diff --git a/split_file_writer.cc b/split_file_writer.cc
new file mode 100644
index 0000000..e868947
--- /dev/null
+++ b/split_file_writer.cc
@@ -0,0 +1,113 @@
+// Copyright (c) 2010 The Chromium OS Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#include "update_engine/split_file_writer.h"
+#include <algorithm>
+
+using std::min;
+
+namespace chromeos_update_engine {
+
+int SplitFileWriter::Open(const char* path, int flags, mode_t mode) {
+  int first_result = first_file_writer_->Open(first_path_,
+  first_flags_,
+  first_mode_);
+  if (first_result < 0) {
+    LOG(ERROR) << "Error opening first file " << first_path_;
+    return first_result;
+  }
+  int second_result = second_file_writer_->Open(path, flags, mode);
+  if (second_result < 0) {
+    LOG(ERROR) << "Error opening second file " << path;
+    first_file_writer_->Close();
+    return second_result;
+  }
+  return second_result;
+}
+
+namespace {
+ssize_t PerformWrite(FileWriter* writer, const void* bytes, size_t count) {
+  int rc = writer->Write(bytes, count);
+  if (rc < 0) {
+    LOG(ERROR) << "Write failed to file.";
+    return rc;
+  }
+  if (rc != static_cast<int>(count)) {
+    LOG(ERROR) << "Not all bytes successfully written to file.";
+    return -EIO;
+  }
+  return rc;
+}
+}
+
+ssize_t SplitFileWriter::Write(const void* bytes, size_t count) {
+  const size_t original_count = count;
+  
+  // This first block is trying to read the first sizeof(uint64_t)
+  // bytes, which are the number of bytes that should be written
+  // to the first FileWriter.
+  if (bytes_received_ < static_cast<off_t>(sizeof(uint64_t))) {
+    // Write more to the initial buffer
+    size_t bytes_to_copy = min(count,
+                               sizeof(first_length_buf_) - bytes_received_);
+    memcpy(&first_length_buf_[bytes_received_], bytes, bytes_to_copy);
+    bytes_received_ += bytes_to_copy;
+    count -= bytes_to_copy;
+    bytes = static_cast<const void*>(
+        static_cast<const char*>(bytes) + bytes_to_copy);
+
+    // See if we have all we need
+    if (bytes_received_ == sizeof(first_length_buf_)) {
+      // Parse first number
+      uint64_t big_endian_first_length;
+      memcpy(&big_endian_first_length, first_length_buf_,
+             sizeof(big_endian_first_length));
+      first_length_ = be64toh(big_endian_first_length);
+    }
+    if (count == 0)
+      return original_count;
+  }
+  CHECK_GE(bytes_received_, static_cast<off_t>(sizeof(uint64_t)));
+
+  // This block of code is writing to the first FileWriter.
+  if (bytes_received_ - static_cast<off_t>(sizeof(uint64_t)) < first_length_) {
+    // Write to first FileWriter
+    size_t bytes_to_write = min(
+        first_length_ -
+        (bytes_received_ - static_cast<off_t>(sizeof(uint64_t))),
+        static_cast<off_t>(count));
+      
+    int rc = PerformWrite(first_file_writer_, bytes, bytes_to_write);
+    if (rc != static_cast<int>(bytes_to_write))
+      return rc;
+    
+    bytes_received_ += bytes_to_write;
+    count -= bytes_to_write;
+    bytes = static_cast<const void*>(
+        static_cast<const char*>(bytes) + bytes_to_write);
+    if (count == 0)
+      return original_count;
+  }
+
+  CHECK_GE(static_cast<off_t>(bytes_received_),
+           first_length_ + static_cast<off_t>(sizeof(uint64_t)));
+  // Write to second FileWriter
+  int rc = PerformWrite(second_file_writer_, bytes, count);
+  if (rc != static_cast<int>(count))
+    return rc;
+  return original_count;
+}
+
+int SplitFileWriter::Close() {
+  int first_result = first_file_writer_->Close();
+  if (first_result < 0)
+    LOG(ERROR) << "Error Close()ing first file.";
+  int second_result = second_file_writer_->Close();
+  if (second_result < 0)
+    LOG(ERROR) << "Error Close()ing second file.";
+  // Return error if either had returned error.
+  return second_result < 0 ? second_result : first_result;
+}
+
+}  // namespace chromeos_update_engine