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I'd like to watch disk activity on my USB external hard drive. I know that I can use iotop to monitor disk I/O for each running process, but is there a way to get a measure per filesystem?

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3 Answers 3

up vote 17 down vote accepted

I'm not skilled in this area, but iostat comes to mind. You can install it with the sysstat package. Good luck!

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example usage: iostat -d 10 /dev/sda will give you the io utilization in 10 second intervals of /dev/sda. I'd almost use watch iostat -d /dev/sda over its interval option. Edit: beat me to it while I was typing :) – aperson Sep 3 '10 at 21:24
I just read that the first set of values reported are statistics from what has occurred since system startup, so it turns out not to make much sense to use watch. Continuous reporting with something like iostat -dk 10 returns more meaningful numbers. – ændrük Sep 3 '10 at 21:41
Hmm... I just don't like that it fills my entire scrollback. – aperson Sep 3 '10 at 22:08

dstat is better that iostat for monitoring strictly disk activity.

I am running the following command while moving files from one harddrive to another

dstat -D sda,sdc

for more info, have a look at this page

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Would like to add in the comments of Leon Nardella's answer on Sep 3 '10 at 20:47, but lack the rep. (And after completing this I find it is lengthy enough that it should probably be its own answer anyway.) So...

Using iostat from the sysstat package provides a single snapshot of results since startup. Use of the interval parameter will append the results for only the last interval to the output. 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. 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.

I've found this most effective when combined with the watch command and indicating to only collect for a single interval of statistics. For example: watch -t -n 0.1 iostat -d -t -y 5 1 gives a refresh every 5.1 seconds of activity statistics for the last 5 seconds. To break down the options and parameters...

  • The first -t tells watch to omit the header. This is to avoid confusion that otherwise the header will include "Every 0.1s" which does not represent the snapshot of data.
  • The -n 0.1 tells watch to run the following command every 0.1 seconds. 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. It will run the command 0.1 seconds after the prior instance completes.
  • The -d tells iostat to only display device utilization, relevant since the question was concerning disk activity. Alternately to monitor a specific disk the device name could be inserted prior to the interval parameters.
  • The second -t switch tells iostat to include the time of the refresh in the statistics. This is useful since the earlier omission of the watch header removed the time display that would have been there.
  • The -y switch omits the first screen of "since boot" statistics from the interval display. Without this the result would be a display of the statistics since boot updating at the interval of the watch command.
  • The 5 1 are the iostat interval parameters. In this case capture 5 seconds of statistics once (the 1). Because the -y switch was used this will only present a single screen of data.

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. 5 seconds later the new data will replace the old, watch will wait 0.1 seconds, wash, rinse, repeat...

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