diff --git a/db/db_bench.cc b/db/db_bench.cc
index ca05b2b20..a98e7c7d3 100644
--- a/db/db_bench.cc
+++ b/db/db_bench.cc
@@ -898,6 +898,7 @@ class ReportFileOpEnv : public EnvWrapper {
         return rv;
       }
 
+      Status Truncate(uint64_t size) override { return target_->Truncate(size); }
       Status Close() override { return target_->Close(); }
       Status Flush() override { return target_->Flush(); }
       Status Sync() override { return target_->Sync(); }
diff --git a/db/db_iter_test.cc b/db/db_iter_test.cc
index 73ef91b6a..90ec90831 100644
--- a/db/db_iter_test.cc
+++ b/db/db_iter_test.cc
@@ -1884,6 +1884,8 @@ TEST_F(DBIterWithMergeIterTest, InnerMergeIterator2) {
   ASSERT_EQ(db_iter_->value().ToString(), "4");
 }
 
+#if !(defined NDEBUG) || !defined(OS_WIN)
+
 TEST_F(DBIterWithMergeIterTest, InnerMergeIteratorDataRace1) {
   // Test Prev() when one child iterator is at its end but more rows
   // are added.
@@ -2234,6 +2236,7 @@ TEST_F(DBIterWithMergeIterTest, InnerMergeIteratorDataRace8) {
 
   rocksdb::SyncPoint::GetInstance()->DisableProcessing();
 }
+#endif // #if !(defined NDEBUG) || !defined(OS_WIN)
 }  // namespace rocksdb
 
 int main(int argc, char** argv) {
diff --git a/db/fault_injection_test.cc b/db/fault_injection_test.cc
index 87028e381..7ce18c8e7 100644
--- a/db/fault_injection_test.cc
+++ b/db/fault_injection_test.cc
@@ -141,6 +141,7 @@ class TestWritableFile : public WritableFile {
                             FaultInjectionTestEnv* env);
   virtual ~TestWritableFile();
   virtual Status Append(const Slice& data) override;
+  virtual Status Truncate(uint64_t size) override { return target_->Truncate(size); }
   virtual Status Close() override;
   virtual Status Flush() override;
   virtual Status Sync() override;
diff --git a/include/rocksdb/env.h b/include/rocksdb/env.h
index 093292f66..6300a4703 100644
--- a/include/rocksdb/env.h
+++ b/include/rocksdb/env.h
@@ -393,6 +393,12 @@ class RandomAccessFile {
   virtual Status Read(uint64_t offset, size_t n, Slice* result,
                       char* scratch) const = 0;
 
+  // Used by the file_reader_writer to decide if the ReadAhead wrapper
+  // should simply forward the call and do not enact buffering or locking.
+  virtual bool ShouldForwardRawRequest() const {
+    return false;
+  }
+
   // Tries to get an unique ID for this file that will be the same each time
   // the file is opened (and will stay the same while the file is open).
   // Furthermore, it tries to make this ID at most "max_size" bytes. If such an
@@ -413,7 +419,6 @@ class RandomAccessFile {
               // compatibility.
   };
 
-
   enum AccessPattern { NORMAL, RANDOM, SEQUENTIAL, WILLNEED, DONTNEED };
 
   virtual void Hint(AccessPattern pattern) {}
@@ -438,7 +443,35 @@ class WritableFile {
   }
   virtual ~WritableFile();
 
+  // Indicates if the class makes use of unbuffered I/O
+  virtual bool UseOSBuffer() const {
+    return true;
+  }
+
+  const size_t c_DefaultPageSize = 4 * 1024;
+
+  // This is needed when you want to allocate
+  // AlignedBuffer for use with file I/O classes
+  // Used for unbuffered file I/O when UseOSBuffer() returns false
+  virtual size_t GetRequiredBufferAlignment() const {
+    return c_DefaultPageSize;
+  }
+
   virtual Status Append(const Slice& data) = 0;
+
+  // Positioned write for unbuffered access default forward
+  // to simple append as most of the tests are buffered by default
+  virtual Status PositionedAppend(const Slice& /* data */, uint64_t /* offset */) {
+    return Status::NotSupported();
+  }
+
+  // Truncate is necessary to trim the file to the correct size
+  // before closing. It is not always possible to keep track of the file
+  // size due to whole pages writes. The behavior is undefined if called
+  // with other writes to follow.
+  virtual Status Truncate(uint64_t size) {
+    return Status::OK();
+  }
   virtual Status Close() = 0;
   virtual Status Flush() = 0;
   virtual Status Sync() = 0; // sync data
@@ -839,6 +872,10 @@ class WritableFileWrapper : public WritableFile {
   explicit WritableFileWrapper(WritableFile* t) : target_(t) { }
 
   Status Append(const Slice& data) override { return target_->Append(data); }
+  Status PositionedAppend(const Slice& data, uint64_t offset) override {
+    return target_->PositionedAppend(data, offset);
+  }
+  Status Truncate(uint64_t size) override { return target_->Truncate(size); }
   Status Close() override { return target_->Close(); }
   Status Flush() override { return target_->Flush(); }
   Status Sync() override { return target_->Sync(); }
diff --git a/port/win/env_win.cc b/port/win/env_win.cc
index 70c68b8fb..2668ad1d2 100644
--- a/port/win/env_win.cc
+++ b/port/win/env_win.cc
@@ -30,6 +30,7 @@
 #include "util/iostats_context_imp.h"
 #include "util/rate_limiter.h"
 #include "util/sync_point.h"
+#include "util/aligned_buffer.h"
 
 #include "util/thread_status_updater.h"
 #include "util/thread_status_util.h"
@@ -161,15 +162,6 @@ inline int fsync(HANDLE hFile) {
   return 0;
 }
 
-inline size_t TruncateToPageBoundary(size_t page_size, size_t s) {
-  s -= (s & (page_size - 1));
-  assert((s % page_size) == 0);
-  return s;
-}
-
-// Roundup x to a multiple of y
-inline size_t Roundup(size_t x, size_t y) { return ((x + y - 1) / y) * y; }
-
 // SetFileInformationByHandle() is capable of fast pre-allocates.
 // However, this does not change the file end position unless the file is
 // truncated and the pre-allocated space is not considered filled with zeros.
@@ -492,7 +484,6 @@ class WinMmapFile : public WritableFile {
 
       size_t n = std::min(left, avail);
       memcpy(dst_, src, n);
-      IOSTATS_ADD(bytes_written, n);
       dst_ += n;
       src += n;
       left -= n;
@@ -502,6 +493,12 @@ class WinMmapFile : public WritableFile {
     return Status::OK();
   }
 
+  // Means Close() will properly take care of truncate
+  // and it does not need any additional information
+  virtual Status Truncate(uint64_t size) override {
+    return Status::OK();
+  }
+
   virtual Status Close() override {
     Status s;
 
@@ -612,94 +609,6 @@ class WinMmapFile : public WritableFile {
   }
 };
 
