iostat -x -g: group %util has division problem, so keeps shrinking with each report

iostat.c を読む: write_stats() は各レポートでデバイスごとにグループの dev_tp を 1 回インクリメントし、write_plain_ext_stat() は dev_in_grp = dev_tp - T_GROUP を %util の除数として導出するため、除数は間隔ごとに増加する。報告者の 8 行のパッチは、グループ統計がゼロにされたときにグループの dev_tp を T_GROUP にリセットし、h == hl でガードされるので、--pretty のマルチパス出力は正しいままになる。ビルド (./configure --disable-nls --disable-sensors && make) と、負荷をかけた状態での iostat -y -x -m -g 10 の実行によって検証する: グループの %util は、最初のレポートだけでなく、後続のすべてのレポートで sum(disk %util)/4 と等しくなければならない。

Hello,
I have found an issue under -g Group Mode where the summary total of util% doesn't add up correctly, due to how it calculates the average. Below, I'll show the commands exhibiting the problem, and then the commands showing the fixed output after compiling with the attached patch file.

Full disclosure: I am not a C developer, and I both diagnosed this issue and created the patch using claude. I did compile and test the patch on my own machine however, and it fixed the issue for me.

System info

• iostat -V: sysstat version 12.7.9
• Linux 7.2.8-200.fc44.x86_64 (Fedora 44)
• Also reproduced on current master (da7ff231, built from source).

Steps to reproduce

1. run a light workload with fio to stimulate some disk activity: fio --name=light --filename=./fiotest.tmp --size=256m --rw=randrw --bs=64k --rate=1m,1m --ioengine=psync --direct=1 --time_based --runtime=45
2. watch with iostat in another terminal: iostat -y -x -m -g home_disks nvme0n1 nvme2n1 nvme3n1 nvme5n1 10

Expected results
%util summary line is the average of its member disks' utilization, ie the sum/num disks

Actual results
Here is my output from the Steps to reproduce commands, I let iostat generate 3 reports with 10s interval while fio was running:

Linux 7.2.8-200.fc44.x86_64 (server.home)      10/04/2026      _x86_64_        (48 CPU)

avg-cpu:  %user   %nice %system %iowait  %steal   %idle
           2.51    0.00    0.93    0.12    0.00   96.43

Device            r/s     rMB/s   rrqm/s  %rrqm r_await rareq-sz     w/s     wMB/s   wrqm/s  %wrqm w_await wareq-sz     d/s     dMB/s   drqm/s  %drqm d_await dareq-sz     f/s f_await  aqu-sz  %util
nvme0n1          6.00      0.94     6.10  50.41    3.57   159.67    2.60      0.02     0.00   0.00    0.92     8.00    0.00      0.00     0.00   0.00    0.00     0.00    1.30    0.77    0.02   2.40
nvme2n1          0.40      0.30     2.00  83.33    7.00   756.00    2.60      0.02     0.00   0.00    1.96     8.00    0.00      0.00     0.00   0.00    0.00     0.00    1.30    1.31    0.01   0.78
nvme3n1         10.90      0.57     2.60  19.26    1.26    53.69    2.60      0.02     0.00   0.00    0.54     7.23    0.00      0.00     0.00   0.00    0.00     0.00    1.30    0.85    0.02   1.57
nvme5n1          0.70      0.55     4.20  85.71    7.00   807.43    2.60      0.02     0.00   0.00    1.65     7.23    0.00      0.00     0.00   0.00    0.00     0.00    1.30    1.31    0.01   0.93
 home_disks     18.00      2.35    14.90  45.29    2.38   133.93   10.40      0.08     0.00   0.00    1.27     7.62    0.00      0.00     0.00   0.00    0.00     0.00    5.20    1.06    0.06   0.71


avg-cpu:  %user   %nice %system %iowait  %steal   %idle
           2.42    0.00    0.79    0.09    0.00   96.70

Device            r/s     rMB/s   rrqm/s  %rrqm r_await rareq-sz     w/s     wMB/s   wrqm/s  %wrqm w_await wareq-sz     d/s     dMB/s   drqm/s  %drqm d_await dareq-sz     f/s f_await  aqu-sz  %util
nvme0n1          4.30      0.24     1.40  24.56    3.95    58.05   12.20      0.88     3.90  24.22    0.19    73.44    0.00      0.00     0.00   0.00    0.00     0.00    0.20    0.50    0.02   1.92
nvme2n1          0.50      0.38     2.70  84.38    7.00   786.40   12.80      0.88     3.90  23.35    0.18    70.00    0.00      0.00     0.00   0.00    0.00     0.00    0.20    1.00    0.01   0.54
nvme3n1          0.40      0.30     2.10  84.00    7.00   770.00   11.00      0.66     3.30  23.08    0.19    61.53    0.00      0.00     0.00   0.00    0.00     0.00    0.20    1.00    0.01   0.43
nvme5n1          0.00      0.00     0.00   0.00    0.00     0.00   11.00      0.66     3.30  23.08    0.20    61.53    0.00      0.00     0.00   0.00    0.00     0.00    0.20    1.00    0.00   0.17
 home_disks      5.20      0.93     6.20  54.39    4.48   182.85   47.00      3.07    14.40  23.45    0.19    66.93    0.00      0.00     0.00   0.00    0.00     0.00    0.80    0.88    0.03   0.25


