vSAN Datastore Capacity Increase Due to Reactive Rebalancing - VMware vSAN

Products

• VMware vSAN

Issue/Introduction

Introduction:

This article describes transient vSAN datastore capacity increases caused by vSAN Reactive Rebalancing in environments running VMware vSAN 8.0.x.

Symptoms:

• Abnormal vSAN datastore space consumption (e.g., a >5% unexpected capacity spike).
• Capacity increase occurs rapidly without significant VM growth, snapshot creation, or infrastructure changes.
• Skyline Health actively displays a "Storage space" warning.
• ESXi /var/log/clomd.log entries indicate active REACTIVE_REBALANCE tasks and report high transiantCapGenerated values.
  

  clomd.log

  2026-08-13T03:38:59.536Z No(29) clomd[2100441]: [Originator@6876 opID=########] CLOMReconfigure: Reconfiguring 65db7b6a-####-####-####-######### workItem type REACTIVE_REBALANCE
2026-08-13T03:38:59.544Z No(29) clomd[2100441]: [Originator@6876 opID=########] CLOMReconfigure: exit: obj 65db7b6a-####-####-####-######### transiantCapGenerated - total: 226307866624, site1: 0, site2: 0, workItem type REACTIVE_REBALANCE configDelay 0 newConfigGenerated 1 newCompWitnessOnly 0 status Success

Environment

VMware vSAN 8.x

Cause

• vSAN Reactive Rebalancing is an automated, self-healing mechanism that triggers when individual datastore capacity devices (disks) near the 80% utilization threshold.
• To safely move data off highly utilized disks, vSAN utilizes a "make-before-break" approach. It builds the new data component on a destination disk before deleting the original copy. During this transfer window, both copies consume space, generating transient capacity overhead.
• Additionally, vSAN may reconfigure these objects into smaller concatenations to fit the fragmented free space, which generates new file system metadata overhead as the data is moved.

Resolution

 

1. Monitor: Use the vSphere Client to track active data movement (Cluster > Monitor > vSAN > Resyncing Objects) and transient capacity allocation (Cluster > Monitor > vSAN > Capacity, expanding the Operations space and System overhead metrics). Alternatively, use the vSAN Resync Dashboard (RVC vsan.resync_dashboard).
2. Verify Cleanup: Inspect the /var/log/clomd.log for successful CLEANUP task completion, which confirms the automatic reclamation of transient space once the data transfer finishes.
3. Proactive Management: Maintain overall cluster utilization below 70% to prevent uneven disk distribution and automated reactive rebalancing.
4. Scaling: If overall cluster utilization frequently exceeds the 70% threshold, evaluate cluster capacity and consider expanding cluster resources (adding disks or nodes).