iostat: await vs. svctm

Question

In the iostat manpage I have found these two similar columns:

await
    The average time (in milliseconds) for I/O requests issued to the device to be  served.  This
    includes the time spent by the requests in queue and the time spent servicing them.

svctm
    The  average  service time (in milliseconds) for I/O requests that were issued to the device.
    Warning! Do not trust this field any more.  This field will be removed in  a  future  sysstat
    version.

Are these columns meant to represent the same thing? I seem that sometimes they agree, but sometimes not:

avg-cpu:  %user   %nice %system %iowait  %steal   %idle
           4.44    0.02    1.00    0.36    0.00   94.19

Device:         rrqm/s   wrqm/s     r/s     w/s    rkB/s    wkB/s avgrq-sz avgqu-sz   await r_await w_await  svctm  %util
sda               0.07     0.96    0.28    1.28     8.98    47.45    72.13     0.02   11.36   11.49   11.34   5.71   0.89

avg-cpu:  %user   %nice %system %iowait  %steal   %idle
           8.00    0.00    2.50    2.50    0.00   87.00

Device:         rrqm/s   wrqm/s     r/s     w/s    rkB/s    wkB/s avgrq-sz avgqu-sz   await r_await w_await  svctm  %util
sda               0.00     9.00    2.00    6.00    12.00    68.00    20.00     0.05    6.00    2.00    7.33   6.00   4.80

avg-cpu:  %user   %nice %system %iowait  %steal   %idle
           4.57    0.00    0.51    0.00    0.00   94.92

Device:         rrqm/s   wrqm/s     r/s     w/s    rkB/s    wkB/s avgrq-sz avgqu-sz   await r_await w_await  svctm  %util
sda               0.00     0.00    0.00    0.00     0.00     0.00     0.00     0.00    0.00    0.00    0.00   0.00   0.00

avg-cpu:  %user   %nice %system %iowait  %steal   %idle
          13.93    0.00    1.99    1.49    0.00   82.59

Device:         rrqm/s   wrqm/s     r/s     w/s    rkB/s    wkB/s avgrq-sz avgqu-sz   await r_await w_await  svctm  %util
sda               0.00    29.00    0.00    4.00     0.00   132.00    66.00     0.03    7.00    0.00    7.00   7.00   2.80

Other than the obvious warning that svctm is depreciated, what is the difference between these two columns?

Solution

On linux iostat, the await column (average wait) is showing the average time spent by an I/O request computed from its very beginning toward its end.

The svctm column (service time) should display the average time spent servicing the request, i.e. the time spent "outside" the OS. It should be equal or smaller than the previous one as the request might have lost time waiting in a queue if the device is already busy and doesn't accept more concurrent requests.

Unlike most if not all other Unix / Unix like implementations, the Linux kernel doesn't measure the actual service time so iostat on that platform is trying to derive it from existing statistics but fails as this just cannot be done outside trivial use cases.

See this blog and the interesting discussions that follows for details.

Comments

[dotancohen]

Thanks. I now understand that await = svctm + however_long_in_queue, just as the fine manual states!