Attinder Pal Singh gives a practical look at cgroup v2 support in SQL Server 2025 preview and SQL Server 2022 CU 20 on Linux, helping DBAs and engineers adopt better resource controls and avoid OOM errors on Kubernetes and other container platforms.

Smarter Resource Management for Containers and Beyond

Authors: Engineering: Andrew Carter (Lead), Nicolas Blais-Miko. Product Manager: Attinder Pal Singh and Amit Khandelwal

SQL Server now supports cgroup v2 on Linux with SQL Server 2025 preview and SQL Server 2022 CU 20. This improvement means more accurate, reliable resource management for SQL Server instances—especially those running inside containers on platforms like Docker, Kubernetes, Azure Kubernetes Service (AKS), and OpenShift.

Why Does cgroup v2 Matter?

Linux control groups (cgroups) let you set, enforce, and monitor resource usage (CPU, memory) for processes and containers. Version 2 of cgroup provides:

  • Unified hierarchy
  • Stricter resource isolation
  • Reliable enforcement and better metrics

Read more on cgroup v2 in the Linux Kernel docs.

How to Check Your cgroup Version

Run this command:

stat -fc %T /sys/fs/cgroup/
  • If the result is cgroup2fs, you’re running cgroup v2
  • If it is cgroup, you’re running cgroup v1

Switching to cgroup v2

  • Use a current Linux distro with cgroup v2 enabled by default
  • To enable manually:
    1. Add systemd.unified_cgroup_hierarchy=1 to GRUB
    2. Run sudo update-grub

Real-World Scenario: SQL Server in Kubernetes

Before SQL Server 2022 CU 20: SQL Server containers sometimes exceeded memory limits, leading to out-of-memory (OOM) errors, despite those limits being set. Example:

  • Node: 4 CPUs, 16 GB RAM
  • Pod config: memory limited to 3Gi
  • SQL Server still saw and used up to ~12.8GB (80% of node total), risking OOM

Errorlog snippet:

Microsoft SQL Server 2022 (RTM-CU19)… Detected 12792 MB of RAM, 12313 MB of available memory…

Why Only 80% Memory?

By default, SQL Server’s memory.memorylimit parameter is set to 80% physical memory to help prevent host OOM. See official docs.

What Changes With cgroup v2 Support?

Starting in SQL Server 2022 CU 20 and SQL Server 2025 preview, the memory limits set by the orchestration platform (Kubernetes, Docker, etc.) are now respected reliably.

  • SQL Server now sees the correct per-container memory, based on cgroup v2 limits.
  • Example errorlog with 3Gi limit:

Microsoft SQL Server 2022 (RTM-CU20)… Detected 2458 MB of RAM, 1932 MB of available memory…

This reflects 80% of 3Gi—so SQL Server fully honors resource control in containerized environments on Linux.

How to Validate Your Deployment

  • Check your cgroup version as above
  • Confirm SQL Server errorlog detects the correct RAM
  • Container resource config: kubectl get pod ... -o jsonpath="{.status.qosClass}{.spec.containers[*].resources.limits.memory}"

Learn More

Summary

The cgroup v2 support in SQL Server for Linux enables smarter, predictable Quality of Service enforcement, especially for containerized solutions. DBAs and DevOps engineers deploying SQL Server on AKS or on-premises now have greater confidence in resource isolation and platform resilience.

Authors: Andrew Carter, Nicolas Blais-Miko, Attinder Pal Singh, Amit Khandelwal

Last updated: Aug 12, 2025

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