vSAN Cluster constantly resyncing due to Automatic rebalance activity

Products

• VMware vSAN 6.x
• VMware vSAN 7.x
• VMware vSAN 8.x

Issue/Introduction

Symptoms:

• You see resyncing activity due to automatic rebalancing in Cluster -> Monitor -> vSAN -> Resyncing Objects: 

• You see the the resyncing activity is related to a specific host or subset of hosts in the cluster. 
• The Bytes left to resync may reduce over time, but you observe that additional rebalance activity is later added, so it never reaches 0.

Environment

• VMware vSAN 8.x
• VMware vSAN 7.x
• VMware vSAN 6.x

Cause

• The occurs in primarily 2 situations:

1. 1. This occurs due to misconfiguration of the disk groups. 
   2. There is a disk failure(s) that is reducing the available size of one of more disk groups/pools. 
       

• In order for Automatic Rebalance to perform best, the disk groups/pools in the cluster should be of a consistent size within and across all hosts in the cluster.
  
  
  

Validation:

• You can check the disk group health as follows:   Choose vSAN Cluster ->  Configure -> vSAN - Disk Management 
  
  
  • If any groups/pools are unhealthy, it will display an unhealthy status for the host with the problem disk(s): 
    
    

 

...then:  Choose the Host -> View Disks to display the detail of the disks contained in the disk group (to both identify disk sizes and/or absent/failed disks):

Resolution

Workaround:

• • To avoid the constant resync activity in the the short term, disable Automatic Rebalance until you are able to remediate the inconsistent disk groups and/or failed disks. See TechDocs: Configure Automatic Rebalance in vSAN Cluster

Final Resolution:

• Replace any failed/absent disks, as needed. See KBs:
  • Troubleshooting vSAN OSA disk issues
  • Disk failure in vSAN ESA Cluster
  • The vSAN disk appears as Absent in vSAN Disk Management
    
    
• Reconfigure the disk groups across the vSAN cluster, as needed, so that the disk groups have consistent sizes within and across all hosts in the cluster. See TechDocs:
  
  • • Remove Disk Groups or Devices from vSAN
    • Create a Disk Group or Storage Pool in vSAN Cluster

Additional Information