This read-only 64-bit counter tracks the total number of bytes written to this disk device since the host last booted, functioning as the wrap-resistant counterpart to the 32-bit diskIONWritten. Admins use it for the same reason as diskIONReadX, to get accurate write-throughput figures on busy disks without worrying about 32-bit counter wraparound between polls. A monitoring system tracking a high-write database server's disk would use diskIONWrittenX rather than diskIONWritten to reliably calculate write throughput over time.