diff --git a/lib/storage/partition.go b/lib/storage/partition.go
index dbd2e5f35f..49235333fd 100644
--- a/lib/storage/partition.go
+++ b/lib/storage/partition.go
@@ -808,8 +808,8 @@ func (pt *partition) partsMerger(mergerFunc func(isFinal bool) error) error {
 	}
 }
 
-func (pt *partition) maxOutPartRows() uint64 {
-	freeSpace := mustGetFreeDiskSpace(pt.bigPartsPath)
+func maxRowsByPath(path string) uint64 {
+	freeSpace := mustGetFreeDiskSpace(path)
 
 	// Calculate the maximum number of rows in the output merge part
 	// by dividing the freeSpace by the number of concurrent
@@ -817,7 +817,11 @@ func (pt *partition) maxOutPartRows() uint64 {
 	// This assumes each row is compressed into 1 byte. Production
 	// simulation shows that each row usually occupies up to 0.5 bytes,
 	// so this is quite safe assumption.
-	return freeSpace / uint64(mergeWorkers)
+	maxRows := freeSpace / uint64(mergeWorkers)
+	if maxRows > maxRowsPerBigPart {
+		maxRows = maxRowsPerBigPart
+	}
+	return maxRows
 }
 
 func mustGetFreeDiskSpace(path string) uint64 {
@@ -861,10 +865,7 @@ type freeSpaceEntry struct {
 }
 
 func (pt *partition) mergeBigParts(isFinal bool) error {
-	maxRows := pt.maxOutPartRows()
-	if maxRows > maxRowsPerBigPart {
-		maxRows = maxRowsPerBigPart
-	}
+	maxRows := maxRowsByPath(pt.bigPartsPath)
 
 	pt.partsLock.Lock()
 	pws := getPartsToMerge(pt.bigParts, maxRows, isFinal)
@@ -883,7 +884,15 @@ func (pt *partition) mergeBigParts(isFinal bool) error {
 }
 
 func (pt *partition) mergeSmallParts(isFinal bool) error {
-	maxRows := uint64(maxRowsPerSmallPart * defaultPartsToMerge)
+	maxRows := maxRowsByPath(pt.smallPartsPath)
+	if maxRows > maxRowsPerSmallPart {
+		// The output part may go to big part,
+		// so make sure it as enough space.
+		maxBigPartRows := maxRowsByPath(pt.bigPartsPath)
+		if maxRows > maxBigPartRows {
+			maxRows = maxBigPartRows
+		}
+	}
 
 	pt.partsLock.Lock()
 	pws := getPartsToMerge(pt.smallParts, maxRows, isFinal)
diff --git a/lib/storage/partition_test.go b/lib/storage/partition_test.go
index f87d7d64fc..ef2c4ce8a9 100644
--- a/lib/storage/partition_test.go
+++ b/lib/storage/partition_test.go
@@ -6,11 +6,8 @@ import (
 	"testing"
 )
 
-func TestPartitionMaxOutPartRows(t *testing.T) {
-	pt := &partition{
-		bigPartsPath: ".",
-	}
-	n := pt.maxOutPartRows()
+func TestPartitionMaxRowsByPath(t *testing.T) {
+	n := maxRowsByPath(".")
 	if n < 1e3 {
 		t.Fatalf("too small number of rows can be created in the current directory: %d", n)
 	}