def disk_usage
      size_ptr = FFI::MemoryPointer.new(:uint64)
      rc = Native.sd_journal_get_usage(@ptr, size_ptr)

      raise JournalError, rc if rc < 0
      size_ptr.read_uint64
    end
def disk_usage
      size_ptr = FFI::MemoryPointer.new(:uint64)
      rc = Native.sd_journal_get_usage(@ptr, size_ptr)

      raise JournalError, rc if rc < 0
      size_ptr.read_uint64
    end
Get the number of bytes the Journal is currently using on disk.
 If {Systemd::Journal::Flags::LOCAL_ONLY} was passed when opening the
 journal,  this value will only reflect the size of journal files of the
 local host, otherwise of all hosts.
 @return [Integer] size in bytes