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Oracle monitoring with the vmstat wa column

Oracle Database Tips by Donald BurlesonApril 25,  2015

The vmstat wait (wa) column details are available for monitoring Oracle on AIX servers, and show waits on pending disk I/O.

When using the vmstat utility to monitor Oracle databases, we generally monitor for the CPU runqueue (the vmstat "ru" column), plus RAM paging.  We can also look at the vmstat wait (wa) column, available only on vmstat implementations for AIX.

For complete details, vmstat monitoring best practices in book "Oracle Shell Scripting" by Oracle ACE Jon Emmons.

The vmstat wa column is the percentage of time that the CPU was idle with pending local disk I/O and NFS-mounted disks. If there is at least one outstanding I/O to a disk when wait is running, the time is classified as waiting for I/O.

The vmstat wa column is unique to the IBM AIX implementation of vmstat:

vmstat for AIX

In the IBM AIX operating environment, the output from vmstat will appear like this, with the wa column displayed on the far right:

root> vmstat 2 5

kthr     memory             page              faults        cpu    
----- ----------- ------------------------ ------------ -----------
 r  b   avm    fre  re  pi  po  fr   sr  cy  in     sy  cs  us sy id wa
 7  5 220214   141   0   0   0  42   53   0 1724 12381 2206 19 46 28  7
 9  5 220933   195   0   0   1 216  290   0 1952 46118 2712 27 55 13  5
13  5 220646   452   0   0   1  33   54   0 2130 86185 3014 30 59  8  3
 6  5 220228   672   0   0   0   0    0   0 1929 25068 2485 25 49 16 10

Unless asynchronous I/O is being used by the process, an I/O request to disk causes the calling process to block (or sleep) until the request has been completed. Once an I/O request for a process completes, it is placed on the run queue. If the I/Os were completing faster, more CPU time could be used.

A vmstat wa column value of over 25 percent could indicate that the disk subsystem might not be balanced properly, or it might be the result of a disk-intensive workload.

For information on the change made to wa, see Wait I/O time reporting.


 

 

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