avg-cpu:  %user   %nice %system %iowait  %steal   %idle
           2.52    0.00    1.01    0.05    0.00   96.43

Device            r/s     rMB/s   rrqm/s  %rrqm r_await rareq-sz     w/s     wMB/s   wrqm/s  %wrqm w_await wareq-sz     d/s     dMB/s   drqm/s  %drqm d_await dareq-sz     f/s f_await  aqu-sz  %util
nvme0n1          0.70      0.70     4.90  87.50    7.00  1024.00    0.00      0.00     0.00   0.00    0.00     0.00    0.00      0.00     0.00   0.00    0.00     0.00    0.00    0.00    0.00   0.49
nvme2n1          0.30      0.20     1.40  82.35    4.67   685.33    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    0.00    0.00   0.14
nvme3n1          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      0.00     0.00   0.00    0.00     0.00    0.00    0.00    0.00   0.00
nvme5n1          0.30      0.20     1.50  83.33    7.00   668.00    0.00      0.00     0.00   0.00    0.00     0.00    0.00      0.00     0.00   0.00    0.00     0.00    0.00    0.00    0.00   0.21
 home_disks      1.30      1.10     7.80  85.71    6.46   863.69    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    0.00    0.01   0.05

The table below adds up each report's four per-disk %util values and compares their average (sum / 4, which is what the home_disks line should show) with the value iostat actually printed.

The rest of the columns besides %util add up correctly.

What claude thinks is happening

In write_stats(), the group's dev_tp is incremented once per device on every report ((g->dev_tp)++ in the h == hl branch) and is never reset. write_plain_ext_stat() then computes dev_in_grp = d->dev_tp - T_GROUP and divides xds->util by it, so the divisor grows by the number of devices each report.

The attached patch (iostat-group-util.patch, 8 lines) resets the group's dev_tp to T_GROUP when the group's stats are zeroed at the start of a report. The reset is only done on the first pass (h == hl), so the multi-pass --pretty layout still counts correctly.

It thinks the regression was caused by 6857aa71, first released in 12.1.6 in 2019. I don't have the coding skills to personally validate the above explanation, other than to say that the patch worked.

iostat-group-util.patch

Building and testing the patch
I built and tested the attached patch like so:

git clone https://github.com/sysstat/sysstat.git
cd sysstat
git apply iostat-group-util.patch
 ./configure --disable-nls --disable-sensors
 make
./iostat -y -x -m -g home_disks nvme0n1 nvme2n1 nvme3n1 nvme5n1 10

New output showing the average is calculated correctly
Same test as before:

Linux 7.2.8-200.fc44.x86_64 (server.home)      10/04/26        _x86_64_        (48 CPU)

avg-cpu:  %user   %nice %system %iowait  %steal   %idle
           2.48    0.00    0.91    0.20    0.00   96.42

Device            r/s     rMB/s   rrqm/s  %rrqm r_await rareq-sz     w/s     wMB/s   wrqm/s  %wrqm w_await wareq-sz     d/s     dMB/s   drqm/s  %drqm d_await dareq-sz     f/s f_await  aqu-sz  %util
nvme0n1          7.40      0.58     3.10  29.52    3.26    80.70   13.70      0.48     2.20  13.84    0.07    36.03    0.00      0.00     0.00   0.00    0.00     0.00    0.90    0.44    0.03   2.52
nvme2n1          3.60      0.75     4.90  57.65    4.97   212.78   12.30      0.48     2.20  15.17    0.27    40.13    0.00      0.00     0.00   0.00    0.00     0.00    0.90    0.89    0.02   2.05
nvme3n1          4.20      0.34     2.00  32.26    4.88    83.62   14.30      0.95     3.70  20.56    0.34    67.72    0.00      0.00     0.00   0.00    0.00     0.00    0.80    1.12    0.03   2.47
nvme5n1          4.40      0.51     3.10  41.33    3.89   118.64   17.80      0.95     3.70  17.21    0.16    54.40    0.00      0.00     0.00   0.00    0.00     0.00    0.80    0.88    0.02   1.98
 home_disks     19.60      2.18    13.10  40.06    4.06   114.10   58.10      2.86    11.80  16.88    0.21    50.33    0.00      0.00     0.00   0.00    0.00     0.00    3.40    0.82    0.09   2.25


avg-cpu:  %user   %nice %system %iowait  %steal   %idle
           2.45    0.00    0.94    0.16    0.00   96.45

