diff --git a/source/core/StarBTreeDatabase.cpp b/source/core/StarBTreeDatabase.cpp
index 19b0bd2..f66bea1 100644
--- a/source/core/StarBTreeDatabase.cpp
+++ b/source/core/StarBTreeDatabase.cpp
@@ -1,6 +1,7 @@
 #include "StarBTreeDatabase.hpp"
 #include "StarSha256.hpp"
 #include "StarVlqEncoding.hpp"
+#include "StarLogging.hpp"
 
 namespace Star {
 
@@ -243,7 +244,7 @@ uint32_t BTreeDatabase::freeBlockCount() {
     indexBlockIndex = indexBlock.nextFreeBlock;
   }
 
-  count += m_availableBlocks.size() + m_pendingFree.size();
+  count += m_availableBlocks.size();
 
   // Include untracked blocks at the end of the file in the free count.
   count += (m_device->size() - m_deviceSize) / m_blockSize;
@@ -272,7 +273,7 @@ uint32_t BTreeDatabase::leafBlockCount() {
       return true;
     }
 
-    BTreeDatabase* parent;
+    BTreeDatabase* parent = nullptr;
     BlockIndex leafBlockCount = 0;
   };
 
@@ -293,8 +294,8 @@ void BTreeDatabase::rollback() {
 
   m_availableBlocks.clear();
   m_indexCache.clear();
+  m_uncommittedWrites.clear();
   m_uncommitted.clear();
-  m_pendingFree.clear();
 
   readRoot();
 
@@ -305,7 +306,8 @@ void BTreeDatabase::rollback() {
 void BTreeDatabase::close(bool closeDevice) {
   WriteLocker writeLocker(m_lock);
   if (m_open) {
-    doCommit();
+    if (!tryFlatten())
+      doCommit();
 
     m_indexCache.clear();
 
@@ -536,7 +538,7 @@ auto BTreeDatabase::BTreeImpl::loadIndex(Pointer pointer) -> Index {
   index->pointers.resize(s);
   for (uint32_t i = 0; i < s; ++i) {
     auto& e = index->pointers[i];
-    e.key =buffer.readBytes(parent->m_keySize);
+    e.key = buffer.readBytes(parent->m_keySize);
     e.pointer = buffer.read<BlockIndex>();
   }
 
@@ -896,17 +898,25 @@ void BTreeDatabase::rawReadBlock(BlockIndex blockIndex, size_t blockOffset, char
   if (size <= 0)
     return;
 
-  m_device->readFullAbsolute(HeaderSize + blockIndex * (StreamOffset)m_blockSize + blockOffset, block, size);
+  if (auto buffer = m_uncommittedWrites.ptr(blockIndex))
+    buffer->copyTo(block, blockOffset, size);
+  else
+    m_device->readFullAbsolute(HeaderSize + blockIndex * (StreamOffset)m_blockSize + blockOffset, block, size);
 }
 
-void BTreeDatabase::rawWriteBlock(BlockIndex blockIndex, size_t blockOffset, char const* block, size_t size) const {
+void BTreeDatabase::rawWriteBlock(BlockIndex blockIndex, size_t blockOffset, char const* block, size_t size) {
   if (blockOffset > m_blockSize || size > m_blockSize - blockOffset)
     throw DBException::format("Write past end of block, offset: {} size {}", blockOffset, size);
 
   if (size <= 0)
     return;
 
-  m_device->writeFullAbsolute(HeaderSize + blockIndex * (StreamOffset)m_blockSize + blockOffset, block, size);
+  StreamOffset blockStart = HeaderSize + blockIndex * (StreamOffset)m_blockSize;
+  auto buffer = m_uncommittedWrites.find(blockIndex);
+  if (buffer == m_uncommittedWrites.end())
+    buffer = m_uncommittedWrites.emplace(blockIndex, m_device->readBytesAbsolute(blockStart, m_blockSize)).first;
+
+  buffer->second.writeFrom(block, blockOffset, size);
 }
 
 auto BTreeDatabase::readFreeIndexBlock(BlockIndex blockIndex) -> FreeIndexBlock {
@@ -991,12 +1001,12 @@ auto BTreeDatabase::leafTailBlocks(BlockIndex leafPointer) -> List<BlockIndex> {
 }
 
