vSAN host fails to enter Maintenance Mode with 'Full data migration' option selected

Products

• VMware vSAN

Issue/Introduction

Awareness of vSAN cluster requirements.

Symptoms:
A host will fail to enter maintenance mode using Full Data Evacuation if the cluster is not able to meet the minimum number of fault domains that the storage policy requires which is N+1.

Environment

VMware vSAN 8.0.x
VMware vSAN 7.0.x
VMware vSAN 6.x

Cause

vSAN requires a certain number of hosts to be active with disk groups contributing capacity and resources in vSAN in order to provide fault tolerance. If the requirements cannot be met, vSAN will fail the pre-check it performs when placing a host into maintenance mode. 

See the below documentation for further information:
Managing Fault Domains in Virtual SAN Clusters
Design and Operation Considerations When Using vSAN Fault Domains
Fault Domains

When entering Maintenance Mode a "What-if" scenario will be run. If the result is, that after entering Maintenance Mode only not enough Fault Domains will be available the Full Data migration will fail. 

The clomd.log will provide the related error message:
LOM_CheckClusterResourcesForPolicy: Not enough Upper FD's available. Available: 3, needed: 4
LOM_CheckClusterResourcesForPolicy: Not enough Upper FD's available. Available: 4, needed: 5

Resolution

If full data migration is needed, then a 5th host for RAID5 or 7th host for RAID6 is required to be added to the vSAN cluster first.
* Otherwise the Storage Policy can be changed to FTT=1,FTM=RAID1

Please be aware that during the change from RAID5 or RAID6 to RAID1, new components will be created first and the old ones deleted once the rebuild has completed. Make sure that there is sufficient space in the cluster before proceeding.

Workaround:
Select Maintenance Mode 'Ensure Accessibility' option - note that all data using RAID5/6 Storage Policies will be in a reduced redundancy state until the host has exited Maintenance Mode and the data resynced back to compliance.

Additional Information

A Closer Look at vSAN Maintenance Mode
Place a Member of vSAN Cluster in Maintenance Mode