diff --git a/extent_writer_unittest.cc b/extent_writer_unittest.cc
new file mode 100644
index 0000000..a35ba26
--- /dev/null
+++ b/extent_writer_unittest.cc
@@ -0,0 +1,255 @@
+// 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 <sys/stat.h>
+#include <sys/types.h>
+#include <unistd.h>
+#include <algorithm>
+#include <string>
+#include <vector>
+#include <gtest/gtest.h>
+#include "update_engine/extent_writer.h"
+#include "update_engine/test_utils.h"
+#include "update_engine/utils.h"
+
+using std::min;
+using std::string;
+using std::vector;
+
+namespace chromeos_update_engine {
+
+COMPILE_ASSERT(sizeof(off_t) == 8, off_t_not_64_bit);
+
+namespace {
+const char kPathTemplate[] = "./ExtentWriterTest-file.XXXXXX";
+const size_t kBlockSize = 4096;
+}
+
+class ExtentWriterTest : public ::testing::Test {
+ protected:
+  virtual void SetUp() {
+    memcpy(path_, kPathTemplate, sizeof(kPathTemplate));
+    fd_ = mkstemp(path_);
+    ASSERT_GE(fd_, 0);
+  }
+  virtual void TearDown() {
+    close(fd_);
+    unlink(path_);
+  }
+  int fd() { return fd_; }
+  const char* path() { return path_; }
+  
+  // Writes data to an extent writer in 'chunk_size' chunks with
+  // the first chunk of size first_chunk_size. It calculates what the
+  // resultant file should look like and ensure that the extent writer
+  // wrote the file correctly.
+  void WriteAlignedExtents(size_t chunk_size, size_t first_chunk_size);
+  void TestZeroPad(bool aligned_size);
+ private:
+  int fd_;
+  char path_[sizeof(kPathTemplate)];
+};
+
+TEST_F(ExtentWriterTest, SimpleTest) {
+  vector<Extent> extents;
+  Extent extent;
+  extent.set_start_block(1);
+  extent.set_num_blocks(1);
+  extents.push_back(extent);
+  
+  const string bytes = "1234";
+
+  DirectExtentWriter direct_writer;
+  EXPECT_TRUE(direct_writer.Init(fd(), extents, kBlockSize));
+  EXPECT_TRUE(direct_writer.Write(bytes.data(), bytes.size()));
+  EXPECT_TRUE(direct_writer.End());
+  
+  struct stat stbuf;
+  EXPECT_EQ(0, fstat(fd(), &stbuf));
+  EXPECT_EQ(kBlockSize + bytes.size(), stbuf.st_size);
+  
+  vector<char> result_file;
+  EXPECT_TRUE(utils::ReadFile(path(), &result_file));
+  
+  vector<char> expected_file(kBlockSize);
+  expected_file.insert(expected_file.end(),
+                       bytes.data(), bytes.data() + bytes.size());
+  ExpectVectorsEq(expected_file, result_file);
+}
+
+TEST_F(ExtentWriterTest, ZeroLengthTest) {
+  vector<Extent> extents;
+  Extent extent;
+  extent.set_start_block(1);
+  extent.set_num_blocks(1);
+  extents.push_back(extent);
+
+  DirectExtentWriter direct_writer;
+  EXPECT_TRUE(direct_writer.Init(fd(), extents, kBlockSize));
+  EXPECT_TRUE(direct_writer.Write(NULL, 0));
+  EXPECT_TRUE(direct_writer.End());
+}
+
+TEST_F(ExtentWriterTest, OverflowExtentTest) {
+  WriteAlignedExtents(kBlockSize * 3, kBlockSize * 3);
+}
+
+TEST_F(ExtentWriterTest, UnalignedWriteTest) {
+  WriteAlignedExtents(7, 7);
+}
+
+TEST_F(ExtentWriterTest, LargeUnalignedWriteTest) {
+  WriteAlignedExtents(kBlockSize * 2, kBlockSize / 2);
+}
+
+void ExtentWriterTest::WriteAlignedExtents(size_t chunk_size,
+                                           size_t first_chunk_size) {
+  vector<Extent> extents;
+  Extent extent;
+  extent.set_start_block(1);
+  extent.set_num_blocks(1);
+  extents.push_back(extent);
+  extent.set_start_block(0);
+  extent.set_num_blocks(1);
+  extents.push_back(extent);
+  extent.set_start_block(2);
+  extent.set_num_blocks(1);
+  extents.push_back(extent);
+  
+  vector<char> data(kBlockSize * 3);
+  FillWithData(&data);
+  
+  DirectExtentWriter direct_writer;
+  EXPECT_TRUE(direct_writer.Init(fd(), extents, kBlockSize));
+  
+  size_t bytes_written = 0;
+  while (bytes_written < data.size()) {
+    size_t bytes_to_write = min(data.size() - bytes_written, chunk_size);
+    if (bytes_written == 0) {
+      bytes_to_write = min(data.size() - bytes_written, first_chunk_size);
+    }
+    EXPECT_TRUE(direct_writer.Write(&data[bytes_written], bytes_to_write));
+    bytes_written += bytes_to_write;
+  }
+  EXPECT_TRUE(direct_writer.End());
+  
+  struct stat stbuf;
+  EXPECT_EQ(0, fstat(fd(), &stbuf));
+  EXPECT_EQ(data.size(), stbuf.st_size);
+  
+  vector<char> result_file;
+  EXPECT_TRUE(utils::ReadFile(path(), &result_file));
+  
+  vector<char> expected_file;
+  expected_file.insert(expected_file.end(),
+                       data.begin() + kBlockSize,
+                       data.begin() + kBlockSize * 2);
+  expected_file.insert(expected_file.end(),
+                       data.begin(), data.begin() + kBlockSize);
+  expected_file.insert(expected_file.end(),
+                       data.begin() + kBlockSize * 2, data.end());
+  ExpectVectorsEq(expected_file, result_file);
+}
+
+TEST_F(ExtentWriterTest, ZeroPadNullTest) {
+  TestZeroPad(true);
+}
+
+TEST_F(ExtentWriterTest, ZeroPadFillTest) {
+  TestZeroPad(false);
+}
+
+void ExtentWriterTest::TestZeroPad(bool aligned_size) {
+  vector<Extent> extents;
+  Extent extent;
+  extent.set_start_block(1);
+  extent.set_num_blocks(1);
+  extents.push_back(extent);
+  extent.set_start_block(0);
+  extent.set_num_blocks(1);
+  extents.push_back(extent);
+  
+  vector<char> data(kBlockSize * 2);
+  FillWithData(&data);
+  
+  DirectExtentWriter direct_writer;
+  ZeroPadExtentWriter zero_pad_writer(&direct_writer);
+
+  EXPECT_TRUE(zero_pad_writer.Init(fd(), extents, kBlockSize));
+  size_t bytes_to_write = data.size();
+  const size_t missing_bytes = (aligned_size ? 0 : 9);
+  bytes_to_write -= missing_bytes;
+  lseek64(fd(), kBlockSize - missing_bytes, SEEK_SET);
+  EXPECT_EQ(3, write(fd(), "xxx", 3));
+  ASSERT_TRUE(zero_pad_writer.Write(&data[0], bytes_to_write));
+  EXPECT_TRUE(zero_pad_writer.End());
+  
+  struct stat stbuf;
+  EXPECT_EQ(0, fstat(fd(), &stbuf));
+  EXPECT_EQ(data.size(), stbuf.st_size);
+  
+  vector<char> result_file;
+  EXPECT_TRUE(utils::ReadFile(path(), &result_file));
+  
+  vector<char> expected_file;
+  expected_file.insert(expected_file.end(),
+                       data.begin() + kBlockSize,
+                       data.begin() + kBlockSize * 2);
+  expected_file.insert(expected_file.end(),
+                       data.begin(), data.begin() + kBlockSize);
+  if (missing_bytes) {
+    memset(&expected_file[kBlockSize - missing_bytes], 0, missing_bytes);
+  }
+
+  ExpectVectorsEq(expected_file, result_file);
+}
+
+TEST_F(ExtentWriterTest, SparseFileTest) {
+  vector<Extent> extents;
+  Extent extent;
+  extent.set_start_block(1);
+  extent.set_num_blocks(1);
+  extents.push_back(extent);
+  extent.set_start_block(kSparseHole);
+  extent.set_num_blocks(2);
+  extents.push_back(extent);
+  extent.set_start_block(0);
+  extent.set_num_blocks(1);
+  extents.push_back(extent);
+  const int block_count = 4;
+  const int on_disk_count = 2;
+
+  vector<char> data(17);
+  FillWithData(&data);
+
+  DirectExtentWriter direct_writer;
+  EXPECT_TRUE(direct_writer.Init(fd(), extents, kBlockSize));
+  
+  size_t bytes_written = 0;
+  while (bytes_written < (block_count * kBlockSize)) {
+    size_t bytes_to_write = min(block_count * kBlockSize - bytes_written,
+                                data.size());
+    EXPECT_TRUE(direct_writer.Write(&data[0], bytes_to_write));
+    bytes_written += bytes_to_write;
+  }
+  EXPECT_TRUE(direct_writer.End());
+  
+  // check file size, then data inside
+  ASSERT_EQ(2 * kBlockSize, FileSize(path()));
+  
+  vector<char> resultant_data;
+  EXPECT_TRUE(utils::ReadFile(path(), &resultant_data));
+  
+  // Create expected data
+  vector<char> expected_data(on_disk_count * kBlockSize);
+  vector<char> big(block_count * kBlockSize);
+  for (vector<char>::size_type i = 0; i < big.size(); i++) {
+    big[i] = data[i % data.size()];
+  }
+  memcpy(&expected_data[kBlockSize], &big[0], kBlockSize);
+  memcpy(&expected_data[0], &big[3 * kBlockSize], kBlockSize);
+  ExpectVectorsEq(expected_data, resultant_data);
+}
+
+}  // namespace chromeos_update_engine