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ntcong
Asked: 2013-06-25 23:27:29 +0800 CST

vmstat spend writing/reading explaination

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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 ?

linux

2 Answers

  1. Clearly a dodgy miscalculation, or you have multiple disks, or multiple files were written over that period, or cached. I would suggest not relying on this stat.

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  2. Best Answer

    As stated on iostat documentation:

    Field 4 -- # of milliseconds spent reading

      This is the total number of milliseconds spent by all reads (as
  measured from __make_request() to end_that_request_last()).

    So this is the total number of ms by all reads, there's many read in one sec, that's why the total reading in ms is big

    In my case:

    "read_count": 340,
    "read_delay": 1436,
    "read_per_secs": "85.9M",

    So in average each read requires 4ms to complete, which seems to be correct.

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