diff --git a/CHANGELOG-3.4.md b/CHANGELOG-3.4.md
index ebb4ef69d98..54bae2953db 100644
--- a/CHANGELOG-3.4.md
+++ b/CHANGELOG-3.4.md
@@ -34,6 +34,7 @@ See [code changes](https://github.com/coreos/etcd/compare/v3.3.0...v3.4.0) and [
   - e.g. a node is removed from cluster, or [`raftpb.MsgProp` arrives at current leader while there is an ongoing leadership transfer](https://github.com/coreos/etcd/issues/8975).
 - Add [`snapshot`](https://github.com/coreos/etcd/pull/9118) package for easier snapshot workflow (see [`godoc.org/github.com/etcd/snapshot`](https://godoc.org/github.com/coreos/etcd/snapshot) for more).
 - Improve [functional tester](https://github.com/coreos/etcd/tree/master/functional) coverage: [proxy layer to run network fault tests in CI](https://github.com/coreos/etcd/pull/9081), [TLS is enabled both for server and client](https://github.com/coreos/etcd/pull/9534), [liveness mode](https://github.com/coreos/etcd/issues/9230), [shuffle test sequence](https://github.com/coreos/etcd/issues/9381), [membership reconfiguration failure cases](https://github.com/coreos/etcd/pull/9564), [disastrous quorum loss and snapshot recover from a seed member](https://github.com/coreos/etcd/pull/9565), [embedded etcd](https://github.com/coreos/etcd/pull/9572).
+- Improve [index compaction blocking](https://github.com/coreos/etcd/pull/9511) by using a copy on write clone to avoid holding the lock for the traversal of the entire index.
 
 ### Breaking Changes
 
diff --git a/mvcc/index.go b/mvcc/index.go
index 626de3825fc..16e223be225 100644
--- a/mvcc/index.go
+++ b/mvcc/index.go
@@ -185,27 +185,34 @@ func (ti *treeIndex) RangeSince(key, end []byte, rev int64) []revision {
 
 func (ti *treeIndex) Compact(rev int64) map[revision]struct{} {
 	available := make(map[revision]struct{})
-	var emptyki []*keyIndex
 	if ti.lg != nil {
 		ti.lg.Info("compact tree index", zap.Int64("revision", rev))
 	} else {
 		plog.Printf("store.index: compact %d", rev)
 	}
-	// TODO: do not hold the lock for long time?
-	// This is probably OK. Compacting 10M keys takes O(10ms).
 	ti.Lock()
-	defer ti.Unlock()
-	ti.tree.Ascend(compactIndex(rev, available, &emptyki))
-	for _, ki := range emptyki {
-		item := ti.tree.Delete(ki)
-		if item == nil {
-			if ti.lg != nil {
-				ti.lg.Panic("failed to delete during compaction")
-			} else {
-				plog.Panic("store.index: unexpected delete failure during compaction")
+	clone := ti.tree.Clone()
+	ti.Unlock()
+
+	clone.Ascend(func(item btree.Item) bool {
+		keyi := item.(*keyIndex)
+		//Lock is needed here to prevent modification to the keyIndex while
+		//compaction is going on or revision added to empty before deletion
+		ti.Lock()
+		keyi.compact(rev, available)
+		if keyi.isEmpty() {
+			item := ti.tree.Delete(keyi)
+			if item == nil {
+				if ti.lg != nil {
+					ti.lg.Panic("failed to delete during compaction")
+				} else {
+					plog.Panic("store.index: unexpected delete failure during compaction")
+				}
 			}
 		}
-	}
+		ti.Unlock()
+		return true
+	})
 	return available
 }
 
@@ -222,17 +229,6 @@ func (ti *treeIndex) Keep(rev int64) map[revision]struct{} {
 	return available
 }
 
-func compactIndex(rev int64, available map[revision]struct{}, emptyki *[]*keyIndex) func(i btree.Item) bool {
-	return func(i btree.Item) bool {
-		keyi := i.(*keyIndex)
-		keyi.compact(rev, available)
-		if keyi.isEmpty() {
-			*emptyki = append(*emptyki, keyi)
-		}
-		return true
-	}
-}
-
 func (ti *treeIndex) Equal(bi index) bool {
 	b := bi.(*treeIndex)
 
diff --git a/mvcc/index_bench_test.go b/mvcc/index_bench_test.go
new file mode 100644
index 00000000000..5d2e5e3f49b
--- /dev/null
+++ b/mvcc/index_bench_test.go
@@ -0,0 +1,42 @@
+// Copyright 2018 The etcd Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package mvcc
+
+import (
+	"testing"
+
+	"go.uber.org/zap"
+)
+
+func BenchmarkIndexCompact1(b *testing.B)       { benchmarkIndexCompact(b, 1) }
+func BenchmarkIndexCompact100(b *testing.B)     { benchmarkIndexCompact(b, 100) }
+func BenchmarkIndexCompact10000(b *testing.B)   { benchmarkIndexCompact(b, 10000) }
+func BenchmarkIndexCompact100000(b *testing.B)  { benchmarkIndexCompact(b, 100000) }
+func BenchmarkIndexCompact1000000(b *testing.B) { benchmarkIndexCompact(b, 1000000) }
+
+func benchmarkIndexCompact(b *testing.B, size int) {
+	log := zap.NewNop()
+	kvindex := newTreeIndex(log)
+
+	bytesN := 64
+	keys := createBytesSlice(bytesN, size)
+	for i := 1; i < size; i++ {
+		kvindex.Put(keys[i], revision{main: int64(i), sub: int64(i)})
+	}
+	b.ResetTimer()
+	for i := 1; i < b.N; i++ {
+		kvindex.Compact(int64(i))
+	}
+}
diff --git a/mvcc/kvstore.go b/mvcc/kvstore.go
index f03b6311e4f..832de9e35d5 100644
--- a/mvcc/kvstore.go
+++ b/mvcc/kvstore.go
@@ -217,17 +217,18 @@ func (s *store) HashByRev(rev int64) (hash uint32, currentRev int64, compactRev
 
 func (s *store) Compact(rev int64) (<-chan struct{}, error) {
 	s.mu.Lock()
-	defer s.mu.Unlock()
 	s.revMu.Lock()
-	defer s.revMu.Unlock()
-
 	if rev <= s.compactMainRev {
 		ch := make(chan struct{})
 		f := func(ctx context.Context) { s.compactBarrier(ctx, ch) }
 		s.fifoSched.Schedule(f)
+		s.mu.Unlock()
+		s.revMu.Unlock()
 		return ch, ErrCompacted
 	}
 	if rev > s.currentRev {
+		s.mu.Unlock()
+		s.revMu.Unlock()
 		return nil, ErrFutureRev
 	}
 
@@ -245,6 +246,8 @@ func (s *store) Compact(rev int64) (<-chan struct{}, error) {
 	// ensure that desired compaction is persisted
 	s.b.ForceCommit()
 
+	s.mu.Unlock()
+	s.revMu.Unlock()
 	keep := s.kvindex.Compact(rev)
 	ch := make(chan struct{})
 	var j = func(ctx context.Context) {