diff --git a/extent_writer.cc b/extent_writer.cc
new file mode 100644
index 0000000..1ae565b
--- /dev/null
+++ b/extent_writer.cc
@@ -0,0 +1,67 @@
+// Copyright (c) 2009 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/extent_writer.h"
+#include <errno.h>
+#include <unistd.h>
+#include <algorithm>
+
+using std::min;
+
+namespace chromeos_update_engine {
+
+namespace {
+// Returns true on success.
+bool WriteAll(int fd, const void *buf, size_t count) {
+  const char* c_buf = reinterpret_cast<const char*>(buf);
+  ssize_t bytes_written = 0;
+  while (bytes_written < static_cast<ssize_t>(count)) {
+    ssize_t rc = write(fd, c_buf + bytes_written, count - bytes_written);
+    TEST_AND_RETURN_FALSE_ERRNO(rc >= 0);
+    bytes_written += rc;
+  }
+  return true;
+}
+}
+
+bool DirectExtentWriter::Write(const void* bytes, size_t count) {
+  if (count == 0)
+    return true;
+  const char* c_bytes = reinterpret_cast<const char*>(bytes);
+  size_t bytes_written = 0;
+  while (count - bytes_written > 0) {
+    TEST_AND_RETURN_FALSE(next_extent_index_ < extents_.size());
+    uint64 bytes_remaining_next_extent =
+        extents_[next_extent_index_].num_blocks() * block_size_ -
+        extent_bytes_written_;
+    CHECK_NE(bytes_remaining_next_extent, 0);
+    size_t bytes_to_write =
+        static_cast<size_t>(min(static_cast<uint64>(count - bytes_written),
+                                bytes_remaining_next_extent));
+    TEST_AND_RETURN_FALSE(bytes_to_write > 0);
+
+    if (extents_[next_extent_index_].start_block() != kSparseHole) {
+      const off64_t offset =
+          extents_[next_extent_index_].start_block() * block_size_ +
+          extent_bytes_written_;
+      TEST_AND_RETURN_FALSE_ERRNO(lseek64(fd_, offset, SEEK_SET) !=
+                                  static_cast<off64_t>(-1));
+      TEST_AND_RETURN_FALSE(
+          WriteAll(fd_, c_bytes + bytes_written, bytes_to_write));
+    }
+    bytes_written += bytes_to_write;
+    extent_bytes_written_ += bytes_to_write;
+    if (bytes_remaining_next_extent == bytes_to_write) {
+      // We filled this extent
+      CHECK_EQ(extent_bytes_written_,
+               extents_[next_extent_index_].num_blocks() * block_size_);
+      // move to next extent
+      extent_bytes_written_ = 0;
+      next_extent_index_++;
+    }
+  }
+  return true;
+}
+
+}  // namespace chromeos_update_engine