1The following is a demonstration of the iopending tool,
2
3Here we run it with a sample interval of 1 second,
4
5   # iopending 1
6   Tracing... Please wait.
7   2006 Jan  6 20:21:59,  load: 0.02,  disk_r:      0 KB,  disk_w:      0 KB
8
9              value  ------------- Distribution ------------- count
10                < 0 |                                         0
11                  0 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ 1010
12                  1 |                                         0
13
14   2006 Jan  6 20:22:00,  load: 0.03,  disk_r:      0 KB,  disk_w:      0 KB
15
16              value  ------------- Distribution ------------- count
17                < 0 |                                         0
18                  0 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ 1000
19                  1 |                                         0
20
21   2006 Jan  6 20:22:01,  load: 0.03,  disk_r:      0 KB,  disk_w:      0 KB
22
23              value  ------------- Distribution ------------- count
24                < 0 |                                         0
25                  0 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@ 1000
26                  1 |                                         0
27
28   ^C
29
30The iopending tool samples at 1000 Hz, and prints a distribution of how many
31disk events were "pending" completion. In the above example the disks are
32quiet - for all the samples there are zero disk events pending.
33
34
35
36Now iopending is run with no arguments. It will default to an interval of 5
37seconds,
38
39   # iopending
40   Tracing... Please wait.
41   2006 Jan  6 19:15:41,  load: 0.03,  disk_r:   3599 KB,  disk_w:      0 KB
42
43              value  ------------- Distribution ------------- count
44                < 0 |                                         0
45                  0 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@@     4450
46                  1 |@@@                                      390
47                  2 |@                                        80
48                  3 |                                         40
49                  4 |                                         20
50                  5 |                                         30
51                  6 |                                         0
52
53   ^C
54
55In the above output there was a little disk activity. For 390 samples there
56was 1 I/O event pending; for 80 samples there was 2, and so on.
57
58
59
60
61In the following example iopending is run during heavy disk activity. We
62print output every 10 seconds,
63
64   # iopending 10
65   Tracing... Please wait.
66   2006 Jan  6 20:58:07,  load: 0.03,  disk_r:  25172 KB,  disk_w:  33321 KB
67
68              value  ------------- Distribution ------------- count
69                < 0 |                                         0
70                  0 |@@@@@@@@@                                2160
71                  1 |@@@@@@@@@@@@@@@@@@@@@@@@@@@              6720
72                  2 |@@@@                                     1000
73                  3 |                                         50
74                  4 |                                         30
75                  5 |                                         20
76                  6 |                                         10
77                  7 |                                         10
78                  8 |                                         10
79                  9 |                                         0
80
81   2006 Jan  6 20:58:17,  load: 0.05,  disk_r:   8409 KB,  disk_w:  12449 KB
82
83              value  ------------- Distribution ------------- count
84                < 0 |                                         0
85                  0 |@@@@@@@@@@@@@@@@@@@@@@@@@@@@@            7260
86                  1 |@@@@@@@                                  1700
87                  2 |@                                        300
88                  3 |                                         0
89                  4 |                                         10
90                  5 |                                         10
91                  6 |                                         10
92                  7 |                                         20
93                  8 |                                         0
94                  9 |                                         0
95                 10 |                                         0
96                 11 |                                         0
97                 12 |                                         0
98                 13 |                                         0
99                 14 |                                         0
100                 15 |                                         0
101                 16 |                                         0
102                 17 |                                         10
103                 18 |                                         20
104                 19 |                                         0
105                 20 |                                         0
106                 21 |                                         0
107                 22 |                                         0
108                 23 |                                         0
109                 24 |                                         0
110                 25 |                                         0
111                 26 |                                         0
112                 27 |                                         0
113                 28 |                                         0
114                 29 |                                         0
115                 30 |                                         0
116                 31 |                                         10
117              >= 32 |@@@                                      650
118
119   ^C
120
121In the first output, most of the time (67%) there was 1 event pending,
122and for a short time there were 8 events pending. In the second output we
123see many samples were off the scale - 650 samples at 32 or more pending
124events. For this sample I had typed "sync" in another window, which
125queued many disk events immediately which were eventually completed.
126
127