-// This class is to manage an aligned user
-// allocated buffer for unbuffered I/O purposes
-// though it does not make a difference if you need a buffer.
-class AlignedBuffer {
-  const size_t alignment_;
-  std::unique_ptr<char[]> buf_;
-  size_t capacity_;
-  size_t cursize_;
-  char* bufstart_;
-
- public:
-  explicit AlignedBuffer(size_t alignment)
-      : alignment_(alignment), capacity_(0), cursize_(0), bufstart_(nullptr) {
-    assert(alignment > 0);
-    assert((alignment & (alignment - 1)) == 0);
-  }
-
-  size_t GetAlignment() const { return alignment_; }
-
-  size_t GetCapacity() const { return capacity_; }
-
-  size_t GetCurrentSize() const { return cursize_; }
-
-  const char* GetBufferStart() const { return bufstart_; }
-
-  void Clear() { cursize_ = 0; }
-
-  // Allocates a new buffer and sets bufstart_ to the aligned first byte
-  void AllocateNewBuffer(size_t requestedCapacity) {
-    size_t size = Roundup(requestedCapacity, alignment_);
-    buf_.reset(new char[size + alignment_]);
-
-    char* p = buf_.get();
-    bufstart_ = reinterpret_cast<char*>(
-        (reinterpret_cast<uintptr_t>(p) + (alignment_ - 1)) &
-        ~static_cast<uintptr_t>(alignment_ - 1));
-    capacity_ = size;
-    cursize_ = 0;
-  }
-
-  // Used for write
-  // Returns the number of bytes appended
-  size_t Append(const char* src, size_t append_size) {
-    size_t buffer_remaining = capacity_ - cursize_;
-    size_t to_copy = std::min(append_size, buffer_remaining);
-
-    if (to_copy > 0) {
-      memcpy(bufstart_ + cursize_, src, to_copy);
-      cursize_ += to_copy;
-    }
-    return to_copy;
-  }
-
-  size_t Read(char* dest, size_t offset, size_t read_size) const {
-    assert(offset < cursize_);
-    size_t to_read = std::min(cursize_ - offset, read_size);
-    if (to_read > 0) {
-      memcpy(dest, bufstart_ + offset, to_read);
-    }
-    return to_read;
-  }
-
-  /// Pad to alignment
-  void PadToAlignmentWith(int padding) {
-    size_t total_size = Roundup(cursize_, alignment_);
-    size_t pad_size = total_size - cursize_;
-
-    if (pad_size > 0) {
-      assert((pad_size + cursize_) <= capacity_);
-      memset(bufstart_ + cursize_, padding, pad_size);
-      cursize_ += pad_size;
-    }
-  }
-
-  // After a partial flush move the tail to the beginning of the buffer
-  void RefitTail(size_t tail_offset, size_t tail_size) {
-    if (tail_size > 0) {
-      memmove(bufstart_, bufstart_ + tail_offset, tail_size);
-    }
-    cursize_ = tail_size;
-  }
-
-  // Returns place to start writing
-  char* GetDestination() { return bufstart_ + cursize_; }
-
-  void SetSize(size_t cursize) { cursize_ = cursize; }
-};
-
 class WinSequentialFile : public SequentialFile {
  private:
   const std::string filename_;
@@ -734,7 +643,7 @@ class WinSequentialFile : public SequentialFile {
     // Windows ReadFile API accepts a DWORD.
     // While it is possible to read in a loop if n is > UINT_MAX
     // it is a highly unlikely case.
-    if (n > UINT_MAX) {      
+    if (n > UINT_MAX) {
       return IOErrorFromWindowsError(filename_, ERROR_INVALID_PARAMETER);
     }
 
@@ -747,8 +656,6 @@ class WinSequentialFile : public SequentialFile {
       return IOErrorFromWindowsError(filename_, GetLastError());
     }
 
-    IOSTATS_ADD(bytes_read, r);
-
     *result = Slice(scratch, r);
 
     return s;
@@ -791,12 +698,13 @@ class WinRandomAccessFile : public RandomAccessFile {
       : filename_(fname),
         hFile_(hFile),
         use_os_buffer_(options.use_os_buffer),
-        buffer_(alignment),
+        buffer_(),
         buffered_start_(0) {
     assert(!options.use_mmap_reads);
 
     // Unbuffered access, use internal buffer for reads
     if (!use_os_buffer_) {
+      buffer_.Alignment(alignment);
       // Random read, no need in a big buffer
       // We read things in database blocks which are likely to be similar to
       // the alignment we use.
@@ -826,7 +734,7 @@ class WinRandomAccessFile : public RandomAccessFile {
       // Let's see if at least some of the requested data is already
       // in the buffer
       if (offset >= buffered_start_ &&
-          offset < (buffered_start_ + buffer_.GetCurrentSize())) {
+          offset < (buffered_start_ + buffer_.CurrentSize())) {
         size_t buffer_offset = offset - buffered_start_;
         r = buffer_.Read(dest, buffer_offset, left);
         assert(r >= 0);
@@ -839,7 +747,7 @@ class WinRandomAccessFile : public RandomAccessFile {
       // Still some left or none was buffered
       if (left > 0) {
         // Figure out the start/end offset for reading and amount to read
-        const size_t alignment = buffer_.GetAlignment();
+        const size_t alignment = buffer_.Alignment();
         const size_t start_page_start =
             TruncateToPageBoundary(alignment, offset);
         const size_t end_page_start =
@@ -847,21 +755,18 @@ class WinRandomAccessFile : public RandomAccessFile {
         const size_t actual_bytes_toread =
             (end_page_start - start_page_start) + alignment;
 
-        if (buffer_.GetCapacity() < actual_bytes_toread) {
+        if (buffer_.Capacity() < actual_bytes_toread) {
           buffer_.AllocateNewBuffer(actual_bytes_toread);
         } else {
           buffer_.Clear();
         }
 
         SSIZE_T read = 0;
-        {
-          IOSTATS_TIMER_GUARD(read_nanos);
-          read = pread(hFile_, buffer_.GetDestination(), actual_bytes_toread,
-                       start_page_start);
-        }
+        read = pread(hFile_, buffer_.Destination(), actual_bytes_toread,
+                      start_page_start);
 
         if (read > 0) {
-          buffer_.SetSize(read);
+          buffer_.Size(read);
           buffered_start_ = start_page_start;
 
           // Let's figure out how much we read from the users standpoint
@@ -884,7 +789,6 @@ class WinRandomAccessFile : public RandomAccessFile {
       }
     }
 
-    IOSTATS_ADD_IF_POSITIVE(bytes_read, n - left);
     *result = Slice(scratch, (r < 0) ? 0 : n - left);
 
     if (r < 0) {
@@ -893,6 +797,10 @@ class WinRandomAccessFile : public RandomAccessFile {
     return s;
   }
 
+  virtual bool ShouldForwardRawRequest() const override {
+    return true;
+  }
+
   virtual void Hint(AccessPattern pattern) override {}
 
   virtual Status InvalidateCache(size_t offset, size_t length) override {
@@ -915,33 +823,23 @@ class WinRandomAccessFile : public RandomAccessFile {
 class WinWritableFile : public WritableFile {
  private:
   const std::string filename_;
-  HANDLE hFile_;
-  AlignedBuffer buffer_;
-
-  uint64_t filesize_;      // How much data is actually written disk
-  uint64_t reservedsize_;  // how far we have reserved space
-
-  bool pending_sync_;
-
-  RateLimiter* rate_limiter_;
-
-  const bool use_os_buffer_;  // Used to indicate unbuffered access, the file
-                              // must be opened as unbuffered if false
+  HANDLE            hFile_;
+  const bool        use_os_buffer_;  // Used to indicate unbuffered access, the file
+  const uint64_t    alignment_;
+  // must be opened as unbuffered if false
+  uint64_t          filesize_;      // How much data is actually written disk
+  uint64_t          reservedsize_;  // how far we have reserved space
 
  public:
   WinWritableFile(const std::string& fname, HANDLE hFile, size_t alignment,
                   size_t capacity, const EnvOptions& options)
       : filename_(fname),
         hFile_(hFile),
-        buffer_(alignment),
+        use_os_buffer_(options.use_os_buffer),
+        alignment_(alignment),
         filesize_(0),
-        reservedsize_(0),
-        pending_sync_(false),
-        rate_limiter_(options.rate_limiter),
-        use_os_buffer_(options.use_os_buffer) {
+        reservedsize_(0) {
     assert(!options.use_mmap_writes);
-
-    buffer_.AllocateNewBuffer(capacity);
   }
 
