Monitor Data Guard I/O Activity using iostat Donald K. Burleson

Also see these important notes on Data Guard redo transport waits

The performance of the log transfer service to a standby site depends on the I/O subsystem and the speed of I/O activity on the disk where archived redo logs are being written. To determine if the I/O on the standby host is the bottleneck in the log transfer process, the iostat operating system utility, or a variant of it depending on the operating system, can be used. The iostat utility shows a detailed map of the I/O activity on the disks. The important columns for log transfer service are:

• Kw/S (Average KB written per second)
   

• Wait
   

• %w (Occupancy of wait queue)
   

• %b (device busy)
   

• asvc_t (average service time)

If there is a lot of I/O wait in the system, the disks should be configured properly, using RAID or any other technology, to boost the efficiency of I/O activity.

The following is sample output from iostat –xn 5 30 that shows I/O statistics collected every 5 seconds for 30 intervals. Note a %w and asvc_t for the device c0t0d0. This disk contains the log archive destination for the physical standby database "appsdb". This I/O wait will result in a delay in completion of the transfer of archived log files to the standby host.

DBA@jrsp01 iostat -xn 5 20

I/O Activity on the Standby Host

     r/s    w/s   kr/s   kw/s wait actv wsvc_t asvc_t  %w  %b device
   56.6  382.8 1079.1 3991.9  3.3  1.9    7.6    4.3  81  97 c0t0d0
    0.0    0.0    0.0    0.0  0.0  0.0    0.0    0.0   0   0 c0t2d0
    0.0    0.0    0.0    0.0  0.0  0.0    0.0    0.0   0   0 c0t3d0