It will take 5 seconds for iostat to collect the data, it will then be displayed in watch, and 0.1 seconds later watch will trigger the iostat command again. Because the -y switch was used this will only present a single screen of data. In this case capture 5 seconds of statistics once (the 1). The 5 1 are the iostat interval parameters.Without this the result would be a display of the statistics since boot updating at the interval of the watch command. The -y switch omits the first screen of "since boot" statistics from the interval display.This is useful since the earlier omission of the watch header removed the time display that would have been there. The second -t switch tells iostat to include the time of the refresh in the statistics.The -d tells iostat to only display device utilization, relevant since the question was concerning disk activity.The -p sda,sdc tells iostat to only display stats for these devices.It will run the command 0.1 seconds after the prior instance completes. This is the smallest interval for watch ( procps-ng 3.3.9) but don't worry, it isn't actually running the command every 0.1 seconds. The -n 0.1 tells watch to run the following command every 0.1 seconds.This is to avoid confusion that otherwise the header will include "Every 0.1s" which does not represent the snapshot of data. The first -t tells watch to omit the header.To break down the options and parameters. This gives a refresh every 5.1 seconds of activity statistics for the last 5 seconds.
For example: watch -t -n 0.1 iostat -p sda,sdc -d -t -y 5 1 I've found this most effective when combined with the watch command and indicating to only collect for a single interval of statistics. Include the -y option to omit the first display of statistics since boot but understand that the command will appear idle for the specified interval while the system collects the first snapshot. Example, iostat 10 will first show the "since boot" values then continue to add the last 10 seconds of statistics to the output, every 10 seconds. Use of the interval parameter will append the results for only the last interval to the output. Using iostat from the sysstat package provides a single snapshot of results since startup.
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