iotop Command Linux: Complete Guide to Monitor Disk IO Usage and Performance

The iotop command is an essential Linux utility that provides real-time monitoring of disk I/O usage by processes and threads. Similar to how top monitors CPU usage, iotop displays which processes are causing the most disk activity on your system. This powerful tool is invaluable for system administrators and developers who need to identify I/O bottlenecks and optimize system performance.

What is iotop Command?

iotop is a Python-based utility that monitors I/O usage information provided by the Linux kernel and displays a table of current I/O usage by processes or threads on the system. It requires kernel 2.6.20 or later with the CONFIG_TASKSTATS and CONFIG_TASK_DELAY_ACCT options enabled.

Key Features of iotop:

• Real-time I/O monitoring
• Process and thread-level visibility
• Read/write statistics
• Total I/O bandwidth usage
• Interactive process management
• Customizable display options

Installing iotop on Different Linux Distributions

Ubuntu/Debian:

sudo apt update
sudo apt install iotop

CentOS/RHEL/Fedora:

# For CentOS/RHEL 7/8
sudo yum install iotop

# For Fedora/CentOS Stream
sudo dnf install iotop

Arch Linux:

sudo pacman -S iotop

openSUSE:

sudo zypper install iotop

Basic iotop Usage and Syntax

The basic syntax for iotop is:

iotop [OPTIONS]

Running iotop with Default Settings:

sudo iotop

Sample Output:

Total DISK READ :       2.50 M/s | Total DISK WRITE :      15.20 M/s
Actual DISK READ:       2.50 M/s | Actual DISK WRITE:      12.40 M/s
  TID  PRIO  USER     DISK READ  DISK WRITE  SWAPIN     IO>    COMMAND
 3254 be/4 root        0.00 B/s   11.85 M/s  0.00 %  89.21 % [kworker/0:1H]
 1853 be/4 mysql       2.50 M/s    3.35 M/s  0.00 %  45.32 % mysqld --defaults-file=/etc/mysql/my.cnf
 2145 be/4 www-data    0.00 B/s    0.00 B/s  0.00 %   5.15 % apache2 -DFOREGROUND
 1254 be/4 root        0.00 B/s    0.00 B/s  0.00 %   2.33 % systemd-journald

Understanding iotop Output

The iotop output consists of two main sections:

Header Section:

• Total DISK READ/WRITE: Total bandwidth for disk reads and writes
• Actual DISK READ/WRITE: Actual bandwidth to/from storage devices

Process Table Columns:

• TID: Thread ID
• PRIO: I/O priority (rt=real-time, be=best-effort, idle=idle)
• USER: User running the process
• DISK READ: Bandwidth of disk reads
• DISK WRITE: Bandwidth of disk writes
• SWAPIN: Percentage of time spent swapping in
• IO: Percentage of time spent in I/O
• COMMAND: Command name/line

Essential iotop Command Options

Display Only Active Processes (-o, –only):

sudo iotop -o

This option shows only processes or threads that are actively performing I/O operations.

Show Processes Instead of Threads (-P, –processes):

sudo iotop -P

Groups threads by process, showing process-level I/O statistics instead of individual threads.

Show Accumulated I/O (-a, –accumulated):

sudo iotop -a

Displays accumulated I/O instead of bandwidth, showing total bytes read/written since iotop started.

Non-Interactive Mode (-b, –batch):

sudo iotop -b -n 3

Runs in batch mode with 3 iterations, useful for scripts and logging.

Set Update Delay (-d, –delay):

sudo iotop -d 5

Sets the delay between updates to 5 seconds (default is 1 second).

Advanced iotop Usage Examples

Monitor Specific User Activity:

sudo iotop -u mysql

Shows I/O activity only for processes owned by the mysql user.

Monitor with Custom Delay and Process View:

sudo iotop -P -d 2 -o

Updates every 2 seconds, shows processes instead of threads, and displays only active processes.

Export I/O Data for Analysis:

sudo iotop -b -n 10 -d 5 -o > io_analysis.txt

Captures 10 snapshots every 5 seconds of active processes and saves to a file.

Monitor High I/O Processes:

sudo iotop -a -o -d 3

Shows accumulated I/O for active processes with 3-second intervals.

Interactive iotop Commands

While iotop is running interactively, you can use these keyboard shortcuts:

Practical Use Cases and Examples

1. Identifying Database Performance Issues:

sudo iotop -u mysql -P -a

Monitor MySQL processes to identify high I/O operations that might indicate inefficient queries or missing indexes.

2. Web Server I/O Analysis:

sudo iotop -u www-data -o -d 1

Monitor Apache/Nginx processes for high I/O activity that could indicate log writing issues or file serving bottlenecks.

3. System Backup Monitoring:

sudo iotop -P -d 5

Monitor backup processes to ensure they’re not overwhelming the system’s I/O capacity.

4. Development Environment Optimization:

sudo iotop -u developer -a -o

Track I/O patterns during development to optimize application performance.

Troubleshooting Common iotop Issues

Permission Denied Error:

If you see “Permission denied” errors, ensure you’re running iotop as root or with sudo:

sudo iotop

CONFIG_TASKSTATS Not Enabled:

If iotop fails to start, check if the required kernel options are enabled:

zcat /proc/config.gz | grep -E "(CONFIG_TASKSTATS|CONFIG_TASK_DELAY_ACCT)"

High CPU Usage by iotop:

Reduce the update frequency to lower CPU usage:

sudo iotop -d 5  # Update every 5 seconds instead of 1

Alternative I/O Monitoring Tools

iostat (from sysstat package):

iostat -x 1

Provides device-level I/O statistics.

atop:

atop -d 1

System and process monitor with I/O information.

pidstat:

pidstat -d 1

Shows I/O statistics for individual processes.

Best Practices for Using iotop

1. Start with basic monitoring: Use sudo iotop -o to see only active processes
2. Combine with other tools: Use alongside top, htop, and iostat for comprehensive monitoring
3. Monitor during peak hours: Run iotop during high-traffic periods to identify bottlenecks
4. Use batch mode for logging: Capture data for later analysis using the -b option
5. Focus on accumulated I/O: Use -a option to understand total I/O impact over time
6. Regular monitoring: Make iotop part of your regular system monitoring routine

Security Considerations

Since iotop requires root privileges to access kernel I/O statistics, consider these security aspects:

• Only grant iotop access to trusted users
• Use sudo with specific iotop permissions instead of full root access
• Monitor iotop usage in multi-user environments
• Consider using restricted environments for iotop access

Conclusion

The iotop command is an indispensable tool for Linux system administrators and developers who need to monitor and optimize disk I/O performance. By understanding its features, options, and best practices, you can effectively identify I/O bottlenecks, optimize system performance, and maintain healthy server environments.

Whether you’re troubleshooting slow database queries, monitoring backup processes, or optimizing application performance, iotop provides the real-time visibility needed to make informed decisions about your system’s I/O usage. Regular use of iotop as part of your monitoring toolkit will help ensure optimal system performance and quick identification of potential issues.

Remember to combine iotop with other system monitoring tools for a comprehensive view of your system’s performance, and always run it with appropriate permissions to access the kernel’s I/O statistics effectively.