 void BTreeDatabase::freeBlock(BlockIndex b) {
-  if (m_uncommitted.contains(b)) {
+  if (m_uncommitted.contains(b))
     m_uncommitted.remove(b);
-    m_availableBlocks.add(b);
-  } else {
-    m_pendingFree.append(b);
-  }
+  if (m_uncommittedWrites.contains(b))
+    m_uncommittedWrites.remove(b);
+
+  m_availableBlocks.add(b);
 }
 
 auto BTreeDatabase::reserveBlock() -> BlockIndex {
@@ -1007,10 +1017,7 @@ auto BTreeDatabase::reserveBlock() -> BlockIndex {
       FreeIndexBlock indexBlock = readFreeIndexBlock(m_headFreeIndexBlock);
       for (auto const& b : indexBlock.freeBlocks)
         m_availableBlocks.add(b);
-      // We cannot make available the block itself, because we must maintain
-      // atomic consistency.  We will need to free this block later and commit
-      // the new free index block chain.
-      m_pendingFree.append(m_headFreeIndexBlock);
+      m_availableBlocks.add(m_headFreeIndexBlock);
       m_headFreeIndexBlock = indexBlock.nextFreeBlock;
     }
 
@@ -1068,65 +1075,168 @@ void BTreeDatabase::readRoot() {
 }
 
 void BTreeDatabase::doCommit() {
-  if (m_availableBlocks.empty() && m_pendingFree.empty() && m_uncommitted.empty())
+  if (m_availableBlocks.empty() && m_uncommitted.empty())
     return;
 
-  if (!m_availableBlocks.empty() || !m_pendingFree.empty()) {
+  if (!m_availableBlocks.empty()) {
     // First, read the existing head FreeIndexBlock, if it exists
     FreeIndexBlock indexBlock = FreeIndexBlock{InvalidBlockIndex, {}};
-    if (m_headFreeIndexBlock != InvalidBlockIndex) {
+
+    auto newBlock = [&]() -> BlockIndex {
+      if (!m_availableBlocks.empty())
+        return m_availableBlocks.takeFirst();
+      else
+        return makeEndBlock();
+    };
+
+    if (m_headFreeIndexBlock != InvalidBlockIndex)
       indexBlock = readFreeIndexBlock(m_headFreeIndexBlock);
-      if (indexBlock.freeBlocks.size() >= maxFreeIndexLength()) {
-        // If the existing head free index block is full, then we should start a
-        // new one and leave it alone
-        indexBlock.nextFreeBlock = m_headFreeIndexBlock;
-        indexBlock.freeBlocks.clear();
-      } else {
-        // If we are copying an existing free index block, the old free index
-        // block will be a newly freed block
-        indexBlock.freeBlocks.append(m_headFreeIndexBlock);
-      }
-    }
+    else
+      m_headFreeIndexBlock = newBlock();
 
-    // Then, we need to write all the available blocks, which are safe to write
-    // to, and the pending free blocks, which are NOT safe to write to, to the
-    // FreeIndexBlock chain.
+    // Then, we need to write all the available blocks to the FreeIndexBlock chain.
     while (true) {
-      if (indexBlock.freeBlocks.size() < maxFreeIndexLength() && (!m_availableBlocks.empty() || !m_pendingFree.empty())) {
-        // If we have room on our current FreeIndexblock, just add a block to
-        // it.  Prioritize the pending free blocks, because we cannot use those
-        // to write to.
-        BlockIndex toAdd;
-        if (m_pendingFree.empty())
-          toAdd = m_availableBlocks.takeFirst();
-        else
-          toAdd = m_pendingFree.takeFirst();
-
+      // If we have room on our current FreeIndexBlock, just add a block to it.
+      if (!m_availableBlocks.empty() && indexBlock.freeBlocks.size() < maxFreeIndexLength()) {
+        BlockIndex toAdd = m_availableBlocks.takeFirst();
         indexBlock.freeBlocks.append(toAdd);
       } else {
-        // If our index block is full OR we are out of blocks to free, then
-        // need to write a new head free index block.
-        if (m_availableBlocks.empty())
-          m_headFreeIndexBlock = makeEndBlock();
-        else
-          m_headFreeIndexBlock = m_availableBlocks.takeFirst();
+        // Update the current head free index block.
         writeFreeIndexBlock(m_headFreeIndexBlock, indexBlock);
 
