Linux iostat command

The iotop is a free and useful Linux top command like monitoring tool meant to monitor disk I/O activity on the server. You must have a kernel version of 2.6.20 or later to use iotop. It requires root privileges to execute. We could also use sar command or iostat to monitor I/O activity on the server, but iotop is different from these tools because it displays columns for the I/O bandwidth utilized by each process/thread during the sampling period. It also shows the percentage of time the thread/process spent while swapping in and while waiting on I/O. Having I/O utilization information based on process IDs instead of disks makes identification of processes causing I/O bottlenecks straightforward.

The iotop is not available by default on most systems. So, let’s see the installation process on Redhat/Centos and Ubuntu/Debian based systems.

Installing iotop on Debian/Ubuntu systems

[email protected]:~# apt-get install iotop
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following NEW packages will be installed:
iotop
0 upgraded, 1 newly installed, 0 to remove and 12 not upgraded.
Need to get 23.8 kB of archives.
After this operation, 127 kB of additional disk space will be used.
Get:1 http://ap-southeast-1.ec2.archive.ubuntu.com/ubuntu/ trusty/universe iotop amd64 0.6-1 [23.8 kB]
Fetched 23.8 kB in 0s (36.7 kB/s)
Selecting previously unselected package iotop.
(Reading database ... 118631 files and directories currently installed.)
Preparing to unpack .../archives/iotop_0.6-1_amd64.deb ...
Unpacking iotop (0.6-1) ...
Processing triggers for man-db (2.6.7.1-1ubuntu1) ...
Setting up iotop (0.6-1) ...

We could query information about the package via dpkg command as shown below:

[email protected]:~# dpkg -p iotop
Package: iotop
Priority: optional
Section: admin
Installed-Size: 124
Maintainer: Ubuntu Developers <[email protected]>
Architecture: amd64
Version: 0.6-1
Depends: python (>= 2.7.1-0ubuntu2), python (<< 2.8)
Size: 23828
Description: simple top-like I/O monitor
iotop does for I/O usage what top(1) does for CPU usage. It watches I/O
usage information output by the Linux kernel and displays a table of
current I/O usage by processes on the system. It is handy for answering
the question "Why is the disk churning so much?".
iotop can only run under a Linux 2.6.20 or later kernel built with the
CONFIG_TASKSTATS, CONFIG_TASK_DELAY_ACCT, CONFIG_TASK_IO_ACCOUNTING and
CONFIG_VM_EVENT_COUNTERS build config options on.
Original-Maintainer: Paul Wise <[email protected]>
Homepage: http://guichaz.free.fr/iotop/

 

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Installing iotop on Redhat/Centos systems

[[email protected] ~]# yum install iotop -y
Loaded plugins: fastestmirror
Loading mirror speeds from cached hostfile
* base: centos.s.uw.edu
* epel: mirror.nes.co.id
* extras: mirrors.usc.edu
* nux-dextop: li.nux.ro
* updates: centos.unixheads.org
Resolving Dependencies
--> Running transaction check
---> Package iotop.noarch 0:0.6-2.el7 will be installed
--> Finished Dependency Resolution
Dependencies Resolved
===========================================================================================================================================================================================
Package Arch Version Repository Size
===========================================================================================================================================================================================
Installing:
iotop noarch 0.6-2.el7 base 52 k
Transaction Summary
================================================================================
Install 1 Package
Total download size: 52 k
Installed size: 156 k
Downloading packages:
iotop-0.6-2.el7.noarch.rpm | 52 kB 00:00:01
Running transaction check
Running transaction test
Transaction test succeeded
Running transaction
Installing : iotop-0.6-2.el7.noarch 1/1
Verifying : iotop-0.6-2.el7.noarch 1/1
Installed:
iotop.noarch 0:0.6-2.el7
Complete!

Let’s query the package via the following rpm command

[[email protected]:~] $ rpm -qa | grep iotop
iotop-0.3.2-7.el6.noarch
[[email protected]:~] $ rpm -qi iotop
Name        : iotop                       
Relocations: (not relocatable)
Version     : 0.3.2                             
Vendor: Red Hat, Inc.
Release     : 7.el6                         
Build Date: Wed 18 Sep 2013 08:02:59 AM UTC
Install Date: Fri 12 Dec 2014 01:51:26 PM UTC      
Build Host: x86-022.build.eng.bos.redhat.com
Group       : Applications/System          
Source RPM: iotop-0.3.2-7.el6.src.rpm
Size        : 146237                           
License: GPLv2
Signature   : RSA/8, Thu 19 Sep 2013 08:09:14 AM UTC, Key ID 199e2f91fd431d51
Packager    : Red Hat, Inc. <http://bugzilla.redhat.com/bugzilla>
URL         : http://guichaz.free.fr/iotop/
Summary     : Top like utility for I/O
Description :
Linux has always been able to show how much I/O was going on
(the bi and bo columns of the vmstat 1 command).
iotop is a Python program with a top like UI used to
show of behalf of which process is the I/O going on.