Device            r/s     rMB/s   rrqm/s  %rrqm r_await rareq-sz     w/s     wMB/s   wrqm/s  %wrqm w_await wareq-sz     d/s     dMB/s   drqm/s  %drqm d_await dareq-sz     f/s f_await  aqu-sz  %util
nvme0n1          5.40      0.18     0.60  10.00    4.56    34.67    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    0.00    0.02   2.42
nvme2n1          1.40      0.18     1.00  41.67    6.36   133.43    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    0.00    0.01   0.87
nvme3n1          2.90      0.24     1.60  35.56    4.24    84.97    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    0.00    0.01   1.23
nvme5n1          1.80      0.21     1.30  41.94    6.50   118.00    0.00      0.00     0.00   0.00    0.00     0.00    0.00      0.00     0.00   0.00    0.00     0.00    0.00    0.00    0.01   1.16
 home_disks     11.50      0.81     4.50  28.12    5.00    72.42    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    0.00    0.06   1.42


avg-cpu:  %user   %nice %system %iowait  %steal   %idle
           2.47    0.00    0.95    0.10    0.00   96.48

Device            r/s     rMB/s   rrqm/s  %rrqm r_await rareq-sz     w/s     wMB/s   wrqm/s  %wrqm w_await wareq-sz     d/s     dMB/s   drqm/s  %drqm d_await dareq-sz     f/s f_await  aqu-sz  %util
nvme0n1          2.90      0.15     0.60  17.14    4.93    51.45   13.10      0.83     4.30  24.71    0.08    64.61    0.00      0.00     0.00   0.00    0.00     0.00    0.20    0.50    0.02   1.59
nvme2n1          1.50      0.50     3.40  69.39    6.87   344.53   11.10      0.83     4.30  27.92    0.20    76.25    0.00      0.00     0.00   0.00    0.00     0.00    0.20    0.50    0.01   1.16
nvme3n1          0.40      0.09     0.60  60.00    7.00   219.00   11.40      1.01     4.60  28.75    0.20    90.81    0.00      0.00     0.00   0.00    0.00     0.00    0.20    0.00    0.01  19.13
nvme5n1          1.60      0.37     2.60  61.90    6.38   236.75   10.90      1.01     4.60  29.68    0.22    94.97    0.00      0.00     0.00   0.00    0.00     0.00    0.20    0.50    0.01   1.23
 home_disks      6.40      1.11     7.20  52.94    5.88   176.94   46.50      3.67    17.80  27.68    0.17    80.93    0.00      0.00     0.00   0.00    0.00     0.00    0.80    0.38    0.05   5.78


avg-cpu:  %user   %nice %system %iowait  %steal   %idle
           2.88    0.00    1.28    0.16    0.00   95.68

Device            r/s     rMB/s   rrqm/s  %rrqm r_await rareq-sz     w/s     wMB/s   wrqm/s  %wrqm w_await wareq-sz     d/s     dMB/s   drqm/s  %drqm d_await dareq-sz     f/s f_await  aqu-sz  %util
nvme0n1          1.40      0.03     0.00   0.00    5.79    25.14    1.40      0.02     0.00   0.00    0.86    12.29    0.00      0.00     0.00   0.00    0.00     0.00    0.70    0.71    0.01   0.95
nvme2n1          0.30      0.10     0.70  70.00    7.00   346.67    1.40      0.02     0.00   0.00    1.64    12.29    0.00      0.00     0.00   0.00    0.00     0.00    0.70    1.29    0.01   0.44
nvme3n1          0.40      0.00     0.00   0.00    6.75     8.00    1.60      0.02     0.00   0.00    0.94    12.00    0.00      0.00     0.00   0.00    0.00     0.00    0.80    1.00    0.00   0.44
nvme5n1          0.70      0.12     0.70  50.00    6.86   170.29    1.60      0.02     0.00   0.00    1.62    12.00    0.00      0.00     0.00   0.00    0.00     0.00    0.80    1.50    0.01   0.72
 home_disks      2.80      0.26     1.40  33.33    6.32    93.43    6.00      0.07     0.00   0.00    1.27    12.13    0.00      0.00     0.00   0.00    0.00     0.00    3.00    1.13    0.03   0.64

Summary table of above output showing the fixed calculations:

I hope this helps, thanks for creating/maintaining this software. Cheers