Change the vSAN storage policy on VMs to save space

Products

• VMware vSAN

Issue/Introduction

In certain situations there may be a need to convert VMs on a vSAN datastore to a different storage policy to save on used space.  If the vSAN datastore is out of space or too close to running out of space, then this storage policy change may not be possible. 

Environment

VMware vSAN (All Versions)

Cause

When VM disk consumption or new VMs are added to a cluster due to normal growth, the available space of the vSAN datastore could diminish.  As normal growth happens, additional storage should be added to the vSAN datastore to compensate for the additional usage if needed.  If this is not done, sometimes the vSAN datastore can trigger the vSAN usage alarm.  To avoid issues, additional space should be added.  If this is not readily possible, then the storage policy of the VMs can be changed to another that might save space.

Resolution

• If running RAID-1, you can convert VMs to RAID-5 or RAID-6 to save space.
• If running RAID-5, you can convert VMs to RAID-6 to save space.
• There is typically low impact on running VMs as the resync is a low priority background task.  
• This VM storage policy conversion needs be done methodically instead of all at once to diminish the extra processing and to lower the amount of required overhead space.
• Convert the smaller VMs first in small groups or one at a time.  As more VMs are converted, more space savings will be seen.  
• Once completing VM storage policy conversions and moving on to the larger VMs, there will be more available space with which to complete the larger conversions.
• The extra space is required as both object layouts (original policy and new policy) are active until the full cutover to the new policy is completed.
• After all the VMs are converted from one RAID to another RAID, the overall space savings can be viewed in Cluster > Monitor > vSAN Capacity.
• There should be no VM impact, provided the policy change is done in small groups of VMs and not to large groups of VMs or the entire set of production VMs at once.
  Note: For latency sensitive applications, like databases, it's best to keep these VMs on a RAID-1 policy as there is less overhead
• There is a minimum number of nodes required when converting to RAID-5 and RAID-6. However, for resiliency, implementing required+1 number of nodes will help during maintenance situations.
• Ensure there is available space for the storage policy conversion prior to attempting the storage policy change.

Additional Information

For additional information, see the following tech doc:

Using vSAN Policies 

 

Comparing the Space Efficiency and Host count Requirements below: