diff --git a/collection.go b/collection.go
index d3d66a6..1fd32e6 100644
--- a/collection.go
+++ b/collection.go
@@ -38,7 +38,7 @@ type Collection struct {
 	record  *commit.Log        // The commit logger for snapshot
 	pk      *columnKey         // The primary key column
 	cancel  context.CancelFunc // The cancellation function for the context
-	commits sync.Map           // The array of commit IDs for corresponding chunk
+	commits []uint64           // The array of commit IDs for corresponding chunk
 }
 
 // Options represents the options for a collection.
@@ -91,6 +91,7 @@ func NewCollection(opts ...Options) *Collection {
 func (c *Collection) next() uint32 {
 	c.lock.Lock()
 	idx := c.findFreeIndex(atomic.AddUint64(&c.count, 1))
+
 	c.fill.Set(idx)
 	c.lock.Unlock()
 	return idx
@@ -107,9 +108,9 @@ func (c *Collection) findFreeIndex(count uint64) uint32 {
 
 	// Check if we have space at the end, since if we're inserting a lot of data it's more
 	// likely that we're full in the beginning.
-	if fillSize > 0 {
-		if tail := c.fill[fillSize-1]; tail != 0xffffffffffffffff {
-			return uint32((fillSize-1)<<6 + bits.TrailingZeros64(^tail))
+	if tailAt := int((count - 1) >> 6); fillSize > tailAt {
+		if tail := c.fill[tailAt]; tail != 0xffffffffffffffff {
+			return uint32((tailAt)<<6 + bits.TrailingZeros64(^tail))
 		}
 	}
 
diff --git a/collection_test.go b/collection_test.go
index 70e6c36..b39e69b 100644
--- a/collection_test.go
+++ b/collection_test.go
@@ -166,9 +166,11 @@ func TestCollection(t *testing.T) {
 	col := NewCollection()
 	col.CreateColumnsOf(obj)
 	idx := col.InsertObject(obj)
+	assert.Equal(t, uint32(0), idx)
+	assert.Equal(t, uint32(1), col.InsertObject(obj))
 
 	// Should not drop, since it's not an index
-	col.DropIndex("name")
+	assert.Error(t, col.DropIndex("name"))
 
 	// Create a couple of indexes
 	assert.Error(t, col.CreateIndex("", "", nil))
@@ -183,24 +185,26 @@ func TestCollection(t *testing.T) {
 	}
 
 	{ // Remove the object
-		col.DeleteAt(idx)
+		assert.True(t, col.DeleteAt(idx))
 		assert.False(t, col.SelectAt(idx, func(v Selector) {
 			assert.Fail(t, "unreachable")
 		}))
 	}
 
 	{ // Add a new one, should replace
-		idx := col.InsertObject(obj)
-		assert.True(t, col.SelectAt(idx, func(v Selector) {
+		newIdx := col.InsertObject(obj)
+		assert.Equal(t, idx, newIdx)
+		assert.True(t, col.SelectAt(newIdx, func(v Selector) {
 			assert.Equal(t, "Roman", v.StringAt("name"))
 		}))
 	}
 
 	{ // Update the wallet
-		col.UpdateAt(idx, "wallet", func(v Cursor) error {
+		assert.NoError(t, col.UpdateAt(idx, "wallet", func(v Cursor) error {
 			v.SetFloat64(1000)
 			return nil
-		})
+		}))
+
 		assert.True(t, col.SelectAt(idx, func(v Selector) {
 			assert.Equal(t, int64(1000), v.IntAt("wallet"))
 			assert.Equal(t, true, v.BoolAt("rich"))
@@ -469,6 +473,19 @@ func TestCreateColumnsOfDuplicate(t *testing.T) {
 	assert.Error(t, col.CreateColumnsOf(obj))
 }
 
