High IO Wait: Diagnostics & Troubleshooting

What to do when the CPU is busy waiting for data

High I/O (input/output) wait refers to a situation where a significant amount of time is spent waiting for disk or storage operations to complete. This can cause system performance degradation and slower response times. High I/O wait usually indicates that the system is experiencing heavy disk activity or facing performance issues with the storage subsystem.

To diagnose and troubleshoot high I/O wait in Linux, you can use various tools and techniques. Here are some commonly used approaches:

  • iostat: The iostat command is a powerful tool for monitoring and analyzing system I/O statistics. It provides information about disk utilization, I/O rates, and other relevant metrics. By running iostat -x 1 or iostat -dmx 1, you can obtain real-time updates on disk activity and identify any disks or partitions with high I/O wait times.

  • iotop: Similar to top for CPU monitoring, iotop helps you track I/O usage on a per-process basis. It displays a list of processes consuming the most I/O resources, making it easier to identify any specific application or service causing high disk activity.

  • sar: The sar (System Activity Reporter) command collects, reports, and analyzes system activity information. It can provide historical data on I/O wait times, CPU usage, and other system performance metrics. Running sar -d or sar -b will show disk-related statistics, including I/O wait times.

  • lsof: The lsof (List Open Files) command lists all open files and the processes accessing them. By examining the output of lsof, you can identify processes that are frequently accessing disks and potentially causing high I/O wait.

High I/O wait Due to Swapping

Often, high I/O wait is actually a sign of high swap usage. In this case, Linux begins swapping to disk to free some memory. Since operations on disk are much much slower than in RAM, the system becomes very slow. In this case, you need to identify, what is causing thehigh memory usage and fix this issue first. Usually, this also fixes the issue with the high I/O wait, when the system stops swapping.

Once you have identified high I/O wait as an issue, you can take several steps to mitigate it:

  • Optimize disk usage: Identify any unnecessary or excessive disk I/O activities, such as frequent writes or reads to temporary files or logs. Adjusting application configurations or implementing caching mechanisms can help reduce unnecessary disk operations.

  • Upgrade hardware: If the high I/O wait is due to hardware limitations, such as slow or overloaded disks, consider upgrading to faster storage devices or increasing the number of disks in a RAID configuration.

  • Tune filesystem options: Adjusting filesystem parameters can improve I/O performance. For example, using a filesystem with better write performance, adjusting I/O scheduler settings, or modifying disk I/O timeout values.

  • Identify and optimize problematic processes: Use the tools mentioned earlier (such as iotop and lsof) to identify processes with high I/O wait times. Investigate the root cause of their disk activity and consider optimizing or redesigning them if they are resource-intensive or inefficient.

  • Monitor and analyze: Continuously monitor the system using the mentioned tools to identify patterns, trends, or specific events that cause high I/O wait. This will help you proactively address any emerging issues.

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