Reload after reading the WAL. (#460)

This causes the head to be GCed at startup,
removing any series that were read from the WAL
but have since been written to a block. In
systems with low ingestion rates, this potentially
could be many many hours of data.

Signed-off-by: Brian Brazil <brian.brazil@robustperception.io>

db.go

@@ -271,12 +271,12 @@ func Open(dir string, l log.Logger, r prometheus.Registerer, opts *Options) (db
 	if err != nil {
 		return nil, err
 	}
-	if err := db.reload(); err != nil {
-		return nil, err
-	}
 	if err := db.head.Init(); err != nil {
 		return nil, errors.Wrap(err, "read WAL")
 	}
+	if err := db.reload(); err != nil {
+		return nil, err
+	}
 
 	go db.run()
 

db_test.go

@@ -27,6 +27,8 @@ import (
 
 	"github.com/oklog/ulid"
 	"github.com/pkg/errors"
+	"github.com/prometheus/client_golang/prometheus"
+	prom_testutil "github.com/prometheus/client_golang/prometheus/testutil"
 	"github.com/prometheus/tsdb/chunks"
 	"github.com/prometheus/tsdb/index"
 	"github.com/prometheus/tsdb/labels"
@@ -1295,11 +1297,15 @@ func TestInitializeHeadTimestamp(t *testing.T) {
 		testutil.Ok(t, err)
 		testutil.Ok(t, w.Close())
 
-		db, err := Open(dir, nil, nil, nil)
+		r := prometheus.NewRegistry()
+
+		db, err := Open(dir, nil, r, nil)
 		testutil.Ok(t, err)
 
 		testutil.Equals(t, int64(6000), db.head.MinTime())
 		testutil.Equals(t, int64(15000), db.head.MaxTime())
+		// Check that old series has been GCed.
+		testutil.Equals(t, 1.0, prom_testutil.ToFloat64(db.head.metrics.series))
 	})
 }