Add GetTemperature on existing files (#9498)

Summary:
For tiered storage

Pull Request resolved: https://github.com/facebook/rocksdb/pull/9498

Test Plan: Just API placeholders for now

Reviewed By: jay-zhuang

Differential Revision: D33993094

Pulled By: pdillinger

fbshipit-source-id: 3cf19a450c7232e05306e94018559b26e9fd35db

include/rocksdb/file_system.h

@@ -701,6 +701,13 @@ class FSSequentialFile {
     return IOStatus::NotSupported("PositionedRead");
   }
 
+  // EXPERIMENTAL
+  // When available, returns the actual temperature for the file. This is
+  // useful in case some outside process moves a file from one tier to another,
+  // though the temperature is generally expected not to change while a file is
+  // open.
+  virtual Temperature GetTemperature() const { return Temperature::kUnknown; }
+
   // If you're adding methods here, remember to add them to
   // SequentialFileWrapper too.
 };
@@ -818,6 +825,13 @@ class FSRandomAccessFile {
     return IOStatus::NotSupported("InvalidateCache not supported.");
   }
 
+  // EXPERIMENTAL
+  // When available, returns the actual temperature for the file. This is
+  // useful in case some outside process moves a file from one tier to another,
+  // though the temperature is generally expected not to change while a file is
+  // open.
+  virtual Temperature GetTemperature() const { return Temperature::kUnknown; }
+
   // If you're adding methods here, remember to add them to
   // RandomAccessFileWrapper too.
 };
@@ -1102,6 +1116,13 @@ class FSRandomRWFile {
 
   virtual IOStatus Close(const IOOptions& options, IODebugContext* dbg) = 0;
 
+  // EXPERIMENTAL
+  // When available, returns the actual temperature for the file. This is
+  // useful in case some outside process moves a file from one tier to another,
+  // though the temperature is generally expected not to change while a file is
+  // open.
+  virtual Temperature GetTemperature() const { return Temperature::kUnknown; }
+
   // If you're adding methods here, remember to add them to
   // RandomRWFileWrapper too.
 
@@ -1418,6 +1439,9 @@ class FSSequentialFileWrapper : public FSSequentialFile {
                           IODebugContext* dbg) override {
     return target_->PositionedRead(offset, n, options, result, scratch, dbg);
   }
+  Temperature GetTemperature() const override {
+    return target_->GetTemperature();
+  }
 
  private:
   FSSequentialFile* target_;
@@ -1466,6 +1490,9 @@ class FSRandomAccessFileWrapper : public FSRandomAccessFile {
   IOStatus InvalidateCache(size_t offset, size_t length) override {
     return target_->InvalidateCache(offset, length);
   }
+  Temperature GetTemperature() const override {
+    return target_->GetTemperature();
+  }
 
  private:
   std::unique_ptr<FSRandomAccessFile> guard_;
@@ -1629,6 +1656,9 @@ class FSRandomRWFileWrapper : public FSRandomRWFile {
   IOStatus Close(const IOOptions& options, IODebugContext* dbg) override {
     return target_->Close(options, dbg);
   }
+  Temperature GetTemperature() const override {
+    return target_->GetTemperature();
+  }
 
  private:
   FSRandomRWFile* target_;