+ int toflush = written_since_last_sync > 2*1024*1024 ?
+ 2*1024*1024 : written_since_last_sync &~ 4095; // write max 2MB at once
+ dest_pos = lseek(m_fd_dest, 0, SEEK_CUR);
+ dest_pos -= toflush;
+ posix_fadvise(m_fd_dest, dest_pos, toflush, POSIX_FADV_DONTNEED);
+ written_since_last_sync -= toflush;