# Periodically Print I/O Block Time

This section describes how to track the amount of time each block I/O requests spends waiting for completion. This is useful in determining whether there are too many outstanding block I/O operations at any given time.

ioblktime.stp

computes the average waiting time for block I/O per device, and prints a list every 10 seconds. As always, you can revise this refresh rate by editing the specified value in `probe timer.s(10), end {`.

In some cases, there can be too many outstanding block I/O operations, at which point the script can exceed the default number of `MAXMAPENTRIES`. `MAXMAPENTRIES` is the maximum number of rows in an array if the array size is not specified explicitly when declared. If the script exceeds the default `MAXMAPENTRIES` value of 2048, run the script again with the `stap` option `-DMAXMAPENTRIES=10000`.

Sample Output

        device  rw total (us)      count   avg (us)
           sda   W       9659          6       1609
          dm-0   W      20278          6       3379
          dm-0   R      20524          5       4104
           sda   R      19277          5       3855

displays the device name, operations performed (`rw`), total wait time of all operations (`total(us)`), number of operations (`count`), and average wait time for all those operations (`avg (us)`). The times tallied by the script are in microseconds.

script examples monitoring I/O block time
examples of SystemTap scripts monitoring I/O block time
monitoring I/O block time examples of SystemTap scripts
I/O block time, monitoring examples of SystemTap scripts
printing I/O block time (periodically) examples of SystemTap scripts