ElasticSearch service: Identifying Cgroup v2 vs. Hybrid Mode on OEL 9 and OEL 8

Products

• VMware Smart Assurance

Issue/Introduction

Users migrating Watch4Net (W4N) 7.9 or higher from Oracle Enterprise Linux (OEL) 8.x to OEL 9.x may notice differences in how control groups (cgroups) are reported.

Specifically:

• The command grep cgroup /proc/filesystems returns both cgroup and cgroup2, leading to the assumption that the system is in "Hybrid" mode.
• Concerns regarding whether the Elasticsearch service is compatible with the cgroup mode on OEL 9.7 or later.

Environment

Watch4net|M&R 7.9

Cause

The reporting of cgroup and cgroup2 in /proc/filesystems only indicates the file systems the kernel is capable of supporting. It does not reflect the active governing mode of the operating system.

• OEL 8.x: Defaults to cgroup v1 or Hybrid mode.
• OEL 9.x: Defaults to pure cgroup v2 (Unified Hierarchy).

Resolution

M&R 7.9 Elasticsearch is compatible with the default pure cgroup v2 mode found in OEL 9.x. To verify the active cgroup mode on your server, do not rely on /proc/filesystems. Instead, use the following command:

cat /proc/self/cgroup

 

Identification Criteria:

OS Version	Cgroup Mode	Expected cat /proc/self/cgroup Output
OEL 8.x	Hybrid / v1	Multiple lines mapping controllers (memory, cpu, blkio) to different IDs (e.g., 12:memory:..., 11:blkio:...).
OEL 9.x	Pure v2	A single line: 0::/user.slice/user-1004.slice/...