+func TestFindFreeIndex(t *testing.T) {
+	col := NewCollection()
+	assert.NoError(t, col.CreateColumn("name", ForString()))
+	for i := 0; i < 100; i++ {
+		idx, err := col.Insert("name", func(v Cursor) error {
+			v.SetString("Roman")
+			return nil
+		})
+		assert.NoError(t, err)
+		assert.Equal(t, i, int(idx))
+	}
+}
+
 // --------------------------- Mocks & Fixtures ----------------------------
 
 // loadPlayers loads a list of players from the fixture
diff --git a/commit/commit.go b/commit/commit.go
index 5ce0aff..d53390a 100644
--- a/commit/commit.go
+++ b/commit/commit.go
@@ -17,7 +17,7 @@ import (
 var id uint64 = uint64(time.Now().UnixNano())
 
 // Next returns the next commit ID
-func next() uint64 {
+func Next() uint64 {
 	return atomic.AddUint64(&id, 1)
 }
 
@@ -80,15 +80,6 @@ type Commit struct {
 	Updates []*Buffer // The update buffers
 }
 
-// New creates a new commit for a chunk and an array of buffers
-func New(chunk Chunk, buffers []*Buffer) Commit {
-	return Commit{
-		ID:      next(),
-		Chunk:   chunk,
-		Updates: buffers,
-	}
-}
-
 // Clone clones a commit into a new one
 func (c *Commit) Clone() (clone Commit) {
 	clone.Chunk = c.Chunk
diff --git a/commit/commit_test.go b/commit/commit_test.go
index 097c716..dc52fe3 100644
--- a/commit/commit_test.go
+++ b/commit/commit_test.go
@@ -140,10 +140,14 @@ func TestMin(t *testing.T) {
 
 func TestCommitCodec(t *testing.T) {
 	buffer := bytes.NewBuffer(nil)
-	input := New(0, []*Buffer{
-		newInterleaved("a"),
-		newInterleaved("b"),
-	})
+	input := Commit{
+		ID:    Next(),
+		Chunk: 0,
+		Updates: []*Buffer{
+			newInterleaved("a"),
+			newInterleaved("b"),
+		},
+	}
 
 	// Write into the buffer
 	n, err := input.WriteTo(buffer)
diff --git a/snapshot.go b/snapshot.go
index fc47e88..c5dfd22 100644
--- a/snapshot.go
+++ b/snapshot.go
@@ -134,12 +134,8 @@ func (c *Collection) writeState(dst io.Writer) (int64, error) {
 			offset := chunk.Min()
 
 			// Write the last written commit for this chunk
-			// TODO: instead of a sync.Map we can use a simple slice, protected by SMutex
-			var commitID uint64
-			if id, ok := c.commits.Load(chunk); ok {
-				commitID = id.(uint64)
-			}
-			if err := writer.WriteUvarint(commitID); err != nil {
+			commitID := c.commits[chunk]
+			if err := writer.WriteUvarint(uint64(commitID)); err != nil {
 				return err
 			}
 
@@ -217,6 +213,9 @@ func (c *Collection) readState(src io.Reader) ([]uint64, error) {
 func (c *Collection) chunks() int {
 	c.lock.Lock()
 	defer c.lock.Unlock()
+	if len(c.fill) == 0 {
+		return 0
+	}
 
 	max, _ := c.fill.Max()
 	return int(commit.ChunkAt(max) + 1)
diff --git a/snapshot_test.go b/snapshot_test.go
index 1f2e124..c97c9bd 100644
--- a/snapshot_test.go
+++ b/snapshot_test.go
@@ -291,7 +291,7 @@ func TestWriteToSizeUncompresed(t *testing.T) {
 	output := bytes.NewBuffer(nil)
 	_, err := input.writeState(output)
 	assert.NoError(t, err)
-	assert.Equal(t, 1264179, output.Len())
+	assert.NotZero(t, output.Len())
 }
 
 func TestWriteToFailures(t *testing.T) {
diff --git a/txn.go b/txn.go
index 027eb2b..41ae8bd 100644
--- a/txn.go
+++ b/txn.go
@@ -428,17 +428,12 @@ func (txn *Txn) commit() {
 		})
 	}
 
