iostat syntax for disk utilization report
iostat -d -x interval count
-d : Display the device utilization report (d == disk)
-x : Display extended statistics including disk utilization
interval : It is time period in seconds between two samples . iostat 2 will give data at each 2 seconds interval.
count : It is the number of times the data is needed . iostat 2 5 will give data at 2 seconds interval 5 times
Display 3 reports of extended statistics at 5 second intervals for disk
Type the following command:
# iostat -d -x 5 3
Output:
[root@vari Desktop]# iostat -d -x 5 3
Linux 2.6.18-128.7.1.el5xen (vari.taashee.com) 08/26/2009 _i686_ (2 CPU)
Device: rrqm/s wrqm/s r/s w/s rsec/s wsec/s avgrq-sz avgqu-sz await svctm %util
sda 2.41 12.42 3.27 5.37 134.84 142.70 32.13 0.12 14.04 1.83 1.58
Device: rrqm/s wrqm/s r/s w/s rsec/s wsec/s avgrq-sz avgqu-sz await svctm %util
sda 0.00 1.60 0.00 0.40 0.00 16.00 40.00 0.00 0.00 0.00 0.00
Device: rrqm/s wrqm/s r/s w/s rsec/s wsec/s avgrq-sz avgqu-sz await svctm %util
sda 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
Where,
rrqm/s : The number of read requests merged per second that were queued to the hard disk
wrqm/s : The number of write requests merged per second that were queued to the hard disk
r/s : The number of read requests per second
w/s : The number of write requests per second
rsec/s : The number of sectors read from the hard disk per second
wsec/s : The number of sectors written to the hard disk per second
avgrq-sz : The average size (in sectors) of the requests that were issued to the device.
avgqu-sz : The average queue length of the requests that were issued to the device
await : The average time (in milliseconds) for I/O requests issued to the device to be served. This includes the time spent by the requests in queue and the time spent servicing them.
svctm : The average service time (in milliseconds) for I/O requests that were issued to the device
%util : Percentage of CPU time during which I/O requests were issued to the device (bandwidth utilization for the device). Device saturation occurs when this value is close to 100%.
How do I interpret the output result for optimization?
First you need to note down following values from the iostat output:
1. The average service time (svctm)
2. Percentage of CPU time during which I/O requests were issued (%util)
3. See if a hard disk reports consistently high reads/writes (r/s and w/s)
If any one of these are high, you need to take one of the following action:
Get high speed disk and controller for file system (for example move from SATA I to SAS 15k disk)
Tune software or application or kernel or file system for better disk utilization
Use RAID array to spread the file system