   ~WinWritableFile() {
@@ -950,106 +848,84 @@ class WinWritableFile : public WritableFile {
     }
   }
 
-  virtual Status Append(const Slice& data) override {
-    const char* src = data.data();
+  // Indicates if the class makes use of unbuffered I/O
+  virtual bool UseOSBuffer() const override {
+    return use_os_buffer_;
+  }
 
-    assert(data.size() < INT_MAX);
+  virtual size_t GetRequiredBufferAlignment() const override {
+    return alignment_;
+  }
 
-    size_t left = data.size();
-    Status s;
-    pending_sync_ = true;
+  virtual Status Append(const Slice& data) override {
 
-    // This would call Alloc() if we are out of blocks
-    PrepareWrite(GetFileSize(), left);
+    // Used for buffered access ONLY
+    assert(use_os_buffer_);
+    assert(data.size() < std::numeric_limits<int>::max());
 
-    // Flush only when I/O is buffered
-    if (use_os_buffer_ &&
-        (buffer_.GetCapacity() - buffer_.GetCurrentSize()) < left) {
-      if (buffer_.GetCurrentSize() > 0) {
-        s = Flush();
-        if (!s.ok()) {
-          return s;
-        }
-      }
+    Status s;
 
-      if (buffer_.GetCapacity() < c_OneMB) {
-        size_t desiredCapacity = buffer_.GetCapacity() * 2;
-        desiredCapacity = std::min(desiredCapacity, c_OneMB);
-        buffer_.AllocateNewBuffer(desiredCapacity);
-      }
+    DWORD bytesWritten = 0;
+    if (!WriteFile(hFile_, data.data(),
+        data.size(), &bytesWritten, NULL)) {
+      auto lastError = GetLastError();
+      s = IOErrorFromWindowsError(
+        "Failed to WriteFile: " + filename_,
+        lastError);
+    } else {
+      assert(size_t(bytesWritten) == data.size());
+      filesize_ += data.size();
     }
 
-    // We always use the internal buffer for the unbuffered I/O
-    // or we simply use it for its original purpose to accumulate many small
-    // chunks
-    if (!use_os_buffer_ || (buffer_.GetCapacity() >= left)) {
-      while (left > 0) {
-        size_t appended = buffer_.Append(src, left);
-        left -= appended;
-        src += appended;
+    return s;
+  }
 
-        if (left > 0) {
-          s = Flush();
-          if (!s.ok()) {
-            break;
-          }
+  virtual Status PositionedAppend(const Slice& data, uint64_t offset) override {
+    Status s;
 
-          size_t cursize = buffer_.GetCurrentSize();
-          size_t capacity = buffer_.GetCapacity();
+    SSIZE_T ret = pwrite(hFile_, data.data(), 
+      data.size(), offset);
 
-          // We double the buffer here because
-          // Flush calls do not keep up with the incoming bytes
-          // This is the only place when buffer is changed with unbuffered I/O
-          if (cursize == 0 && capacity < c_OneMB) {
-            size_t desiredCapacity = capacity * 2;
-            desiredCapacity = std::min(desiredCapacity, c_OneMB);
-            buffer_.AllocateNewBuffer(desiredCapacity);
-          }
-        }
-      }
+    // Error break
+    if (ret < 0) {
+      auto lastError = GetLastError();
+      s = IOErrorFromWindowsError(
+        "Failed to pwrite for: " + filename_, lastError);
     } else {
-      // Writing directly to file bypassing what is in the buffer
-      assert(buffer_.GetCurrentSize() == 0);
-      // Use rate limiter for normal I/O very large request if available
-      s = WriteBuffered(src, left);
+      // With positional write it is not clear at all
+      // if this actually extends the filesize
+      assert(size_t(ret) == data.size());
+      filesize_ += data.size();
     }
+    return s;
+  }
 
+  // Need to implement this so the file is truncated correctly
+  // when buffered and unbuffered mode
+  virtual Status Truncate(uint64_t size) override {
+    Status s =  ftruncate(filename_, hFile_, size);
+    if (s.ok()) {
+      filesize_ = size;
+    }
     return s;
   }
 
   virtual Status Close() override {
-    Status s;
 
-    // If there is any data in the cache not written we need to deal with it
-    const size_t cursize = buffer_.GetCurrentSize();
-    const uint64_t final_size = filesize_ + cursize;
-
-    if (cursize > 0) {
-      // If OS buffering is on, we just flush the remainder, otherwise need
-      if (!use_os_buffer_) {
-        s = WriteUnbuffered();
-      } else {
-        s = WriteBuffered(buffer_.GetBufferStart(), cursize);
-      }
-    }
+    Status s;
 
-    if (s.ok()) {
-      s = ftruncate(filename_, hFile_, final_size);
-    }
+    assert(INVALID_HANDLE_VALUE != hFile_);
 
-    // Sync data if buffer was flushed
-    if (s.ok() && (cursize > 0) && fsync(hFile_) < 0) {
+    if (fsync(hFile_) < 0) {
       auto lastError = GetLastError();
       s = IOErrorFromWindowsError("fsync failed at Close() for: " + filename_,
-                                  lastError);
+        lastError);
     }
 
     if (FALSE == ::CloseHandle(hFile_)) {
-      if (s.ok()) {
-        auto lastError = GetLastError();
-        s = IOErrorFromWindowsError("CloseHandle failed for: " + filename_,
-                                    lastError);
-      }
+      auto lastError = GetLastError();
+      s = IOErrorFromWindowsError("CloseHandle failed for: " + filename_,
+                                  lastError);
     }
 
     hFile_ = INVALID_HANDLE_VALUE;
@@ -1057,36 +933,18 @@ class WinWritableFile : public WritableFile {
   }
 
   // write out the cached data to the OS cache
+  // This is now taken care of the WritableFileWriter
   virtual Status Flush() override {
-    Status status;
-
-    if (buffer_.GetCurrentSize() > 0) {
-      if (!use_os_buffer_) {
-        status = WriteUnbuffered();
-      } else {
-        status =
-            WriteBuffered(buffer_.GetBufferStart(), buffer_.GetCurrentSize());
-        if (status.ok()) {
-          buffer_.SetSize(0);
-        }
-      }
-    }
-    return status;
+    return Status::OK();
   }
 
   virtual Status Sync() override {
-    Status s = Flush();
-    if (!s.ok()) {
-      return s;
-    }
-
+    Status s;
     // Calls flush buffers
-    if (pending_sync_ && fsync(hFile_) < 0) {
+    if (fsync(hFile_) < 0) {
       auto lastError = GetLastError();
       s = IOErrorFromWindowsError("fsync failed at Sync() for: " + filename_,
                                   lastError);
-    } else {
-      pending_sync_ = false;
     }
     return s;
   }
@@ -1094,7 +952,12 @@ class WinWritableFile : public WritableFile {
   virtual Status Fsync() override { return Sync(); }
 
   virtual uint64_t GetFileSize() override {
-    return filesize_ + buffer_.GetCurrentSize();
+    // Double accounting now here with WritableFileWriter
+    // and this size will be wrong when unbuffered access is used
+    // but tests implement their own writable files and do not use WritableFileWrapper
+    // so we need to squeeze a square peg through
+    // a round hole here.
+    return filesize_;
   }
 
