diff --git a/wal.go b/wal.go
index 5b85ca3c96..b06425e076 100644
--- a/wal.go
+++ b/wal.go
@@ -299,6 +299,8 @@ func (w *WAL) entry(et WALEntryType, flag byte, buf []byte) error {
 		sz    = int64(6 + 4 + len(buf))
 		newsz = w.curN + sz
 	)
+	// XXX(fabxc): this currently cuts a new file whenever the WAL was newly opened.
+	// Probably fine in general but may yield a lot of short files in some cases.
 	if w.cur == nil || w.curN > w.segmentSize || newsz > w.segmentSize && sz <= w.segmentSize {
 		if err := w.cut(); err != nil {
 			return err
@@ -431,6 +433,8 @@ func NewWALReader(rs ...io.ReadCloser) *WALReader {
 }
 
 // At returns the last decoded entry of labels or samples.
+// The returned slices are only valid until the next call to Next(). Their elements
+// have to be copied to preserve them.
 func (r *WALReader) At() ([]labels.Labels, []refdSample) {
 	return r.labels, r.samples
 }
diff --git a/wal_test.go b/wal_test.go
index 773fc4e085..8442dbddc7 100644
--- a/wal_test.go
+++ b/wal_test.go
@@ -8,6 +8,8 @@ import (
 	"os"
 	"testing"
 
+	"github.com/fabxc/tsdb/labels"
+
 	"github.com/coreos/etcd/pkg/fileutil"
 	"github.com/stretchr/testify/require"
 )
@@ -117,52 +119,88 @@ func TestWAL_cut(t *testing.T) {
 
 // Symmetrical test of reading and writing to the WAL via its main interface.
 func TestWAL_Log_Restore(t *testing.T) {
+	const (
+		numMetrics = 5000
+		iterations = 5
+		stepSize   = 100
+	)
 	// Generate testing data. It does not make semantical sense but
 	// for the purpose of this test.
-	series, err := readPrometheusLabels("testdata/20k.series", 10000)
+	series, err := readPrometheusLabels("testdata/20k.series", numMetrics)
 	require.NoError(t, err)
 
-	var samples []refdSample
-	for i := 0; i < 200000; i++ {
-		samples = append(samples, refdSample{
-			ref: uint64(i % 10000),
-			t:   int64(i * 2),
-			v:   rand.Float64(),
-		})
-	}
-
 	dir, err := ioutil.TempDir("", "test_wal_log_restore")
 	require.NoError(t, err)
 	defer os.RemoveAll(dir)
 
-	w, err := OpenWAL(dir, nil, 0)
-	require.NoError(t, err)
+	var (
+		recordedSeries  [][]labels.Labels
+		recordedSamples [][]refdSample
+	)
+	var totalSamples int
 
-	// Set smaller segment size so we can actually write several files.
-	w.segmentSize = 300 * 1000
+	// Open WAL a bunch of times, validate all previous data can be read,
+	// write more data to it, close it.
+	for k := 0; k < numMetrics; k += numMetrics / iterations {
+		w, err := OpenWAL(dir, nil, 0)
+		require.NoError(t, err)
 
-	for i := 0; i < len(series); i += 100 {
-		require.NoError(t, w.Log(series[i:i+100], samples[i*10:(i+100)*10]))
-	}
+		// Set smaller segment size so we can actually write several files.
+		w.segmentSize = 1000 * 1000
 
-	require.NoError(t, w.Close())
+		r := w.Reader()
 
-	w, err = OpenWAL(dir, nil, 0)
-	r := w.Reader()
+		var (
+			resultSeries  [][]labels.Labels
+			resultSamples [][]refdSample
+		)
 
-	var i, j int
+		for r.Next() {
+			lsets, smpls := r.At()
 
-	for r.Next() {
-		lsets, smpls := r.At()
-
-		if l := len(lsets); l > 0 {
-			require.Equal(t, series[i:i+l], lsets)
-			i += l
+			if len(lsets) > 0 {
+				clsets := make([]labels.Labels, len(lsets))
+				copy(clsets, lsets)
+				resultSeries = append(resultSeries, clsets)
+			}
+			if len(smpls) > 0 {
+				csmpls := make([]refdSample, len(smpls))
+				copy(csmpls, smpls)
+				resultSamples = append(resultSamples, csmpls)
+			}
 		}
-		if l := len(smpls); l > 0 {
-			require.Equal(t, samples[j:j+l], smpls)
-			j += l
+		require.NoError(t, r.Err())
+
+		require.Equal(t, recordedSamples, resultSamples)
+		require.Equal(t, recordedSeries, resultSeries)
+
+		series := series[k : k+(numMetrics/iterations)]
+
+		// Insert in batches and generate different amounts of samples for each.
+		for i := 0; i < len(series); i += stepSize {
+			var samples []refdSample
+
+			for j := 0; j < i*10; j++ {
+				samples = append(samples, refdSample{
+					ref: uint64(j % 10000),
+					t:   int64(j * 2),
+					v:   rand.Float64(),
+				})
+			}
+
+			lbls := series[i : i+stepSize]
+
+			require.NoError(t, w.Log(lbls, samples))
+
+			if len(lbls) > 0 {
+				recordedSeries = append(recordedSeries, lbls)
+			}
+			if len(samples) > 0 {
+				recordedSamples = append(recordedSamples, samples)
+				totalSamples += len(samples)
+			}
 		}
+
+		require.NoError(t, w.Close())
 	}
-	require.NoError(t, r.Err())
 }