How to use I/O-related Counters for Troubleshooting ClickHouse Performance

Table of Contents

Introduction

ClickHouse provides several I/O-related performance counters that can be used to monitor and troubleshoot database performance. Here are some of the most important counters and how to use them:

  1. Disk Read/Write Speed: The system.disk. performance counters provide information about the disk read and write speed of the database. This can be useful for identifying I/O bottlenecks and determining whether disk performance is impacting query execution. To monitor disk read/write speed, you can use the following SQL query:
SELECT *<br>FROM system.metrics<br>WHERE metric LIKE 'system.disk.%'

2. Block Cache Hit Ratio: ClickHouse uses a block cache to improve query performance by caching frequently accessed data in memory. The system.cache.hit_rate performance counter provides information about the hit rate of the block cache, which represents the percentage of queries that are served from the cache rather than from disk. A high hit rate indicates that the block cache is effective in improving query performance. To monitor block cache hit rate, you can use the following SQL query:

SELECT *<br>FROM system.metrics<br>WHERE metric = 'system.cache.hit_rate'

3. I/O Wait Time: The system.io_wait. performance counters provide information about the amount of time spent waiting for I/O operations to complete. High I/O wait times can indicate disk performance issues or other I/O-related bottlenecks that may be impacting query execution. To monitor I/O wait time, you can use the following SQL query:

SELECT *<br>FROM system.metrics<br>WHERE metric LIKE 'system.io_wait.%'

4. Read/Write Queue Length: The system.io_queue. performance counters provide information about the length of the read and write I/O queues. A high queue length can indicate that the database is unable to keep up with the volume of I/O requests, potentially leading to slower query execution and decreased performance. To monitor read/write queue length, you can use the following SQL query:

SELECT *<br>FROM system.metrics<br>WHERE metric LIKE 'system.io_queue.%'

Conclusion

By monitoring these I/O-related performance counters, you can gain insight into the performance and behavior of your database, as well as troubleshoot issues related to I/O bottlenecks, disk performance, and query execution. You can use these counters to identify areas for optimization and improvement, such as optimizing disk configurations, improving block cache efficiency, or optimizing query execution plans.

To read more about troubleshooting I/O, do consider reading the below articles

About Shiv Iyer 225 Articles
Open Source Database Systems Engineer with a deep understanding of Optimizer Internals, Performance Engineering, Scalability and Data SRE. Shiv currently is the Founder, Investor, Board Member and CEO of multiple Database Systems Infrastructure Operations companies in the Transaction Processing Computing and ColumnStores ecosystem. He is also a frequent speaker in open source software conferences globally.