         // If we're out of blocks to free, then we're done
-        if (m_availableBlocks.empty() && m_pendingFree.empty())
+        if (m_availableBlocks.empty())
           break;
 
-        indexBlock.nextFreeBlock = m_headFreeIndexBlock;
-        indexBlock.freeBlocks.clear();
+        // If our head free index block is full, then
+        // need to write a new head free index block.
+        if (indexBlock.freeBlocks.size() >= maxFreeIndexLength()) {
+          indexBlock.nextFreeBlock = m_headFreeIndexBlock;
+          indexBlock.freeBlocks.clear();
+
+          m_headFreeIndexBlock = newBlock();
+          writeFreeIndexBlock(m_headFreeIndexBlock, indexBlock);
+        }
       }
     }
   }
 
+  
+  commitWrites(); 
   writeRoot();
-
   m_uncommitted.clear();
 }
 
+void BTreeDatabase::commitWrites() {
+  for (auto& write : m_uncommittedWrites)
+    m_device->writeFullAbsolute(HeaderSize + write.first * (StreamOffset)m_blockSize, write.second.ptr(), m_blockSize);
+
+  m_device->sync();
+  m_uncommittedWrites.clear();
+}
+
+bool BTreeDatabase::tryFlatten() {
+  if (m_headFreeIndexBlock == InvalidBlockIndex || m_rootIsLeaf || !m_device->isWritable())
+    return false;
+  
+  BlockIndex freeBlockCount = 0;
+  BlockIndex indexBlockIndex = m_headFreeIndexBlock;
+  while (indexBlockIndex != InvalidBlockIndex) {
+    FreeIndexBlock indexBlock = readFreeIndexBlock(indexBlockIndex);
+    freeBlockCount += 1 + indexBlock.freeBlocks.size();
+    indexBlockIndex = indexBlock.nextFreeBlock;
+  }
+
+  BlockIndex expectedBlockCount = (m_deviceSize - HeaderSize) / m_blockSize;
+  float free = float(freeBlockCount) / float(expectedBlockCount);
+  if (free < 0.05f)
+    return false;
+
+  Logger::info("[BTreeDatabase] File '{}' is {:.2f}% free space, flattening", m_device->deviceName(), free * 100.f);
+
+  indexBlockIndex = m_headFreeIndexBlock;
+  {
+    List<BlockIndex> availableBlocksList;
+    do {
+      FreeIndexBlock indexBlock = readFreeIndexBlock(indexBlockIndex);
+      availableBlocksList.appendAll(indexBlock.freeBlocks);
+      availableBlocksList.append(indexBlockIndex);
+      indexBlockIndex = indexBlock.nextFreeBlock;
+    } while (indexBlockIndex != InvalidBlockIndex);
+    m_headFreeIndexBlock = InvalidBlockIndex;
+
+    sort(availableBlocksList);
+    for (auto& availableBlock : availableBlocksList)
+      m_availableBlocks.insert(m_availableBlocks.end(), availableBlock);
+  }
+
+  BlockIndex count = 1; // 1 to include root index
+
+  double start = Time::monotonicTime();
+  auto index = m_impl.loadIndex(m_impl.rootPointer());
+  if (flattenVisitor(index, count)) {
+    m_impl.deleteIndex(index);
+    index->self = InvalidBlockIndex;
+    m_root = m_impl.storeIndex(index);
+  }
+
+  m_availableBlocks.clear();
+  m_device->resize(m_deviceSize = HeaderSize + (StreamOffset)m_blockSize * count);
+
+  m_indexCache.clear();
+  commitWrites();
+  writeRoot();
+  m_uncommitted.clear();
+
+  Logger::info("[BTreeDatabase] Finished flattening '{}' in {:.2f} milliseconds", m_device->deviceName(), (Time::monotonicTime() - start) * 1000.f);
+  return true;
+}
+
+bool BTreeDatabase::flattenVisitor(BTreeImpl::Index& index, BlockIndex& count) {
+  auto pointerCount = index->pointerCount();
+  count += pointerCount;
+  bool canStore = !m_availableBlocks.empty();
+
+  bool needsStore = false;
+  if (m_impl.indexLevel(index) == 0) {
+    for (size_t i = 0; i != pointerCount; ++i) {
+      auto indexPointer = index->pointer(i);
+      auto tailBlocks = leafTailBlocks(indexPointer);
+      if (canStore) {
+        bool leafNeedsStore = m_availableBlocks.first() < indexPointer;
+
+        if (!leafNeedsStore)
+          for (size_t i = 0; !leafNeedsStore && i != tailBlocks.size(); ++i)
+            if (m_availableBlocks.first() < tailBlocks[i])
+              leafNeedsStore = true;
+
+        if (leafNeedsStore) {
+          auto leaf = m_impl.loadLeaf(indexPointer);
+          m_impl.deleteLeaf(leaf);
+          leaf->self = InvalidBlockIndex;
+          index->updatePointer(i, m_impl.storeLeaf(leaf));
+          tailBlocks = leafTailBlocks(leaf->self);
+          needsStore = true;
+        }
+        canStore = !m_availableBlocks.empty();
+      }
+      count += tailBlocks.size();
+    }
+  } else {
+    for (size_t i = 0; i != pointerCount; ++i) {
+      auto childIndex = m_impl.loadIndex(index->pointer(i));
+      if (canStore && flattenVisitor(childIndex, count)) {
+        m_impl.deleteIndex(childIndex);
+        childIndex->self = InvalidBlockIndex;
+        index->updatePointer(i, m_impl.storeIndex(childIndex));
+        canStore = !m_availableBlocks.empty();
+        needsStore = true;
+      }
+    }
+  }
+  return needsStore || (canStore && m_availableBlocks.first() < index->self);
+}
+
 void BTreeDatabase::checkIfOpen(char const* methodName, bool shouldBeOpen) const {
   if (shouldBeOpen && !m_open)
     throw DBException::format("BTreeDatabase method '{}' called when not open, must be open.", methodName);
@@ -1146,7 +1256,7 @@ void BTreeDatabase::checkKeySize(ByteArray const& k) const {
 }
 
