vmstat spend writing/reading explaination

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http://serverfault.com – I ran vmstat -D for 1s interval and take the different of milli reading/writing between two interval. Then I start copy a file to test. bytes write per secs is 23.9M/s, and mili writing is 7244. As vmstat documentation, mili writing is time spend writing in miliseconds, but why the disk spend 7.3s to write in just 1s ? (HowTos)