Postgres' pg_total_relation_size returning unexpected table size for a zfs-based tablespace

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25-10-2024

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When working with PostgreSQL, developers often rely on the pg_total_relation_size function to determine the total size of a database table, including all its indexes. However, many users have reported receiving unexpected results when using this function in conjunction with a ZFS-based tablespace. This article will delve into the nuances of how PostgreSQL calculates table sizes, particularly within a ZFS environment, and provide valuable insights and practical examples for better understanding.

The Problem Scenario

Users have encountered discrepancies when executing the following PostgreSQL SQL command:

SELECT pg_total_relation_size('your_table_name');

When run, this command sometimes returns sizes that do not correspond with what is expected, especially when using tablespaces managed by ZFS.

Analyzing the Issue

1. Understanding pg_total_relation_size: The pg_total_relation_size function computes the total size of a specified table and its associated indexes, toast tables, and any other related information. This function is generally reliable; however, its accuracy can be influenced by how data is stored, especially in advanced storage systems like ZFS.

2. ZFS and PostgreSQL: ZFS (Zettabyte File System) provides a rich feature set, including snapshots, compression, and data integrity checks. While these features are beneficial, they may also lead to inconsistencies in size calculations because the underlying file system manages data differently than traditional file systems.

3. Potential Causes of Size Discrepancies:

   • Snapshots: If you have ZFS snapshots in place, the size reported might include data that is no longer actively in use but is still retained in snapshots.
   • Compression: ZFS may compress data. While this can save space, PostgreSQL's calculations typically do not account for this compression when determining size.
   • Delayed Allocation: ZFS can implement delayed writes, leading to temporary size discrepancies as data may not be flushed to disk yet.

Practical Example and Explanation

To better illustrate this, let’s consider an example where you have a table named employees:

SELECT pg_total_relation_size('employees');

In a ZFS-managed tablespace, this command may return a size that seems larger than the sum of the actual stored data due to the above-mentioned ZFS features.

Steps to Diagnose the Issue

1. Check Active Snapshots: Run the following command to list snapshots related to your ZFS pool:

   zfs list -t snapshot
   

   If you find multiple snapshots, these can contribute to the size reported by pg_total_relation_size.

2. Review Compression Settings: Investigate whether the ZFS file system is set to use compression:

   zfs get compression your_pool_name
   

   Depending on your configuration, the effective size of the data may differ from what PostgreSQL is reporting.

3. Use ZFS Utilities: Tools like zfs list can provide insights into the actual usage of space at the filesystem level.

Conclusion

PostgreSQL's pg_total_relation_size is a powerful function for monitoring table sizes, but discrepancies can arise when working with advanced file systems like ZFS. Understanding the interplay between PostgreSQL's data management and the underlying storage system's capabilities is essential for accurate size assessments. By monitoring snapshots, reviewing compression settings, and leveraging ZFS tools, you can obtain a clearer picture of your database's actual size.

Additional Resources

• PostgreSQL Documentation
• ZFS Documentation
• Understanding PostgreSQL Performance

By following the insights and steps outlined in this article, readers can better understand and navigate the complexities surrounding table size discrepancies in PostgreSQL, particularly in a ZFS environment.