   virtual Status Allocate(off_t offset, off_t len) override {
@@ -1104,7 +967,7 @@ class WinWritableFile : public WritableFile {
     // Make sure that we reserve an aligned amount of space
     // since the reservation block size is driven outside so we want
     // to check if we are ok with reservation here
-    size_t spaceToReserve = Roundup(offset + len, buffer_.GetAlignment());
+    size_t spaceToReserve = Roundup(offset + len, alignment_);
     // Nothing to do
     if (spaceToReserve <= reservedsize_) {
       return status;
@@ -1117,133 +980,6 @@ class WinWritableFile : public WritableFile {
     }
     return status;
   }
-
- private:
-  // This method writes to disk the specified data and makes use of the rate
-  // limiter
-  // if available
-  Status WriteBuffered(const char* data, size_t size) {
-    Status s;
-    assert(use_os_buffer_);
-    const char* src = data;
-    size_t left = size;
-
-    size_t actually_written = 0;
-
-    while (left > 0) {
-      size_t bytes_allowed = RequestToken(left, false);
-
-      DWORD bytesWritten = 0;
-      if (!WriteFile(hFile_, src, bytes_allowed, &bytesWritten, NULL)) {
-        auto lastError = GetLastError();
-        s = IOErrorFromWindowsError(
-            "Failed to write buffered via rate_limiter: " + filename_,
-            lastError);
-        break;
-      } else {
-        actually_written += bytesWritten;
-        src += bytesWritten;
-        left -= bytesWritten;
-      }
-    }
-
-    IOSTATS_ADD(bytes_written, actually_written);
-    filesize_ += actually_written;
-
-    return s;
-  }
-
-  // This flushes the accumulated data in the buffer. We pad data with zeros if
-  // necessary to the whole page.
-  // However, during automatic flushes padding would not be necessary.
-  // We always use RateLimiter if available. We move (Refit) any buffer bytes
-  // that are left over the
-  // whole number of pages to be written again on the next flush because we can
-  // only write on aligned
-  // offsets.
-  Status WriteUnbuffered() {
-    Status s;
-
-    assert(!use_os_buffer_);
-    size_t alignment = buffer_.GetAlignment();
-    assert((filesize_ % alignment) == 0);
-
-    // Calculate whole page final file advance if all writes succeed
-    size_t file_advance =
-        TruncateToPageBoundary(alignment, buffer_.GetCurrentSize());
-
-    // Calculate the leftover tail, we write it here padded with zeros BUT we
-    // will write
-    // it again in the future either on Close() OR when the current whole page
-    // fills out
-    size_t leftover_tail = buffer_.GetCurrentSize() - file_advance;
-
-    // Round up and pad
-    buffer_.PadToAlignmentWith(0);
-
-    const char* src = buffer_.GetBufferStart();
-    size_t left = buffer_.GetCurrentSize();
-    uint64_t file_offset = filesize_;
-    size_t actually_written = 0;
-
-    while (left > 0) {
-      // Request how much is allowed. If this is less than one alignment we may
-      // be blocking a lot on every write
-      // because we can not write less than one alignment (page) unit thus check
-      // the configuration.
-      size_t bytes_allowed = RequestToken(left, true);
-      SSIZE_T ret = pwrite(hFile_, buffer_.GetBufferStart() + actually_written,
-                           bytes_allowed, file_offset);
-
-      // Error break
-      if (ret < 0) {
-        auto lastError = GetLastError();
-        s = IOErrorFromWindowsError(
-            "Failed to pwrite for unbuffered: " + filename_, lastError);
-        buffer_.SetSize(file_advance + leftover_tail);
-        break;
-      }
-      actually_written += ret;
-      file_offset += ret;
-      left -= ret;
-    }
-
-    IOSTATS_ADD(bytes_written, actually_written);
-
-    if (s.ok()) {
-      // Move the tail to the beginning of the buffer
-      // This never happens during normal Append but rather during
-      // explicit call to Flush()/Sync() or Close()
-      buffer_.RefitTail(file_advance, leftover_tail);
-      // This is where we start writing next time which may or not be
-      // the actual file size on disk. They match if the buffer size
-      // is a multiple of whole pages otherwise filesize_ is leftover_tail
-      // behind
-      filesize_ += file_advance;
-    }
-    return s;
-  }
-
-  // This truncates the request to a single burst bytes
-  // and then goes through the request to make sure we are
-  // satisfied in the order of the I/O priority
-  size_t RequestToken(size_t bytes, bool align) const {
-    if (rate_limiter_ && io_priority_ < Env::IO_TOTAL) {
-      bytes = std::min(
-          bytes, static_cast<size_t>(rate_limiter_->GetSingleBurstBytes()));
-
-      if (align) {
-        // Here we may actually require more than burst and block
-        // but we can not write less than one page at a time on unbuffered
-        // thus we may want not to use ratelimiter s
-        size_t alignment = buffer_.GetAlignment();
-        bytes = std::max(alignment, TruncateToPageBoundary(alignment, bytes));
-      }
-
-      rate_limiter_->Request(bytes, io_priority_);
-    }
-    return bytes;
-  }
 };
 
 class WinDirectory : public Directory {
@@ -2092,7 +1828,7 @@ class WinEnv : public Env {
       ThreadPool* thread_pool_;
       size_t thread_id_;  // Thread count in the thread.
 
-      explicit BGThreadMetadata(ThreadPool* thread_pool, size_t thread_id)
+      BGThreadMetadata(ThreadPool* thread_pool, size_t thread_id)
           : thread_pool_(thread_pool), thread_id_(thread_id) {}
     };
 