 

Now that we’ve installed iotop and also verified the install we’ll now focus on its usage.

Example1: Using iotop without any options will display all processes running on the system including those not performing any I/O operations. To restrict the output to only those processes which are currently performing I/O, use the -o option.

[[email protected]:~] $ sudo iotop -o
Total DISK READ: 0.00 B/s | Total DISK WRITE: 54.43 K/s
  TID  PRIO  USER     DISK READ  DISK WRITE  SWAPIN     IO>    COMMAND
 1074 be/3 root        0.00 B/s   27.21 K/s  0.00 %  0.10 % [jbd2/dm-5-8]
54963 be/4 root        0.00 B/s    6.80 M/s  0.00 %  0.00 % rsync --server -vlogDtprxze.iLsfxC . /var/www/html/pub/linux_build/
16153 be/4 root        0.00 B/s    3.89 K/s  0.00 %  0.00 % java -Dgosh.args=-sc telnetd -p6666 start -Xms512m -Xmx1536m -X~anager/bundle-cache/ -b /opt/asrmanager/felix-framework/bundle/

 

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So, the above iotop output only shows the processes engaged in performing any I/O activities at the instant when iotop reported I/O bandwidth utilization on the system.

I’ll briefly describe the different fields reported on by iotop:

  • TID: PID of process
  • PRIO: the I/O priority at which the process is running. This can be modified using the ionice command.
  • USER: The user id which owns the process
  • DISK READ: data read per second
  • DISK WRITE: data written per second
  • SWAPIN: Time spent swapping, i.e., copying of memory pages back and forth between memory and swap.
  • IO: Time spent by the process waiting for I/O during the sampling interval. While debugging I/O performance issues, take note that a high value of IO column will result in a performance bottleneck.
  • COMMAND: The command being executed by the process.

We can filter out the output further to limit it to a user with the -u option or even a process with the -p option which I’ll demonstrate in the next few examples.

Example2: Here is an example to filter out all processes owned by grid user which are presently performing an I/O operation.

[[email protected]:~] $ sudo iotop -o -u grid
Total DISK READ: 107.11 K/s | Total DISK WRITE: 95.00 K/s
  TID  PRIO  USER     DISK READ  DISK WRITE  SWAPIN     IO>    COMMAND
15883 rt/4 grid      953.70 B/s    0.00 B/s  0.00 %  0.04 % ocssd.bin
15875 rt/4 grid      953.70 B/s    0.00 B/s  0.00 %  0.03 % ocssd.bin
15879 rt/4 grid      953.70 B/s    0.00 B/s  0.00 %  0.02 % ocssd.bin
15877 rt/4 grid        0.00 B/s  476.85 B/s  0.00 %  0.02 % ocssd.bin
15872 rt/4 grid        0.00 B/s  476.85 B/s  0.00 %  0.02 % ocssd.bin
15881 rt/4 grid        0.00 B/s  476.85 B/s  0.00 %  0.01 % ocssd.bin
87309 be/4 grid        0.00 B/s    3.73 K/s  0.00 %  0.00 % gipcd.bin
22947 be/4 grid        0.00 B/s    7.45 K/s  0.00 %  0.00 % tnslsnr LISTENER -inherit

 

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Example 3: In this case, only the I/O operations being performed by process id 15883 are reported.

[[email protected]:~] $ sudo iotop -o -p 15883
Total DISK READ: 2.99 K/s | Total DISK WRITE: 148.73 K/s
  TID  PRIO  USER     DISK READ  DISK WRITE  SWAPIN     IO>    COMMAND
15883 rt/4 grid     1022.16 B/s    0.00 B/s  0.00 %  0.23 % ocssd.bin

 

Example 4: Like top command, we can also use iotop in batch mode so that we could make use of the displayed output in scripts or reporting purposes. We use the -b option with iotop to indicate batch mode. The -b option is usually accompanied by -n option which shows the number of iterations iotop should run before stopping.

So, if I needed to check I/O activity performed on the server only once i.e., for just one sample interval, I could type the following:

[[email protected]linuxnix:~] $ sudo iotop -b -n 1 -o
Total DISK READ: 429.62 K/s | Total DISK WRITE: 211.50 K/s
  TID  PRIO  USER     DISK READ  DISK WRITE  SWAPIN      IO    COMMAND
55044 be/4 obryann1  423.01 K/s  211.50 K/s  0.00 %  3.01 % ora_ckpt_bryann12
15875 rt/4 grid        6.61 K/s    0.00 B/s  0.00 %  1.43 % ocssd.bin

I hope this article has helped you to understand how we may use iotop instead of sar or iostat when we need to identify an I/O bottleneck based on a process instead of a disk device.