From 2cb0f4e5def56effc7f42e377768d61f5a8c5161 Mon Sep 17 00:00:00 2001
From: Sebastian Messmer <mail@smessmer.de>
Date: Thu, 5 Mar 2015 22:16:57 +0100
Subject: [PATCH] BlockStore only writes changed data back if it actually was
 changed

---
 implementations/ondisk/OnDiskBlock.cpp | 13 +++++++++----
 implementations/ondisk/OnDiskBlock.h   |  1 +
 implementations/testfake/FakeBlock.cpp |  8 ++++++--
 implementations/testfake/FakeBlock.h   |  1 +
 4 files changed, 17 insertions(+), 6 deletions(-)

diff --git a/implementations/ondisk/OnDiskBlock.cpp b/implementations/ondisk/OnDiskBlock.cpp
index 44911d3e..2ece694f 100644
--- a/implementations/ondisk/OnDiskBlock.cpp
+++ b/implementations/ondisk/OnDiskBlock.cpp
@@ -20,15 +20,15 @@ namespace blockstore {
 namespace ondisk {
 
 OnDiskBlock::OnDiskBlock(const Key &key, const bf::path &filepath, size_t size)
- : Block(key), _filepath(filepath), _data(size) {
+ : Block(key), _filepath(filepath), _data(size), _dataChanged(false) {
 }
 
 OnDiskBlock::OnDiskBlock(const Key &key, const bf::path &filepath, Data &&data)
- : Block(key), _filepath(filepath), _data(std::move(data)) {
+ : Block(key), _filepath(filepath), _data(std::move(data)), _dataChanged(false) {
 }
 
 OnDiskBlock::~OnDiskBlock() {
-  _storeToDisk();
+  flush();
 }
 
 const void *OnDiskBlock::data() const {
@@ -38,6 +38,7 @@ const void *OnDiskBlock::data() const {
 void OnDiskBlock::write(const void *source, uint64_t offset, uint64_t size) {
   assert(offset <= _data.size() && offset + size <= _data.size()); //Also check offset < _data->size() because of possible overflow in the addition
   std::memcpy((uint8_t*)_data.data()+offset, source, size);
+  _dataChanged = true;
 }
 
 size_t OnDiskBlock::size() const {
@@ -61,6 +62,7 @@ unique_ptr<OnDiskBlock> OnDiskBlock::LoadFromDisk(const bf::path &rootdir, const
 }
 
 unique_ptr<OnDiskBlock> OnDiskBlock::CreateOnDisk(const bf::path &rootdir, const Key &key, size_t size) {
+  //TODO Only writeback, if data was actually changed (Block::write() was called)
   auto filepath = rootdir / key.ToString();
   if (bf::exists(filepath)) {
     return nullptr;
@@ -87,7 +89,10 @@ void OnDiskBlock::_storeToDisk() const {
 }
 
 void OnDiskBlock::flush() {
-  _storeToDisk();
+  if (_dataChanged) {
+    _storeToDisk();
+    _dataChanged = false;
+  }
 }
 
 }
diff --git a/implementations/ondisk/OnDiskBlock.h b/implementations/ondisk/OnDiskBlock.h
index db0bf19d..101d0215 100644
--- a/implementations/ondisk/OnDiskBlock.h
+++ b/implementations/ondisk/OnDiskBlock.h
@@ -31,6 +31,7 @@ public:
 private:
   const boost::filesystem::path _filepath;
   Data _data;
+  bool _dataChanged;
 
   OnDiskBlock(const Key &key, const boost::filesystem::path &filepath, size_t size);
   OnDiskBlock(const Key &key, const boost::filesystem::path &filepath, Data &&data);
diff --git a/implementations/testfake/FakeBlock.cpp b/implementations/testfake/FakeBlock.cpp
index 65f48964..6bdc37f3 100644
--- a/implementations/testfake/FakeBlock.cpp
+++ b/implementations/testfake/FakeBlock.cpp
@@ -15,7 +15,7 @@ namespace blockstore {
 namespace testfake {
 
 FakeBlock::FakeBlock(FakeBlockStore *store, const Key &key, shared_ptr<Data> data)
- : Block(key), _store(store), _data(data) {
+ : Block(key), _store(store), _data(data), _dataChanged(false) {
 }
 
 FakeBlock::~FakeBlock() {
@@ -29,6 +29,7 @@ const void *FakeBlock::data() const {
 void FakeBlock::write(const void *source, uint64_t offset, uint64_t size) {
   assert(offset <= _data->size() && offset + size <= _data->size()); //Also check offset < _data->size() because of possible overflow in the addition
   std::memcpy((uint8_t*)_data->data()+offset, source, size);
+  _dataChanged = true;
 }
 
 size_t FakeBlock::size() const {
@@ -36,7 +37,10 @@ size_t FakeBlock::size() const {
 }
 
 void FakeBlock::flush() {
-  _store->updateData(key(), *_data);
+  if(_dataChanged) {
+    _store->updateData(key(), *_data);
+    _dataChanged = false;
+  }
 }
 
 }
diff --git a/implementations/testfake/FakeBlock.h b/implementations/testfake/FakeBlock.h
index 8a1ca87d..04842a12 100644
--- a/implementations/testfake/FakeBlock.h
+++ b/implementations/testfake/FakeBlock.h
@@ -26,6 +26,7 @@ public:
 private:
   FakeBlockStore *_store;
   std::shared_ptr<Data> _data;
+  bool _dataChanged;
 
   DISALLOW_COPY_AND_ASSIGN(FakeBlock);
 };