vSAN -- Enabled: Deduplication & Compression -- Low Ratio / Object Space Reservation at 100%

Products

• VMware vSAN

Issue/Introduction

Symptoms:
Deduplication/Compression ratio can be low when thick objects are being used.

Environment

VMware vSAN 6.x

Cause

• The objects in vSAN are applied policy with 100% Object Space Reservation (OSR).
• The UI the storage values indicate the deduplication/compression did not occur and we mark the space as used.
• This is by design. When OSR is 100%, all of the space is reserved for that VM alone and dedupe/compression will not help as other VMs are not allowed to use this space.
• In many cases the OSR =100 VMDK are overwritten with zero by vmkfstools -w "paht to vmdk" to ensure all the space is consumed and cleared. 

Resolution

These issues has been resolved in 7.0 where the space utilization in OSR=100 is considered as thick. 

However , if you want  space saving on a disk you may need to change the storage policy back to OSR=0 which will free up the unused free space reserved by OSR=100 

The deduplication/compression ratio will increase because we drop the reservation, and report on the actual physical (possibly deduped) usage.

See here for definition of "Object Space Reservation":
https://docs.vmware.com/en/VMware-vSphere/6.7/com.vmware.vsphere.virtualsan.doc/GUID-08911FD3-2462-4C1C-AE81-0D4DBC8F7990.html

Object Space Reservation is changed in the assigned Policy by clicking "Edit":
HTML Client:
https://docs.vmware.com/en/VMware-vSphere/6.7/com.vmware.vsphere.virtualsan.doc/GUID-9A3650CE-36AA-459F-BC9F-D6D6DAAA9EB9.html

Flash Client:
https://docs.vmware.com/en/VMware-vSphere/6.7/com.vmware.vsphere.virtualsan.doc/GUID-EE705FDA-EE34-481E-A84D-1F900AC25FD1.html
(Via clicking "Add rule" Object Space Reservation is being added)