 uint32_t BTreeDatabase::maxFreeIndexLength() const {
-  return (m_blockSize - 2 - sizeof(BlockIndex) - 4) / sizeof(BlockIndex);
+  return (m_blockSize / sizeof(BlockIndex)) - 2 - sizeof(BlockIndex) - 4;
 }
 
 BTreeSha256Database::BTreeSha256Database() {
diff --git a/source/core/StarBTreeDatabase.hpp b/source/core/StarBTreeDatabase.hpp
index f1b88a1..b3a530f 100644
--- a/source/core/StarBTreeDatabase.hpp
+++ b/source/core/StarBTreeDatabase.hpp
@@ -230,7 +230,7 @@ private:
   void updateBlock(BlockIndex blockIndex, ByteArray const& block);
 
   void rawReadBlock(BlockIndex blockIndex, size_t blockOffset, char* block, size_t size) const;
-  void rawWriteBlock(BlockIndex blockIndex, size_t blockOffset, char const* block, size_t size) const;
+  void rawWriteBlock(BlockIndex blockIndex, size_t blockOffset, char const* block, size_t size);
 
   void updateHeadFreeIndexBlock(BlockIndex newHead);
 
@@ -251,6 +251,9 @@ private:
   void writeRoot();
   void readRoot();
   void doCommit();
+  void commitWrites();
+  bool tryFlatten();
+  bool flattenVisitor(BTreeImpl::Index& index, BlockIndex& count);
 
   void checkIfOpen(char const* methodName, bool shouldBeOpen) const;
   void checkBlockIndex(size_t blockIndex) const;
@@ -285,14 +288,14 @@ private:
   bool m_dirty;
 
   // Blocks that can be freely allocated and written to without violating
-  // atomic consistency
+  // atomic consistency.
   Set<BlockIndex> m_availableBlocks;
 
-  // Blocks to be freed on next commit.
-  Deque<BlockIndex> m_pendingFree;
-
   // Blocks that have been written in uncommitted portions of the tree.
   Set<BlockIndex> m_uncommitted;
+
+  // Temporarily holds written data so that it can be rolled back.
+  mutable Map<BlockIndex, ByteArray> m_uncommittedWrites;
 };
 
 // Version of BTreeDatabase that hashes keys with SHA-256 to produce a unique