Disable onboard cache for compaction output

Summary:
FILE_FLAG_WRITE_THROUGH is for disabling device on-board cache in windows API, which should be disabled if user doesn't need system cache.
There was a perf issue related with this, we found during memtable flush, the high percentile latency jumps significantly. During profiling, we found those high latency (P99.9) read requests got queue-jumped by write requests from memtable flush and takes 80ms or even more time to wait, even when SSD overall IO throughput is relatively low.

After enabling FILE_FLAG_WRITE_THROUGH, we rerun the test found high percentile latency drops a lot without observable impact on writes.

Scenario 1: 40MB/s + 40MB/s  R/W compaction throughput

 Original | FILE_FLAG_WRITE_THROUGH | Percentage reduction
---------------------------------------------------------------
P99.9 | 56.897 ms | 35.593 ms | -37.4%
P99 | 3.905 ms | 3.896 ms | -2.8%

Scenario 2:  14MB/s + 14MB/s R/W compaction throughput, cohosted with 100+ other rocksdb instances have manually triggered memtable flush operations (memtable is tiny), creating a lot of randomized the small file writes operations during test.

Original | FILE_FLAG_WRITE_THROUGH | Percentage reduction
---------------------------------------------------------------
P99.9 | 86.227   ms | 50.436 ms | -41.5%
P99 | 8.415   ms | 3.356 ms | -60.1%
Closes facebook#3225

Differential Revision: D6624174

Pulled By: miasantreble

fbshipit-source-id: 321b86aee9d74470840c70e5d0d4fa9880660a91

port/win/env_win.cc

@@ -253,7 +253,7 @@ Status WinEnvIO::OpenWritableFile(const std::string& fname,
   DWORD fileFlags = FILE_ATTRIBUTE_NORMAL;
 
   if (local_options.use_direct_writes && !local_options.use_mmap_writes) {
-    fileFlags = FILE_FLAG_NO_BUFFERING;
+    fileFlags = FILE_FLAG_NO_BUFFERING | FILE_FLAG_WRITE_THROUGH;
   }
 
   // Desired access. We are want to write only here but if we want to memory

port/win/io_win.cc

@@ -880,7 +880,7 @@ inline
 Status WinWritableImpl::SyncImpl() {
   Status s;
   // Calls flush buffers
-  if (fsync(file_data_->GetFileHandle()) < 0) {
+  if (!file_data_->use_direct_io() && fsync(file_data_->GetFileHandle()) < 0) {
     auto lastError = GetLastError();
     s = IOErrorFromWindowsError(
         "fsync failed at Sync() for: " + file_data_->GetName(), lastError);
@@ -961,7 +961,11 @@ Status WinWritableFile::Sync() {
   return SyncImpl();
 }
 
-Status WinWritableFile::Fsync() { return SyncImpl(); }
+Status WinWritableFile::Fsync() {
+  return SyncImpl();
+}
+
+bool WinWritableFile::IsSyncThreadSafe() const { return true; }
 
 uint64_t WinWritableFile::GetFileSize() {
   return GetFileNextWriteOffset();

port/win/io_win.h

@@ -368,6 +368,8 @@ class WinWritableFile : private WinFileData,
 
   virtual Status Fsync() override;
 
+  virtual bool IsSyncThreadSafe() const override;
+
   // Indicates if the class makes use of direct I/O
   // Use PositionedAppend
   virtual bool use_direct_io() const override;