I’ve already written about finding out Linux / UNIX cpu utilization using various tools. You can use same iostat command to find out disk utilization and for monitoring system input/output device loading by observing the time the physical disks are active in relation to their average transfer rates.
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
Linux 2.6.18-53.1.4.el5 (moon.nixcraft.in) 12/17/2007 Device: rrqm/s wrqm/s r/s w/s rsec/s wsec/s avgrq-sz avgqu-sz await svctm %util sda 1.10 39.82 3.41 13.59 309.50 427.48 43.36 0.17 10.03 1.03 1.75 sdb 0.20 18.32 1.15 6.08 117.36 195.25 43.22 0.51 71.14 1.26 0.91 Device: rrqm/s wrqm/s r/s w/s rsec/s wsec/s avgrq-sz avgqu-sz await svctm %util sda 0.00 108.40 1.40 64.40 49.60 1382.40 21.76 0.04 0.67 0.44 2.92 sdb 0.00 37.80 0.00 245.20 0.00 2254.40 9.19 28.91 108.49 1.08 26.36 Device: rrqm/s wrqm/s r/s w/s rsec/s wsec/s avgrq-sz avgqu-sz await svctm %util sda 0.00 97.01 1.00 57.29 39.92 1234.33 21.86 0.03 0.58 0.50 2.89 sdb 0.00 38.32 0.00 288.42 0.00 2623.55 9.10 32.97 122.30 1.15 33.27
- 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:
- The average service time (svctm)
- Percentage of CPU time during which I/O requests were issued (%util)
- 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
For example, from about iostat report it appears that /dev/sdb under load. Hope this information will help you diagnose and optimize disk related issues.
Related: How to find out Linux CPU utilization using vmstat, iostat, mpstat and sar commands.
Please note that command and information discussed here almost applies to any other UNIX like variant.
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