testing.T) {
 	var backupPath string
 	th.sqlDB.QueryRow(t, "SHOW BACKUPS IN 'nodelocal://1/backup'").Scan(&backupPath)
 
-	inc, err = jobs.ScheduledJobDB(th.internalDB()).
-		Load(context.Background(), th.env, inc.ScheduleID())
-	require.NoError(t, err)
-
-	th.env.SetTime(inc.NextRun().Add(time.Second))
-	require.NoError(t, th.executeSchedules())
-	th.waitForSuccessfulScheduledJob(t, inc.ScheduleID())
-
-	inc, err = jobs.ScheduledJobDB(th.internalDB()).
-		Load(context.Background(), th.env, inc.ScheduleID())
-	require.NoError(t, err)
+	for range 3 {
+		inc, err = jobs.ScheduledJobDB(th.internalDB()).
+			Load(context.Background(), th.env, inc.ScheduleID())
+		require.NoError(t, err)
 
-	th.env.SetTime(inc.NextRun().Add(time.Second))
-	require.NoError(t, th.executeSchedules())
-	th.waitForSuccessfulScheduledJob(t, inc.ScheduleID())
+		th.env.SetTime(inc.NextRun().Add(time.Second))
+		require.NoError(t, th.executeSchedules())
+		th.waitForSuccessfulScheduledJob(t, inc.ScheduleID())
+	}
 
 	if blockCompaction != nil {
 		t.Log("executing second full backup before compaction job resolves destination")
@@ -535,7 +529,7 @@ func TestScheduledBackupCompaction(t *testing.T) {
 		fmt.Sprintf("SELECT count(DISTINCT (start_time, end_time)) FROM "+
 			"[SHOW BACKUP FROM '%s' IN 'nodelocal://1/backup']", backupPath),
 	).Scan(&numBackups)
-	require.Equal(t, 4, numBackups)
+	require.Equal(t, 5, numBackups)
 }
 
 func TestBackupCompactionUnsupportedOptions(t *testing.T) {