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High I/O wait values can significantly degrade server performance, leading to service and application disruptions. I/O wait is a metric indicating the percentage of CPU time waiting for I/O operations to complete. I/O performance issues are common on servers with high volumes of disk operations, such as database servers, web servers, etc. This article focuses on diagnostic methods and solutions for high I/O wait issues on CentOS 7 systems.

Diagnostic Procedures

1. Determine Iowait Values Firstly, it's essential to determine if high iowait is indeed a problem. You can do this using the top or vmstat tool. The iowait value is displayed in the wa column.

vmstat 1

2. Identify Busy Disks Using the iotop tool, you can determine which processes are generating the most I/O operations.

iotop -o

3. Check Disk Performance The iostat tool helps identify disk performance and whether any disks are overloaded.

iostat -xm 5

Problem Resolution

1. Disk Performance Optimization

• SSD Utilization: SSDs offer faster access times and better I/O performance compared to HDDs.
• RAID Configuration: Using RAID can improve performance and redundancy. For example, RAID 10 offers a good balance between performance and resilience.

2. File System Tuning

• Use noatime: Mounting disks with the noatime option can reduce disk write operations by not recording the last access time for files.
• Increase Buffer Size: Increasing the disk operation cache memory can help reduce I/O wait.

3. Database Application Optimization

• Indexing: Effective indexing can significantly reduce the need for disk operations by improving query efficiency.
• Database Cache Configuration: Increasing the database cache size can reduce the number of I/O operations required to read data.

4. I/O Operations Monitoring and Scheduling

• ionice: The ionice tool allows setting I/O operation priorities for processes, which can help minimize impact on critical services.
• cgroups: Control groups (cgroups) enable resource allocation, including I/O throughput, among groups of processes.

 

High iowait values indicate that the system is disk-bound. Through diagnostics and identifying problematic areas, followed by implementing appropriate solutions, significant performance improvements can be achieved. The key to success lies in regularly monitoring and adjusting system and application configurations to efficiently utilize available resources.