Monitoring and Troubleshooting Excessive Logical I/Os in ClickHouse

Shiv Iyer ClickHouse, ClickHouse DBA Support, ClickHouse Internals, ClickHouse Performance, clickhouse troubleshooting, Troubleshooting IO

Introduction

Excessive logical I/Os in ClickHouse is a key reason for slow, sub-optimal, and expensive query performance and compromised scalability. In this detailed guide, we share tips & tricks to monitor and troubleshoot logical I/Os in ClickHouse.

Reasons for excessive Logical I/Os in ClickHouse

Excessive logical I/Os in ClickHouse can occur for a number of reasons, including:

  1. Unoptimized queries: Queries that are not optimized can result in excessive logical I/Os. This can occur if the query is not using the appropriate indexes or if the query is not using the most efficient join method.
  2. Insufficient indexing: Insufficient indexing can also result in excessive logical I/Os. This can occur if the table does not have enough indexes to support the queries being run against it.
  3. Data skew: Data skew, where a disproportionate amount of data is stored on a single shard, can result in excessive logical I/Os. This can occur if the data is not evenly distributed across the shards.
  4. Lack of partitioning: Not partitioning the table, can also lead to excessive logical I/Os. Partitioning allows for the data to be distributed across multiple servers and can significantly reduce the number of logical I/Os required to retrieve the data.
  5. Data duplication: Data duplication, where the same data is stored in multiple places, can also result in excessive logical I/Os. This can occur if the data is not properly de-duplicated before being stored in ClickHouse.
  6. Data compression: Data compression, if not done properly, can also lead to excessive logical I/Os. This can occur if the compression algorithm is not appropriate for the data or if the compression level is set too low.
  7. Heavy traffic: Heavy traffic, where large numbers of queries are being run simultaneously, can also result in excessive logical I/Os. This can occur if the number of queries exceeds the capacity of the server or if the queries are not properly optimized.
  8. Disk performance: Disk performance issues can also cause excessive logical I/Os. This can occur if the disk is underpowered or if the disk is experiencing high I/O wait times.

By identifying the cause of the excessive logical I/Os, the appropriate action can be taken to resolve the issue. This can include optimizing queries, adding indexes, partitioning the data, de-duplicating data, adjusting the compression settings, increasing the capacity of the server, or resolving disk performance issues.

Troubleshooting excessive logical I/Os in ClickHouse

Troubleshooting excessive logical I/Os in ClickHouse can involve several steps, including:

  1. Monitor query performance: Use the ClickHouse system tables, such as system.query_log and system.metrics to monitor query performance and identify slow or resource-intensive queries.
  2. Analyze query plans: Use the EXPLAIN statement to analyze the query plans for slow or resource-intensive queries. This can help identify issues such as unoptimized queries, insufficient indexing, or inefficient join methods.
  3. Check for data skew: Use the system.parts table to check for data skew. This table shows the number of rows and bytes for each partition in each table. High values for a single partition can indicate data skew.
  4. Check partitioning: Check if the table is partitioned and if the partition key is appropriate for the queries being run.
  5. Check data duplication: Use the system.merges table to check for data duplication. This table shows the number of rows and bytes for each merge in each table. High values for a single merge can indicate data duplication.
  6. Check compression: Use the system.tables table to check the compression settings for each table. Verify that the compression algorithm and level is appropriate for the data.
  7. Monitor disk performance: Monitor the disk I/O performance and disk space usage to ensure that the disk is not a bottleneck.
  8. Monitor traffic: Monitor the number of queries being run simultaneously and adjust the capacity of the server if necessary.
  9. Optimize the queries: Optimize the queries by changing the join method, adding appropriate indexes, partitioning the data and configuring the compression properly.

Conclusion

By taking these steps, it’s possible to identify the cause of the excessive logical I/Os and take appropriate actions to resolve the issue. This can help improve the performance and scalability of the ClickHouse system.

To know more about Clickhouse Troubleshooting, please do consider reading the below articles: 

 

About Shiv Iyer 216 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.
Contact: Website Facebook Twitter

256 Chapman Road STE 105-4, Newark, New Castle, Delaware, 19702, United States | ©2024 ChistaDATA Inc. All rights reserved. | ClickHouse is a trademark of ClickHouse Corporation. All other trademarks are property of their respective owners. Other product or company names mentioned may be trademarks or trade names of their respective owner.