vSAN objects are Raid0 when Raid5 policy is assigned.

Products

• VMware vSAN

Issue/Introduction

Objects are assigned a Raid5 policy. However, they are running in a Raid0 state with no redundancy. 

- Maintenance mode is allowed to surpass FTT threshold past Raid5 requirements. 

- Can run vSAN cluster with Raid5 with only two hosts actively participating in the cluster. 

- Can place multiple hosts into ensure accessibility maintenance mode, even though normally the policy cannot tolerate it. 

- Objects will not be compliant with policy, if checking VM compliancy. 

 

Scenario: Inaccessible objects may occur after a single failure event (even though policy has Failures To Tolerate 1).

Environment

vSAN 7.x

vSAN 8.x

Cause

When force provisioning is turned on for the vSAN policy. This will allow vSAN to provision out objects or re-provision in a way to ensure they can maintain accessibility, regardless of the normal policy restrictions. 

• This includes, converting Raid5 to Raid0, if not enough hosts are present in order to meet a Raid5 policy. 

 

From the vSAN design guide: 

Force Provisioning

• The Force provisioning policy allows vSAN to violate the NumberOfFailuresToTolerate (FTT) , NumberOfDiskStripesPerObject (SW)
  and FlashReadCacheReservation (FRCR) policy settings during the initial deployment of a virtual machine.
  vSAN will attempt to find a placement that meets all requirements. If it cannot, it will attempt a much simpler placement with
  requirements reduced to FTT=0, SW=1, FRCR=0. This means vSAN will attempt to create an object with just a single mirror. Any
  ObjectSpaceReservation (OSR) policy setting is still honored.

 

Caution when using Force Provisioning : Another special consideration relates to entering Maintenance Mode in full data migration mode, as well as disk/disk group removal with data migration that was introduced in vSAN 6.0. If an object is currently non-compliant due to force
provisioning (either because initial placement or policy reconfiguration could not satisfy the policy requirements), then "Full data
evacuation" of such an object will actually behave like "Ensure Accessibility", i.e. the evacuation will allow the object to have
reduced availability, exposing it a higher risk. This is an important consideration when using force provisioning, and only applies
for non-compliant objects.

Resolution

Force provisioning option for the policy, should be used in certain use cases. To ensure actions are not done to convert objects into Raid0 layout. 

• vSAN will not let you perform an action that will cause inaccessible objects. However, objects in Raid0 are vulnerable to data loss if a failure were to occur when in a Raid0 layout due to having no redundancy. 

 

If Force Provisioning is used, it should only be used in certain circumstances where it is needed. This is to avoid accidental situations, where force provisioning would allow you to bypass the normal safety features in place to keep certain scenario's from happening. 

Additional Information

vSAN Design Guide.

• Page 36. (Force Provisioning)