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      1 The following is a demonstration of the iopending tool,
      2 
      3 Here 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 
     30 The iopending tool samples at 1000 Hz, and prints a distribution of how many
     31 disk events were "pending" completion. In the above example the disks are
     32 quiet - for all the samples there are zero disk events pending.
     33 
     34 
     35 
     36 Now iopending is run with no arguments. It will default to an interval of 5
     37 seconds,
     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 
     55 In the above output there was a little disk activity. For 390 samples there
     56 was 1 I/O event pending; for 80 samples there was 2, and so on. 
     57 
     58 
     59 
     60 
     61 In the following example iopending is run during heavy disk activity. We
     62 print 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 
    121 In the first output, most of the time (67%) there was 1 event pending, 
    122 and for a short time there were 8 events pending. In the second output we
    123 see many samples were off the scale - 650 samples at 32 or more pending
    124 events. For this sample I had typed "sync" in another window, which 
    125 queued many disk events immediately which were eventually completed.
    126 
    127