This counter tracks the cumulative megabytes written to this specific logical unit. A LUN whose write-megabytes counter suddenly grows far faster than its normal baseline can indicate a runaway log file, an unplanned bulk restore, or something writing heavily to that one volume that should not be. An increase of 2,000 in an hour on a LUN that usually sees about 50 megabytes an hour of writes is a strong signal something abnormal is writing to that particular disk.