Remove some more dead code around syncing blob files (#7138)

Summary:
Periodic syncing of blob files is handled by a lower layer, namely by
`WritableFileWriter`; the `NeedsFsync` method of `BlobFile` and the
`last_fsync_` member variable are actually unused and thus can be
removed. See also https://github.com/facebook/rocksdb/pull/7125 .

Pull Request resolved: https://github.com/facebook/rocksdb/pull/7138

Test Plan: `make check`

Reviewed By: zhichao-cao

Differential Revision: D22562981

Pulled By: ltamasi

fbshipit-source-id: c235aad94a7c27120528c9ec270a7a5b9154e49f

utilities/blob_db/blob_file.cc

@@ -103,12 +103,6 @@ void BlobFile::MarkObsolete(SequenceNumber sequence) {
   obsolete_.store(true);
 }
 
-bool BlobFile::NeedsFsync(bool hard, uint64_t bytes_per_sync) const {
-  assert(last_fsync_ <= file_size_);
-  return (hard) ? file_size_ > last_fsync_
-                : (file_size_ - last_fsync_) >= bytes_per_sync;
-}
-
 Status BlobFile::WriteFooterAndCloseLocked(SequenceNumber sequence) {
   BlobLogFooter footer;
   footer.blob_count = blob_count_;
@@ -160,8 +154,6 @@ Status BlobFile::ReadFooter(BlobLogFooter* bf) {
 }
 
 Status BlobFile::SetFromFooterLocked(const BlobLogFooter& footer) {
-  // assume that file has been fully fsync'd
-  last_fsync_.store(file_size_);
   blob_count_ = footer.blob_count;
   expiration_range_ = footer.expiration_range;
   closed_ = true;
@@ -172,7 +164,6 @@ Status BlobFile::Fsync() {
   Status s;
   if (log_writer_.get()) {
     s = log_writer_->Sync();
-    last_fsync_.store(file_size_.load());
   }
   return s;
 }

utilities/blob_db/blob_file.h

@@ -98,9 +98,6 @@ class BlobFile {
   // time when the random access reader was last created.
   std::atomic<std::int64_t> last_access_{-1};
 
-  // last time file was fsync'd/fdatasyncd
-  std::atomic<uint64_t> last_fsync_{0};
-
   bool header_valid_{false};
 
   bool footer_valid_{false};
@@ -184,9 +181,6 @@ class BlobFile {
     return obsolete_sequence_;
   }
 
-  // we will assume this is atomic
-  bool NeedsFsync(bool hard, uint64_t bytes_per_sync) const;
-
   Status Fsync();
 
   uint64_t GetFileSize() const {