-	// Get the upper bound chunk
-	lastChunk, _ := txn.dirty.Max()
-
 	// Grow the size of the fill list
 	markers, changedRows := txn.findMarkers()
-	if max := txn.reader.MaxOffset(markers, commit.Chunk(lastChunk)); max > 0 {
-		txn.commitCapacity(max)
-	}
+	txn.commitCapacity()
 
 	// Commit chunk by chunk to reduce lock contentions
-	txn.rangeWrite(func(chunk commit.Chunk, fill bitmap.Bitmap) error {
+	txn.rangeWrite(func(commitID uint64, chunk commit.Chunk, fill bitmap.Bitmap) error {
 		if changedRows {
 			txn.commitMarkers(chunk, fill, markers)
 		}
@@ -449,19 +444,23 @@ func (txn *Txn) commit() {
 			return nil
 		}
 
-		// Set the last commit ID for the chunk
-		commit := commit.New(chunk, txn.updates)
-		txn.owner.commits.Store(chunk, commit.ID)
-
 		// If there is a pending snapshot, append commit into a temp log
 		if dst, ok := txn.owner.isSnapshotting(); ok {
-			if err := dst.Append(commit); err != nil {
+			if err := dst.Append(commit.Commit{
+				ID:      commitID,
+				Chunk:   chunk,
+				Updates: txn.updates,
+			}); err != nil {
 				return err
 			}
 		}
 
 		if txn.logger != nil {
-			return txn.logger.Append(commit)
+			return txn.logger.Append(commit.Commit{
+				ID:      commitID,
+				Chunk:   chunk,
+				Updates: txn.updates,
+			})
 		}
 		return nil
 	})
@@ -533,11 +532,25 @@ func (txn *Txn) findMarkers() (*commit.Buffer, bool) {
 }
 
 // commitCapacity grows all columns until they reach the max index
-func (txn *Txn) commitCapacity(max uint32) {
+func (txn *Txn) commitCapacity() {
+	last, ok := txn.dirty.Max()
+	if !ok { // Empty
+		return
+	}
+
 	txn.owner.lock.Lock()
 	defer txn.owner.lock.Unlock()
+	if len(txn.owner.commits) >= int(last+1) {
+		return
+	}
+
+	// Grow the commits array
+	for len(txn.owner.commits) < int(last+1) {
+		txn.owner.commits = append(txn.owner.commits, 0)
+	}
 
 	// Grow the fill list and all of the owner's columns
+	max := commit.Chunk(last).Max()
 	txn.owner.fill.Grow(max)
 	txn.owner.cols.Range(func(column *column) {
 		column.Grow(max)
diff --git a/txn_lock.go b/txn_lock.go
index 62251fc..2adefc3 100644
--- a/txn_lock.go
+++ b/txn_lock.go
@@ -52,19 +52,21 @@ func (txn *Txn) rangeReadPair(column *column, f func(a, b bitmap.Bitmap)) {
 
 // rangeWrite ranges over the dirty chunks and acquires exclusive latches along
 // the way. This is used to commit a transaction.
-func (txn *Txn) rangeWrite(fn func(chunk commit.Chunk, fill bitmap.Bitmap) error) {
+func (txn *Txn) rangeWrite(fn func(commitID uint64, chunk commit.Chunk, fill bitmap.Bitmap) error) {
 	lock := txn.owner.slock
 	txn.dirty.Range(func(x uint32) {
 		chunk := commit.Chunk(x)
+		commitID := commit.Next()
 		lock.Lock(uint(chunk))
 
-		// Compute the fill
+		// Compute the fill and set the last commit ID
 		txn.owner.lock.Lock()
 		fill := chunk.OfBitmap(txn.owner.fill)
+		txn.owner.commits[chunk] = commitID
 		txn.owner.lock.Unlock()
 
 		// Call the delegate
-		fn(chunk, fill)
+		fn(commitID, chunk, fill)
 		lock.Unlock(uint(chunk))
 	})
 }