ClickHouse Redo Operations for Data Reliability

Introduction

In ClickHouse, Redo logs are implemented as a write-ahead log (WAL) system, which is used to ensure data consistency and reliability in the event of a system failure. The Redo log records all changes made to the database and provides a mechanism for recovering the database to a consistent state in the event of a crash or system failure.

The Redo log in ClickHouse is stored on disk, separate from the main data file, and it is used to keep track of all changes made to the database. The Redo log is written to disk before the data is written to disk, which ensures that the database can be recovered even if the data file is damaged.

ClickHouse uses the Redo log to perform crash recovery operations, which involves reading the Redo log and replaying any changes that have not been written to disk. This process ensures that the database is returned to a consistent state, and it helps to prevent data loss in the event of a system failure.

The size of the Redo log in ClickHouse can be configured to meet the specific needs of your workloads, and it can be tuned to balance performance and reliability. You can also configure the frequency of Redo log writes to ensure that changes are recorded in the log as quickly as possible, which helps to minimize the amount of data that is lost in the event of a crash.

Overall, the Redo log in ClickHouse is an important feature that provides data reliability and consistency, and it is a key component of the ClickHouse architecture.

Private and public redo threads in ClickHouse

In ClickHouse, the Redo log system uses both private and public Redo threads to manage data changes and ensure the reliability of the database.

Private Redo threads are dedicated to a specific table and are responsible for recording all changes made to that table in the Redo log. Each table in ClickHouse has its own private Redo thread, which helps to ensure that changes are recorded in the log as quickly as possible and that the log is not overwhelmed by changes made to multiple tables.

Public Redo threads, on the other hand, are shared by all tables in the database and are responsible for writing changes from the private Redo threads to disk. The public Redo threads are used to ensure that the Redo log is consistent and that all changes are recorded in the log, even if a system failure occurs.

The use of both private and public Redo threads in ClickHouse helps to balance the performance and reliability of the Redo log system. Private Redo threads allow for fast and efficient recording of changes, while public Redo threads provide a consistent and reliable mechanism for writing changes to disk.

Redo Allocation Latch Performance in ClickHouse

Redo Allocation Latch performance in ClickHouse refers to the performance of the mechanism used to control access to the Redo log when writing changes to the database.

The Redo Allocation Latch is used to ensure that changes to the database are written to the Redo log in a consistent and ordered manner. When a change is made to the database, the Redo Allocation Latch is acquired, which allows the change to be recorded in the Redo log. The Redo Allocation Latch is then released, which allows other changes to be recorded in the log.

The performance of the Redo Allocation Latch is important because it affects the overall performance of the database. If the Redo Allocation Latch is not able to keep up with the rate of changes being made to the database, the database performance may suffer.

To optimize the performance of the Redo Allocation Latch in ClickHouse, it is important to monitor the performance of the Redo log system and adjust the configuration of the Redo log as needed. This may include tuning the size of the Redo log, adjusting the frequency of Redo log writes, and optimizing the performance of the Redo Allocation Latch itself.

Overall, the performance of the Redo Allocation Latch is an important factor in ensuring the reliability and performance of the ClickHouse database, and it is critical to monitor and optimize this performance to ensure optimal database performance.

How log writer writes redo log files from log buffer in ClickHouse?

In ClickHouse, the log writer writes redo log files from the log buffer in a process known as log flushing. This process ensures that changes made to the database are recorded in the Redo log and persist even in the event of a system failure.

The log buffer is a temporary storage area where changes to the database are recorded before they are written to the Redo log. The log writer continuously monitors the log buffer, and when the buffer is full, it writes the changes to the Redo log.

The log writer performs the following steps to write the Redo log files from the log buffer in ClickHouse:

  1. Acquire the Redo Allocation Latch: The log writer acquires the Redo Allocation Latch, which ensures that only one log writer can write to the Redo log at a time.
  2. Copy changes from the log buffer: The log writer copies the changes from the log buffer to the Redo log.
  3. Write the Redo log to disk: The log writer writes the Redo log to disk, which ensures that the changes persist even in the event of a system failure.
  4. Release the Redo Allocation Latch: The log writer releases the Redo Allocation Latch, which allows other log writers to write to the Redo log.

The log flushing process is a critical component of the database system, and it is important to monitor the performance of the log flushing process and optimize the log buffer size and the frequency of log flushes to ensure optimal database performance.

Monitoring Frequency of Log Flushes in ClickHouse

To monitor log flushing in ClickHouse, you can use the following methods:

  1. System Metrics: ClickHouse provides several system metrics that can be used to monitor log flushing performance. You can use the following metrics to monitor log flushing in ClickHouse:
    • replica.log.flush_time: The time taken for log flushing.
    • replica.log.flush_queue: The number of log flushes waiting to be performed.
    • replica.log.generation: The number of log flushes performed.
  2. System Tables: You can also monitor log flushing performance by querying the system tables in ClickHouse. The following system tables can be used to monitor log flushing in ClickHouse:
    • system.metrics: This table provides a comprehensive view of the system metrics in ClickHouse, including the metrics related to log flushing.
    • system.replicas: This table provides information about the replicas in the system, including the Redo log generation information.

You can use the psycopg2 library in Python to query these system tables and monitor log flushing performance in real-time.

  1. Performance Monitoring Tools: ClickHouse provides several performance monitoring tools, including the Monitoring Console, which can be used to monitor the performance of log flushing in ClickHouse.

Conclusion

It is important to monitor log flushing performance to ensure that changes made to the database are recorded in the Redo log and persist even in the event of a system failure. Monitoring log flushing performance can also help identify performance bottlenecks and optimize the database performance

To learn more about updates in ClickHouse, do consider reading the following articles:

About Shiv Iyer 215 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.