Method Read

src/leveldb/util/env_flash.cc:233–276

    delete flash_file_;
  }
  Status Read(uint64_t offset, size_t n, Slice* result, char* scratch) const {
    bool use_flash = false;
    {
      MutexLock l(&mutex_);
      // evenry 30 seconds, check if flash file is identical to dfs file.
      // if so, try open flash file;
      // else, reschedule update it with a higher priority
      if (flash_file_ == NULL && !flash_file_is_checking_ &&
          flash_file_last_check_micros_ + flash_file_check_interval_micros_ <=
              Env::Default()->NowMicros()) {
        flash_file_is_checking_ = true;
        mutex_.Unlock();
        RandomAccessFile* tmp_file = NULL;
        if (flash_env_->FlashFileIdentical(fname_, fsize_)) {
          flash_env_->CacheEnv()->NewRandomAccessFile(local_fname_, &tmp_file, env_opt_);
        }
        mutex_.Lock();
        if (tmp_file != NULL) {
          flash_file_ = tmp_file;
          LOG(INFO) << "[env_flash] switch to local file: " << local_fname_.c_str();
        } else {
          flash_env_->ScheduleUpdateFlash(fname_, fsize_, read_dfs_count_);
          read_dfs_count_ = 0;
        }
        flash_file_is_checking_ = false;
        flash_file_last_check_micros_ = Env::Default()->NowMicros();
      }
      if (flash_file_ != NULL) {
        use_flash = true;
      } else {
        ++read_dfs_count_;
      }
    }

    if (use_flash) {
      Status read_status = flash_file_->Read(offset, n, result, scratch);
      if (read_status.ok()) {
        ssd_read_counter.Inc();
        ssd_read_size_counter.Add(result->size());
      }
      return read_status;
    }
    return dfs_file_->Read(offset, n, result, scratch);
  }

  size_t GetRequiredBufferAlignment() const { return logical_sector_size_; }