[GitHub] [arrow] pitrou commented on a diff in pull request #13640: ARROW-17114: [Python][C++] add O_DIRECT support to writes

[GitBox <gi...@apache.org>]

pitrou commented on code in PR #13640:
URL: https://github.com/apache/arrow/pull/13640#discussion_r924195380


##########
cpp/src/arrow/io/file.cc:
##########
@@ -378,6 +378,77 @@ Status FileOutputStream::Write(const void* data, int64_t length) {
 
 int FileOutputStream::file_descriptor() const { return impl_->fd(); }
 
+// ----------------------------------------------------------------------
+// DirectFileOutputStream, change the Open, Write and Close methods from FileOutputStream
+// Uses DirectIO for writes. Will only write out things in 4096 byte blocks. Buffers leftover bytes
+// in an internal data structure, which will be padded to 4096 bytes and flushed upon call to close.
+
+class DirectFileOutputStream::DirectFileOutputStreamImpl : public OSFile {
+ public:
+  Status Open(const std::string& path, bool append) {
+    const bool truncate = !append;
+    return OpenWritable(path, truncate, append, true /* write_only */, true);
+  }
+  Status Open(int fd) { return OpenWritable(fd); }
+};
+
+DirectFileOutputStream::DirectFileOutputStream() { 
+  uintptr_t mask = (uintptr_t)(4095);
+  uint8_t *mem = static_cast<uint8_t *>(malloc(4096 + 4095));
+  cached_data = reinterpret_cast<uint8_t *>( reinterpret_cast<uintptr_t>(mem+4095) & ~(mask));

Review Comment:
   Indeed, the `open` man page says this about `O_DIRECT`:
   > Try to minimize cache effects of the I/O to and from this file.  In general this will degrade performance, but it is useful in special situations, such as when applications do their own caching.
   
   @marsupialtail Are you seeing any actual benefits from this? In which situations?



-- 
This is an automated message from the Apache Git Service.
To respond to the message, please log on to GitHub and use the
URL above to go to the specific comment.

To unsubscribe, e-mail: github-unsubscribe@arrow.apache.org

For queries about this service, please contact Infrastructure at:
users@infra.apache.org