diff --git a/util/aligned_buffer.h b/util/aligned_buffer.h
new file mode 100644
index 000000000..2244316fe
--- /dev/null
+++ b/util/aligned_buffer.h
@@ -0,0 +1,154 @@
+//  Copyright (c) 2013, Facebook, Inc.  All rights reserved.
+//  This source code is licensed under the BSD-style license found in the
+//  LICENSE file in the root directory of this source tree. An additional grant
+//  of patent rights can be found in the PATENTS file in the same directory.
+//
+// Copyright (c) 2011 The LevelDB Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file. See the AUTHORS file for names of contributors.
+#pragma once
+
+#include <algorithm>
+#include "port/port.h"
+
+namespace rocksdb {
+
+inline size_t TruncateToPageBoundary(size_t page_size, size_t s) {
+  s -= (s & (page_size - 1));
+  assert((s % page_size) == 0);
+  return s;
+}
+
+inline size_t Roundup(size_t x, size_t y) {
+  return ((x + y - 1) / y) * y;
+}
+
+// This class is to manage an aligned user
+// allocated buffer for unbuffered I/O purposes
+// though can be used for any purpose.
+class AlignedBuffer {
+  size_t alignment_;
+  std::unique_ptr<char[]> buf_;
+  size_t capacity_;
+  size_t cursize_;
+  char* bufstart_;
+
+public:
+  AlignedBuffer()
+    : alignment_(),
+      capacity_(0),
+      cursize_(0),
+      bufstart_(nullptr) {
+  }
+
+  AlignedBuffer(AlignedBuffer&& o) ROCKSDB_NOEXCEPT {
+    *this = std::move(o);
+  }
+
+  AlignedBuffer& operator=(AlignedBuffer&& o) ROCKSDB_NOEXCEPT {
+    alignment_ = std::move(o.alignment_);
+    buf_ = std::move(o.buf_);
+    capacity_ = std::move(o.capacity_);
+    cursize_ = std::move(o.cursize_);
+    bufstart_ = std::move(o.bufstart_);
+    return *this;
+  }
+
+  AlignedBuffer(const AlignedBuffer&) = delete;
+
+  AlignedBuffer& operator=(const AlignedBuffer&) = delete;
+
+  size_t Alignment() const {
+    return alignment_;
+  }
+
+  size_t Capacity() const {
+    return capacity_;
+  }
+
+  size_t CurrentSize() const {
+    return cursize_;
+  }
+
+  const char* BufferStart() const {
+    return bufstart_;
+  }
+
+  void Clear() {
+    cursize_ = 0;
+  }
+
+  void Alignment(size_t alignment) {
+    assert(alignment > 0);
+    assert((alignment & (alignment - 1)) == 0);
+    alignment_ = alignment;
+  }
+
+  // Allocates a new buffer and sets bufstart_ to the aligned first byte
+  void AllocateNewBuffer(size_t requestedCapacity) {
+
+    assert(alignment_ > 0);
+    assert((alignment_ & (alignment_ - 1)) == 0);
+
+    size_t size = Roundup(requestedCapacity, alignment_);
+    buf_.reset(new char[size + alignment_]);
+
+    char* p = buf_.get();
+    bufstart_ = reinterpret_cast<char*>(
+      (reinterpret_cast<uintptr_t>(p)+(alignment_ - 1)) &
+      ~static_cast<uintptr_t>(alignment_ - 1));
+    capacity_ = size;
+    cursize_ = 0;
+  }
+  // Used for write
+  // Returns the number of bytes appended
+  size_t Append(const char* src, size_t append_size) {
+    size_t buffer_remaining = capacity_ - cursize_;
+    size_t to_copy = std::min(append_size, buffer_remaining);
+
+    if (to_copy > 0) {
+      memcpy(bufstart_ + cursize_, src, to_copy);
+      cursize_ += to_copy;
+    }
+    return to_copy;
+  }
+
+  size_t Read(char* dest, size_t offset, size_t read_size) const {
+    assert(offset < cursize_);
+    size_t to_read = std::min(cursize_ - offset, read_size);
+    if (to_read > 0) {
+      memcpy(dest, bufstart_ + offset, to_read);
+    }
+    return to_read;
+  }
+
+  /// Pad to alignment
+  void PadToAlignmentWith(int padding) {
+    size_t total_size = Roundup(cursize_, alignment_);
+    size_t pad_size = total_size - cursize_;
+
+    if (pad_size > 0) {
+      assert((pad_size + cursize_) <= capacity_);
+      memset(bufstart_ + cursize_, padding, pad_size);
+      cursize_ += pad_size;
+    }
+  }
+
+  // After a partial flush move the tail to the beginning of the buffer
+  void RefitTail(size_t tail_offset, size_t tail_size) {
+    if (tail_size > 0) {
+      memmove(bufstart_, bufstart_ + tail_offset, tail_size);
+    }
+    cursize_ = tail_size;
+  }
+
+  // Returns place to start writing
+  char* Destination() {
+    return bufstart_ + cursize_;
+  }
+
+  void Size(size_t cursize) {
+    cursize_ = cursize;
+  }
+};
+}
diff --git a/util/db_test_util.h b/util/db_test_util.h
index bb1bfa7cb..dfb2b3e6d 100644
--- a/util/db_test_util.h
+++ b/util/db_test_util.h
@@ -148,6 +148,9 @@ class SpecialEnv : public EnvWrapper {
           return base_->Append(data);
         }
       }
+      Status Truncate(uint64_t size) override {
+        return base_->Truncate(size);
+      }
       Status Close() override {
 // SyncPoint is not supported in Released Windows Mode.
 #if !(defined NDEBUG) || !defined(OS_WIN)
@@ -185,6 +188,7 @@ class SpecialEnv : public EnvWrapper {
           return base_->Append(data);
         }
       }
+      Status Truncate(uint64_t size) override { return base_->Truncate(size); }
       Status Close() override { return base_->Close(); }
       Status Flush() override { return base_->Flush(); }
       Status Sync() override {
@@ -225,6 +229,7 @@ class SpecialEnv : public EnvWrapper {
 #endif
         return s;
       }
+      Status Truncate(uint64_t size) override { return base_->Truncate(size); }
       Status Close() override { return base_->Close(); }
       Status Flush() override { return base_->Flush(); }
       Status Sync() override {
diff --git a/util/env_posix.cc b/util/env_posix.cc
index af2ab8e5d..b856f1d70 100644
--- a/util/env_posix.cc
+++ b/util/env_posix.cc
@@ -494,6 +494,12 @@ class PosixMmapFile : public WritableFile {
     return Status::OK();
   }
 
+  // Means Close() will properly take care of truncate
+  // and it does not need any additional information
+  virtual Status Truncate(uint64_t size) override {
+    return Status::OK();
+  }
+
   virtual Status Close() override {
     Status s;
     size_t unused = limit_ - dst_;
@@ -624,6 +630,12 @@ class PosixWritableFile : public WritableFile {
     return Status::OK();
   }
 
+  // Means Close() will properly take care of truncate
+  // and it does not need any additional information
+  virtual Status Truncate(uint64_t size) override {
+    return Status::OK();
+  }
+
   virtual Status Close() override {
     Status s;
 
diff --git a/util/env_test.cc b/util/env_test.cc
index 0e4feabfd..6fe8ed80b 100644
--- a/util/env_test.cc
+++ b/util/env_test.cc
@@ -989,6 +989,7 @@ TEST_F(EnvPosixTest, WritableFileWrapper) {
     }
 
     Status Append(const Slice& data) override { inc(1); return Status::OK(); }
+    Status Truncate(uint64_t size) override { return Status::OK(); }
     Status Close() override { inc(2); return Status::OK(); }
     Status Flush() override { inc(3); return Status::OK(); }
     Status Sync() override { inc(4); return Status::OK(); }
diff --git a/util/file_reader_writer.cc b/util/file_reader_writer.cc
index 140969843..86d70b62d 100644
--- a/util/file_reader_writer.cc
+++ b/util/file_reader_writer.cc
@@ -20,6 +20,11 @@
 #include "util/sync_point.h"
 
 namespace rocksdb {
+
+namespace {
+  const size_t c_OneMb = (1 << 20);
+}
+
 Status SequentialFileReader::Read(size_t n, Slice* result, char* scratch) {
   Status s = file_->Read(n, result, scratch);
   IOSTATS_ADD(bytes_read, result->size());
@@ -59,81 +64,116 @@ Status WritableFileWriter::Append(const Slice& data) {
     TEST_SYNC_POINT("WritableFileWriter::Append:BeforePrepareWrite");
     writable_file_->PrepareWrite(static_cast<size_t>(GetFileSize()), left);
   }
-  // if there is no space in the cache, then flush
-  if (cursize_ + left > capacity_) {
-    s = Flush();
-    if (!s.ok()) {
-      return s;
+
+  // Flush only when I/O is buffered
+  if (use_os_buffer_ &&
+    (buf_.Capacity() - buf_.CurrentSize()) < left) {
+    if (buf_.CurrentSize() > 0) {
+      s = Flush();
+      if (!s.ok()) {
+        return s;
+      }
     }
-    // Increase the buffer size, but capped at 1MB
-    if (capacity_ < (1 << 20)) {
-      capacity_ *= 2;
-      buf_.reset(new char[capacity_]);
+
+    if (buf_.Capacity() < c_OneMb) {
+      size_t desiredCapacity = buf_.Capacity() * 2;
+      desiredCapacity = std::min(desiredCapacity, c_OneMb);
+      buf_.AllocateNewBuffer(desiredCapacity);
     }
-    assert(cursize_ == 0);
+    assert(buf_.CurrentSize() == 0);
   }
 
