AU: Speed up updates by using buffered writes.

download_action.cc

 // 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/download_action.h"
 #include <errno.h>
 #include <algorithm>
 #include <string>
 #include <vector>
 #include <glib.h>
 #include "update_engine/action_pipe.h"
 #include "update_engine/subprocess.h"
 
 using std::min;
 using std::string;
 using std::vector;
 
 namespace chromeos_update_engine {
 
+// Use a buffer to reduce the number of IOPS on SSD devices.
+const size_t kFileWriterBufferSize = 128 * 1024;  // 128 KiB
+
 DownloadAction::DownloadAction(HttpFetcher* http_fetcher)
     : writer_(NULL),
       http_fetcher_(http_fetcher),
       delegate_(NULL),
       bytes_received_(0) {}
 
 DownloadAction::~DownloadAction() {}
 
 void DownloadAction::PerformAction() {
   http_fetcher_->set_delegate(this);
 
   // Get the InstallPlan and read it
   CHECK(HasInputObject());
   install_plan_ = GetInputObject();
   bytes_received_ = 0;
 
   install_plan_.Dump();
 
   if (writer_) {
     LOG(INFO) << "Using writer for test.";
   } else {
     if (install_plan_.is_full_update) {
       kernel_file_writer_.reset(new DirectFileWriter);
       rootfs_file_writer_.reset(new DirectFileWriter);
-      split_file_writer_.reset(new SplitFileWriter(kernel_file_writer_.get(),
-                                                   rootfs_file_writer_.get()));
+      kernel_buffered_file_writer_.reset(
+          new BufferedFileWriter(kernel_file_writer_.get(),
+                                 kFileWriterBufferSize));
+      rootfs_buffered_file_writer_.reset(
+          new BufferedFileWriter(rootfs_file_writer_.get(),
+                                 kFileWriterBufferSize));
+      split_file_writer_.reset(
+          new SplitFileWriter(kernel_buffered_file_writer_.get(),
+                              rootfs_buffered_file_writer_.get()));
       split_file_writer_->SetFirstOpenArgs(
           install_plan_.kernel_install_path.c_str(),
           O_WRONLY | O_CREAT | O_TRUNC | O_LARGEFILE,
           0644);
       decompressing_file_writer_.reset(
           new GzipDecompressingFileWriter(split_file_writer_.get()));
       writer_ = decompressing_file_writer_.get();
     } else {
       delta_performer_.reset(new DeltaPerformer);
       writer_ = delta_performer_.get();
@@ skipping 84 matching lines @@
 
   FlushLinuxCaches();
 
   // Write the path to the output pipe if we're successful.
   if (code == kActionCodeSuccess && HasOutputPipe())
     SetOutputObject(GetInputObject());
   processor_->ActionComplete(this, code);
 }
 
 };  // namespace {}