Analyzing I/O Performance in Linux
Introduction
Before diving into I/O performance analysis, first read our guide on analyzing CPU performance in Linux. This article builds on that foundation to help you pinpoint which disks are underperforming.
Using the iostat Command
The iostat command provides detailed statistics on CPU and I/O operations. If iostat is not present, install it:
- On RHEL/CentOS:
sudo yum install sysstat - On Ubuntu/Debian:
sudo apt-get install sysstat
Here’s an example output of iostat -x 15:
iostat -x 15
Linux 5.15.0-204.147.6.2.el8uek.x86_64 (techinfobest) 05/27/2024 _x86_64_ (2 CPU)
avg-cpu: %user %nice %system %iowait %steal %idle
1.03 0.20 0.33 0.01 0.16 98.27
Device r/s w/s rkB/s wkB/s rrqm/s wrqm/s %rrqm %wrqm r_await w_await aqu-sz rareq-sz wareq-sz svctm %util
sda 30.00 80.00 15000.00 40000.00 1.00 2.00 5.00 10.00 1.50 3.00 1.00 500.00 500.00 1.50 50.00
sdb 25.00 60.00 12500.00 30000.00 0.50 1.00 2.00 5.00 1.70 2.80 0.90 500.00 500.00 1.70 45.00
sdc 500.00 1000.00 500000.00 1000000.00 5.00 10.00 5.00 10.00 15.00 25.00 15.00 500.00 500.00 5.00 100.00
Key Metrics to Monitor
- r/s and w/s: Read and write requests per second.
- rkB/s and wkB/s: Kilobytes read and written per second.
- r_await and w_await: Average time in milliseconds for read and write requests.
- %util: Percentage of time the device is active. A value close to 100% indicates the disk is at its capacity.
Interpreting the Data
High r_await and w_await values indicate longer wait times, suggesting potential bottlenecks. High %util means the disk is heavily utilized. If sdc shows 100% utilization, it’s operating at full capacity and might be a bottleneck. Compare the performance metrics of sda, sdb, and sdc to identify which disk has the highest read/write times and utilization.
Example Analysis
For instance:
- sda: Low read/write operations but higher
r_awaitandw_await, indicating possible latency issues. - sdb: Moderate operations with slightly higher wait times, suggesting it handles more load but still performs adequately.
- sdc: Highest read/write operations and utilization, indicating it is the most heavily used disk and has reached its performance limit.
Scenario with I/O Issues
Here’s an additional example where sdc shows 100% utilization, indicating an I/O bottleneck:
iostat -x 15
Linux 5.15.0-204.147.6.2.el8uek.x86_64 (techinfobest) 05/27/2024 _x86_64_ (2 CPU)
avg-cpu: %user %nice %system %iowait %steal %idle
1.03 0.20 0.33 0.01 0.16 98.27
Device r/s w/s rkB/s wkB/s rrqm/s wrqm/s %rrqm %wrqm r_await w_await aqu-sz rareq-sz wareq-sz svctm %util
sda 30.00 80.00 15000.00 40000.00 1.00 2.00 5.00 10.00 1.50 3.00 1.00 500.00 500.00 1.50 50.00
sdb 25.00 60.00 12500.00 30000.00 0.50 1.00 2.00 5.00 1.70 2.80 0.90 500.00 500.00 1.70 45.00
sdc 500.00 1000.00 500000.00 1000000.00 5.00 10.00 5.00 10.00 15.00 25.00 15.00 500.00 500.00 5.00 100.00
In this scenario, sdc shows 100% utilization (%util), indicating that it has reached its maximum capacity for read/write throughput and IOPS. This disk is a clear bottleneck and is likely causing performance issues.
The Value of 15 in iostat -x 15
The 15 in the command iostat -x 15 means the command will report the average statistics every 15 seconds. This helps in monitoring performance over time and identifying spikes in I/O activity.
Conclusion
By analyzing iostat -x output, you can identify specific disks with performance issues. High r_await, w_await, and %util values are indicators of potential I/O bottlenecks. Address these by optimizing disk usage, upgrading hardware, or redistributing the load across multiple disks.
For further insights on CPU performance and related I/O issues, visit TechInfoBest.