-  // if the write fits into the cache, then write to cache
-  // otherwise do a write() syscall to write to OS buffers.
-  if (cursize_ + left <= capacity_) {
-    memcpy(buf_.get() + cursize_, src, left);
-    cursize_ += left;
-  } else {
-    while (left != 0) {
-      size_t size = RequestToken(left);
-      {
-        IOSTATS_TIMER_GUARD(write_nanos);
-        s = writable_file_->Append(Slice(src, size));
+  // We never write directly to disk with unbuffered I/O on.
+  // or we simply use it for its original purpose to accumulate many small
+  // chunks
+  if (!use_os_buffer_ || (buf_.Capacity() >= left)) {
+    while (left > 0) {
+      size_t appended = buf_.Append(src, left);
+      left -= appended;
+      src += appended;
+
+      if (left > 0) {
+        s = Flush();
         if (!s.ok()) {
-          return s;
+          break;
         }
-      }
-      IOSTATS_ADD(bytes_written, size);
-      TEST_KILL_RANDOM(rocksdb_kill_odds);
 
-      left -= size;
-      src += size;
+        // We double the buffer here because
+        // Flush calls do not keep up with the incoming bytes
+        // This is the only place when buffer is changed with unbuffered I/O
+        if (buf_.Capacity() < (1 << 20)) {
+          size_t desiredCapacity = buf_.Capacity() * 2;
+          desiredCapacity = std::min(desiredCapacity, c_OneMb);
+          buf_.AllocateNewBuffer(desiredCapacity);
+        }
+      }
     }
+  } else {
+    // Writing directly to file bypassing the buffer
+    assert(buf_.CurrentSize() == 0);
+    s = WriteBuffered(src, left);
   }
+
   TEST_KILL_RANDOM(rocksdb_kill_odds);
   filesize_ += data.size();
   return Status::OK();
 }
 
 Status WritableFileWriter::Close() {
+
+  // Do not quit immediately on failure the file MUST be closed
   Status s;
-  s = Flush();  // flush cache to OS
-  if (!s.ok()) {
+
+  // Possible to close it twice now as we MUST close
+  // in __dtor, simply flushing is not enough
+  // Windows when pre-allocating does not fill with zeros
+  // also with unbuffered access we also set the end of data.
+  if (!writable_file_) {
     return s;
   }
 
+  s = Flush();  // flush cache to OS
+
+  // In unbuffered mode we write whole pages so
+  // we need to let the file know where data ends.
+  Status interim = writable_file_->Truncate(filesize_);
+  if (!interim.ok() && s.ok()) {
+    s = interim;
+  }
+
   TEST_KILL_RANDOM(rocksdb_kill_odds);
-  return writable_file_->Close();
+  interim = writable_file_->Close();
+  if (!interim.ok() && s.ok()) {
+    s = interim;
+  }
+
+  writable_file_.reset();
+
+  return s;
 }
 
+
 // write out the cached data to the OS cache
 Status WritableFileWriter::Flush() {
+  Status s;
   TEST_KILL_RANDOM(rocksdb_kill_odds * REDUCE_ODDS2);
-  size_t left = cursize_;
-  char* src = buf_.get();
-  while (left != 0) {
-    size_t size = RequestToken(left);
-    {
-      IOSTATS_TIMER_GUARD(write_nanos);
-      TEST_SYNC_POINT("WritableFileWriter::Flush:BeforeAppend");
-      Status s = writable_file_->Append(Slice(src, size));
-      if (!s.ok()) {
-        return s;
-      }
+
+  if (buf_.CurrentSize() > 0) {
+    if (use_os_buffer_) {
+      s = WriteBuffered(buf_.BufferStart(), buf_.CurrentSize());
+    } else {
+      s = WriteUnbuffered();
+    }
+    if (!s.ok()) {
+      return s;
     }
-    IOSTATS_ADD(bytes_written, size);
-    TEST_KILL_RANDOM(rocksdb_kill_odds * REDUCE_ODDS2);
-    left -= size;
-    src += size;
   }
-  cursize_ = 0;
 
-  writable_file_->Flush();
+  s = writable_file_->Flush();
+
+  if (!s.ok()) {
+    return s;
+  }
 
   // sync OS cache to disk for every bytes_per_sync_
   // TODO: give log file and sst file different options (log
@@ -147,21 +187,21 @@ Status WritableFileWriter::Flush() {
   // Xfs does neighbor page flushing outside of the specified ranges. We
   // need to make sure sync range is far from the write offset.
   if (!direct_io_ && bytes_per_sync_) {
-    uint64_t kBytesNotSyncRange = 1024 * 1024;  // recent 1MB is not synced.
-    uint64_t kBytesAlignWhenSync = 4 * 1024;    // Align 4KB.
+    const uint64_t kBytesNotSyncRange = 1024 * 1024;  // recent 1MB is not synced.
+    const uint64_t kBytesAlignWhenSync = 4 * 1024;    // Align 4KB.
     if (filesize_ > kBytesNotSyncRange) {
       uint64_t offset_sync_to = filesize_ - kBytesNotSyncRange;
       offset_sync_to -= offset_sync_to % kBytesAlignWhenSync;
       assert(offset_sync_to >= last_sync_size_);
       if (offset_sync_to > 0 &&
           offset_sync_to - last_sync_size_ >= bytes_per_sync_) {
-        RangeSync(last_sync_size_, offset_sync_to - last_sync_size_);
+        s = RangeSync(last_sync_size_, offset_sync_to - last_sync_size_);
         last_sync_size_ = offset_sync_to;
       }
     }
   }
 
-  return Status::OK();
+  return s;
 }
 
 Status WritableFileWriter::Sync(bool use_fsync) {
@@ -214,27 +254,140 @@ Status WritableFileWriter::RangeSync(off_t offset, off_t nbytes) {
   return writable_file_->RangeSync(offset, nbytes);
 }
 
-size_t WritableFileWriter::RequestToken(size_t bytes) {
+size_t WritableFileWriter::RequestToken(size_t bytes, bool align) {
   Env::IOPriority io_priority;
-  if (rate_limiter_&&(io_priority = writable_file_->GetIOPriority()) <
+  if (rate_limiter_ && (io_priority = writable_file_->GetIOPriority()) <
       Env::IO_TOTAL) {
-    bytes = std::min(bytes,
-                     static_cast<size_t>(rate_limiter_->GetSingleBurstBytes()));
+    bytes = std::min(
+      bytes, static_cast<size_t>(rate_limiter_->GetSingleBurstBytes()));
+
+    if (align) {
+      // Here we may actually require more than burst and block
+      // but we can not write less than one page at a time on unbuffered
+      // thus we may want not to use ratelimiter s
+      size_t alignment = buf_.Alignment();
+      bytes = std::max(alignment, TruncateToPageBoundary(alignment, bytes));
+    }
     rate_limiter_->Request(bytes, io_priority);
   }
   return bytes;
 }
 
