This read-only 32-bit counter tracks the total number of bytes written to this disk device since the host last booted. Admins compare it against diskIONRead to understand whether a disk's workload is read-heavy or write-heavy, useful for diagnosing I/O bottlenecks. A disk with diskIONWritten growing much faster than diskIONRead would indicate a write-intensive workload, such as active logging or a database with heavy insert traffic.