Using small disk sizes for vSAN might result in VM failures

Products

• VMware vSAN

Issue/Introduction

• When the physical disks used for vSAN are smaller than 255 GB, virtual machines that run on vSAN datastores might experience disk space issues.
• You see an entry similar to:
  
  There is no more space for virtual disk XX. You might be able to continue this session by freeing disk space on the relevant volume and clicking retry.
  
  Note: When vSAN stores virtual machine objects, it creates components whose default size does not exceed 255 GB. However, when you use disks that are smaller than 255 GB, these storage components might end up being larger than the actual underlying disks. As a result, virtual machines associated with these storage components experience unpredictable behavior and might fail.

Environment

VMware vSAN 6.1.x
VMware vSAN 7.0.x
VMware vSAN 6.0.x
VMware vSAN 5.5.x
VMware vSAN 6.2.x
VMware vSAN 6.5.x
VMware vSAN 8.0.x
VMware vSAN 6.6.x

Resolution

To avoid this issue, adjust the default value of the ClomMaxComponentSizeGB parameter for each host in the vSAN cluster.

1. Browse to the host in the vSphere Web Client navigator.
2. Click the Configure tab.
3. Under System, click Advanced System Settings.
4. Select the VSAN.ClomMaxComponentSizeGB option and click Edit.
5. Change the default value, so that it does not exceed 80% of the smallest disk that you use in the vSAN cluster.
   
   For example, if the smallest disk in the vSAN disk group is 200 GB and you expect virtual machine objects (e.g., vmdks) to grow to 500 GB, adjust VSAN.ClomMaxComponentSizeGB option to 160GB (which is 80% of 200GB).

Notes:

• You must adjust the value of VSAN.ClomMaxComponentSizeGB before any virtual machines are deployed in the vSAN cluster. You should not change this value after virtual machines are provisioned.
• VSAN.ClomMaxCompnentSizeGB value can affect the size of an object deployed in the vSAN cluster. The actual size of the object provisioned is calculated according to this formula:
  
  Maximum object size = MAX (2TB, 12*VSAN.ClomMaxComponentSizeGB)
  
  The maximum object size is 12 times the value of VSAN.ClomMaxComponentSizeGB but not to exceed  62 TB which is the maximum file size supported by vSAN datastore.

What if I had already deployed virtual machines to the vSAN cluster before I made this change?
 

You can preserve object space for the previously deployed objects by applying a storage policy with Object Space Reservation set to 100% with these considerations:

• For virtual machines with single VMDKs, apply the policy to one virtual machine at a time
• For virtual machines with multiple VMDKs, apply the policy to one VMDK at a time

Note: There are no benefits from the default thin provisioning of the vSAN datastore. However, future deployments that do not use the object space reservation policy are thin provisioned, and their objects utilize the maximum component size defined by the VSAN.ClomMaxComponentSizeGB option.