+// This method writes to disk the specified data and makes use of the rate
+// limiter if available
+Status WritableFileWriter::WriteBuffered(const char* data, size_t size) {
+  Status s;
+  assert(use_os_buffer_);
+  const char* src = data;
+  size_t left = size;
+
+  while (left > 0) {
+    size_t allowed = RequestToken(left, false);
+
+    {
+      IOSTATS_TIMER_GUARD(write_nanos);
+      TEST_SYNC_POINT("WritableFileWriter::Flush:BeforeAppend");
+      s = writable_file_->Append(Slice(src, allowed));
+      if (!s.ok()) {
+        return s;
+      }
+    }
+
+    IOSTATS_ADD(bytes_written, allowed);
+    TEST_KILL_RANDOM(rocksdb_kill_odds);
+
+    left -= allowed;
+    src += allowed;
+  }
+  buf_.Size(0);
+  return s;
+}
+
+
+// This flushes the accumulated data in the buffer. We pad data with zeros if
+// necessary to the whole page.
+// However, during automatic flushes padding would not be necessary.
+// We always use RateLimiter if available. We move (Refit) any buffer bytes
+// that are left over the
+// whole number of pages to be written again on the next flush because we can
+// only write on aligned
+// offsets.
+Status WritableFileWriter::WriteUnbuffered() {
+  Status s;
+
+  assert(!use_os_buffer_);
+  const size_t alignment = buf_.Alignment();
+  assert((next_write_offset_ % alignment) == 0);
+
+  // Calculate whole page final file advance if all writes succeed
+  size_t file_advance =
+    TruncateToPageBoundary(alignment, buf_.CurrentSize());
+
+  // Calculate the leftover tail, we write it here padded with zeros BUT we
+  // will write
+  // it again in the future either on Close() OR when the current whole page
+  // fills out
+  size_t leftover_tail = buf_.CurrentSize() - file_advance;
+
+  // Round up and pad
+  buf_.PadToAlignmentWith(0);
+
+  const char* src = buf_.BufferStart();
+  uint64_t write_offset = next_write_offset_;
+  size_t left = buf_.CurrentSize();
+
+  while (left > 0) {
+    // Check how much is allowed
+    size_t size = RequestToken(left, true);
+
+    {
+      IOSTATS_TIMER_GUARD(write_nanos);
+      TEST_SYNC_POINT("WritableFileWriter::Flush:BeforeAppend");
+      // Unbuffered writes must be positional
+      s = writable_file_->PositionedAppend(Slice(src, size), write_offset);
+      if (!s.ok()) {
+        buf_.Size(file_advance + leftover_tail);
+        return s;
+      }
+    }
+
+    IOSTATS_ADD(bytes_written, size);
+    left -= size;
+    src += size;
+    write_offset += size;
+    assert((next_write_offset_ % alignment) == 0);
+  }
+
+  if (s.ok()) {
+    // Move the tail to the beginning of the buffer
+    // This never happens during normal Append but rather during
+    // explicit call to Flush()/Sync() or Close()
+    buf_.RefitTail(file_advance, leftover_tail);
+    // This is where we start writing next time which may or not be
+    // the actual file size on disk. They match if the buffer size
+    // is a multiple of whole pages otherwise filesize_ is leftover_tail
+    // behind
+    next_write_offset_ += file_advance;
+  }
+  return s;
+}
+
+
 namespace {
 class ReadaheadRandomAccessFile : public RandomAccessFile {
  public:
-  ReadaheadRandomAccessFile(std::unique_ptr<RandomAccessFile> file,
-                            size_t readahead_size)
-      : file_(std::move(file)),
-        readahead_size_(readahead_size),
-        buffer_(new char[readahead_size_]),
-        buffer_offset_(0),
-        buffer_len_(0) {}
+   ReadaheadRandomAccessFile(std::unique_ptr<RandomAccessFile>&& file,
+     size_t readahead_size)
+     : file_(std::move(file)),
+       readahead_size_(readahead_size),
+       forward_calls_(file_->ShouldForwardRawRequest()),
+       buffer_(new char[readahead_size_]),
+       buffer_offset_(0),
+       buffer_len_(0) {}
+
+ ReadaheadRandomAccessFile(const ReadaheadRandomAccessFile&) = delete;
+
+ ReadaheadRandomAccessFile& operator=(const ReadaheadRandomAccessFile&) = delete;
 
   virtual Status Read(uint64_t offset, size_t n, Slice* result,
                       char* scratch) const override {
@@ -242,6 +395,14 @@ class ReadaheadRandomAccessFile : public RandomAccessFile {
       return file_->Read(offset, n, result, scratch);
     }
 
+    // On Windows in unbuffered mode this will lead to double buffering
+    // and double locking so we avoid that.
+    // In normal mode Windows caches so much data from disk that we do
+    // not need readahead.
+    if (forward_calls_) {
+      return file_->Read(offset, n, result, scratch);
+    }
+
     std::unique_lock<std::mutex> lk(lock_);
 
     size_t copied = 0;
@@ -249,7 +410,7 @@ class ReadaheadRandomAccessFile : public RandomAccessFile {
     if (offset >= buffer_offset_ && offset < buffer_len_ + buffer_offset_) {
       uint64_t offset_in_buffer = offset - buffer_offset_;
       copied = std::min(static_cast<uint64_t>(buffer_len_) - offset_in_buffer,
-                        static_cast<uint64_t>(n));
+        static_cast<uint64_t>(n));
       memcpy(scratch, buffer_.get() + offset_in_buffer, copied);
       if (copied == n) {
         // fully cached
@@ -259,7 +420,7 @@ class ReadaheadRandomAccessFile : public RandomAccessFile {
     }
     Slice readahead_result;
     Status s = file_->Read(offset + copied, readahead_size_, &readahead_result,
-                           buffer_.get());
+      buffer_.get());
     if (!s.ok()) {
       return s;
     }
@@ -290,20 +451,21 @@ class ReadaheadRandomAccessFile : public RandomAccessFile {
 
  private:
   std::unique_ptr<RandomAccessFile> file_;
-  size_t readahead_size_;
+  size_t               readahead_size_;
+  const bool           forward_calls_;
 
-  mutable std::mutex lock_;
+  mutable std::mutex   lock_;
   mutable std::unique_ptr<char[]> buffer_;
-  mutable uint64_t buffer_offset_;
-  mutable size_t buffer_len_;
+  mutable uint64_t     buffer_offset_;
+  mutable size_t       buffer_len_;
 };
 }  // namespace
 
 std::unique_ptr<RandomAccessFile> NewReadaheadRandomAccessFile(
-    std::unique_ptr<RandomAccessFile> file, size_t readahead_size) {
-  std::unique_ptr<ReadaheadRandomAccessFile> wrapped_file(
-      new ReadaheadRandomAccessFile(std::move(file), readahead_size));
-  return std::move(wrapped_file);
+    std::unique_ptr<RandomAccessFile>&& file, size_t readahead_size) {
+  std::unique_ptr<RandomAccessFile> result(
+    new ReadaheadRandomAccessFile(std::move(file), readahead_size));
+  return result;
 }
 
 }  // namespace rocksdb
diff --git a/util/file_reader_writer.h b/util/file_reader_writer.h
index 616d174a2..f33965dc6 100644
--- a/util/file_reader_writer.h
+++ b/util/file_reader_writer.h
@@ -8,6 +8,8 @@
 // found in the LICENSE file. See the AUTHORS file for names of contributors.
 #pragma once
 #include "rocksdb/env.h"
+#include "util/aligned_buffer.h"
+#include "port/port.h"
 
 namespace rocksdb {
 
@@ -15,7 +17,7 @@ class Statistics;
 class HistogramImpl;
 
 std::unique_ptr<RandomAccessFile> NewReadaheadRandomAccessFile(
-    std::unique_ptr<RandomAccessFile> file, size_t readahead_size);
+  std::unique_ptr<RandomAccessFile>&& file, size_t readahead_size);
 
 class SequentialFileReader {
  private:
@@ -24,6 +26,19 @@ class SequentialFileReader {
  public:
   explicit SequentialFileReader(std::unique_ptr<SequentialFile>&& _file)
       : file_(std::move(_file)) {}
+
+  SequentialFileReader(SequentialFileReader&& o) ROCKSDB_NOEXCEPT {
+    *this = std::move(o);
+  }
+
+  SequentialFileReader& operator=(SequentialFileReader&& o) ROCKSDB_NOEXCEPT {
+    file_ = std::move(o.file_);
+    return *this;
+  }
+
+  SequentialFileReader(SequentialFileReader&) = delete;
+  SequentialFileReader& operator=(SequentialFileReader&) = delete;
+
   Status Read(size_t n, Slice* result, char* scratch);
 
   Status Skip(uint64_t n);
@@ -34,10 +49,10 @@ class SequentialFileReader {
 class RandomAccessFileReader : public RandomAccessFile {
  private:
   std::unique_ptr<RandomAccessFile> file_;
-  Env* env_;
-  Statistics* stats_;
-  uint32_t hist_type_;
-  HistogramImpl* file_read_hist_;
+  Env*            env_;
+  Statistics*     stats_;
+  uint32_t        hist_type_;
+  HistogramImpl*  file_read_hist_;
 
  public:
   explicit RandomAccessFileReader(std::unique_ptr<RandomAccessFile>&& raf,
@@ -51,6 +66,22 @@ class RandomAccessFileReader : public RandomAccessFile {
         hist_type_(hist_type),
         file_read_hist_(file_read_hist) {}
 
+  RandomAccessFileReader(RandomAccessFileReader&& o) ROCKSDB_NOEXCEPT {
+    *this = std::move(o);
+  }
+
+  RandomAccessFileReader& operator=(RandomAccessFileReader&& o) ROCKSDB_NOEXCEPT{
+    file_ = std::move(o.file_);
+    env_ = std::move(o.env_);
+    stats_ = std::move(o.stats_);
+    hist_type_ = std::move(o.hist_type_);
+    file_read_hist_ = std::move(o.file_read_hist_);
+    return *this;
+  }
+
+  RandomAccessFileReader(const RandomAccessFileReader&) = delete;
+  RandomAccessFileReader& operator=(const RandomAccessFileReader&) = delete;
+
   Status Read(uint64_t offset, size_t n, Slice* result, char* scratch) const;
 
   RandomAccessFile* file() { return file_.get(); }
@@ -60,33 +91,47 @@ class RandomAccessFileReader : public RandomAccessFile {
 class WritableFileWriter {
  private:
   std::unique_ptr<WritableFile> writable_file_;
-  size_t cursize_;          // current size of cached data in buf_
-  size_t capacity_;         // max size of buf_
-  unique_ptr<char[]> buf_;  // a buffer to cache writes
-  uint64_t filesize_;
-  bool pending_sync_;
-  bool pending_fsync_;
-  bool direct_io_;
-  uint64_t last_sync_size_;
-  uint64_t bytes_per_sync_;
-  RateLimiter* rate_limiter_;
+  AlignedBuffer           buf_;
+  // Actually written data size can be used for truncate
+  // not counting padding data
+  uint64_t                filesize_;
+  // This is necessary when we use unbuffered access
+  // and writes must happen on aligned offsets
+  // so we need to go back and write that page again
+  uint64_t                next_write_offset_;
+  bool                    pending_sync_;
+  bool                    pending_fsync_;
+  const bool              direct_io_;
+  const bool              use_os_buffer_;
+  uint64_t                last_sync_size_;
+  uint64_t                bytes_per_sync_;
+  RateLimiter*            rate_limiter_;
 
  public:
-  explicit WritableFileWriter(std::unique_ptr<WritableFile>&& file,
-                              const EnvOptions& options)
+  WritableFileWriter(std::unique_ptr<WritableFile>&& file,
+                     const EnvOptions& options)
       : writable_file_(std::move(file)),
-        cursize_(0),
-        capacity_(65536),
-        buf_(new char[capacity_]),
+        buf_(),
         filesize_(0),
+        next_write_offset_(0),
         pending_sync_(false),
         pending_fsync_(false),
         direct_io_(writable_file_->UseDirectIO()),
+        use_os_buffer_(writable_file_->UseOSBuffer()),
         last_sync_size_(0),
         bytes_per_sync_(options.bytes_per_sync),
-        rate_limiter_(options.rate_limiter) {}
+        rate_limiter_(options.rate_limiter) {
+
+    buf_.Alignment(writable_file_->GetRequiredBufferAlignment());
+    buf_.AllocateNewBuffer(65536);
+  }
+
+  WritableFileWriter(const WritableFileWriter&) = delete;
+
+  WritableFileWriter& operator=(const WritableFileWriter&) = delete;
+
+  ~WritableFileWriter() { Close(); }
 
-  ~WritableFileWriter() { Flush(); }
   Status Append(const Slice& data);
 
   Status Flush();
@@ -109,8 +154,13 @@ class WritableFileWriter {
   WritableFile* writable_file() const { return writable_file_.get(); }
 
  private:
+  // Used when os buffering is OFF and we are writing
+  // DMA such as in Windows unbuffered mode
+  Status WriteUnbuffered();
+  // Normal write
+  Status WriteBuffered(const char* data, size_t size);
   Status RangeSync(off_t offset, off_t nbytes);
-  size_t RequestToken(size_t bytes);
+  size_t RequestToken(size_t bytes, bool align);
   Status SyncInternal(bool use_fsync);
 };
 }  // namespace rocksdb
diff --git a/util/file_reader_writer_test.cc b/util/file_reader_writer_test.cc
index 924c171b7..d1f0dcbec 100644
--- a/util/file_reader_writer_test.cc
+++ b/util/file_reader_writer_test.cc
@@ -24,6 +24,9 @@ TEST_F(WritableFileWriterTest, RangeSync) {
       size_ += data.size();
       return Status::OK();
     }
+    virtual Status Truncate(uint64_t size) override {
+      return Status::OK();
+    }
     Status Close() override {
       EXPECT_GE(size_, last_synced_ + kMb);
       EXPECT_LT(size_, last_synced_ + 2 * kMb);
diff --git a/util/memenv.cc b/util/memenv.cc
index 72fdbd242..573737023 100644
--- a/util/memenv.cc
+++ b/util/memenv.cc
@@ -232,7 +232,9 @@ class WritableFileImpl : public WritableFile {
   virtual Status Append(const Slice& data) override {
     return file_->Append(data);
   }
-
+  virtual Status Truncate(uint64_t size) override {
+    return Status::OK();
+  }
   virtual Status Close() override { return Status::OK(); }
   virtual Status Flush() override { return Status::OK(); }
   virtual Status Sync() override { return Status::OK(); }
diff --git a/util/mock_env.cc b/util/mock_env.cc
index 088675071..409e16e3a 100644
--- a/util/mock_env.cc
+++ b/util/mock_env.cc
@@ -250,7 +250,9 @@ class MockWritableFile : public WritableFile {
     }
     return Status::OK();
   }
-
+  virtual Status Truncate(uint64_t size) override {
+    return Status::OK();
+  }
   virtual Status Close() override { return file_->Fsync(); }
 
   virtual Status Flush() override { return Status::OK(); }
diff --git a/util/testutil.h b/util/testutil.h
index 67a2aafad..990a3ba81 100644
--- a/util/testutil.h
+++ b/util/testutil.h
@@ -184,6 +184,10 @@ class StringSink: public WritableFile {
 
   const std::string& contents() const { return contents_; }
 
+  virtual Status Truncate(uint64_t size) override {
+    contents_.resize(size);
+    return Status::OK();
+  }
   virtual Status Close() override { return Status::OK(); }
   virtual Status Flush() override {
     if (reader_contents_ != nullptr) {