vSAN -- Using small disk sizes for vSAN might result in VM failures / vSAN Health Alarm: "vSAN Maximum Component Size"

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

One or both of the following Symptoms are observed:

 

• In the vSphere Client (= Web Client) you observe the following error in relation to one or more VMs:

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.

 

 

• vSAN Health Alarm: "vSAN Maximum Component Size"

 

 

Environment

VMware vSAN (All Versions)

Resolution

Adjust the default value of the VSAN.ClomMaxComponentSizeGB Parameter for each host in the vSAN cluster via:

1.) vSphere Client: Navigate to the vSAN Host

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 of VSAN.ClomMaxComponentSizeGB, so that it does not exceed 80% of the smallest Disk used in the vSAN Cluster

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 on the vSAN Cluster. The actual size of the Object provisioned is calculated according to this formula:
  
  

  Maximum object size = MAX (2TB, 12x VSAN.ClomMaxComponentSizeGB)

  
  The maximum object size is 12 times the value of VSAN.ClomMaxComponentSizeGB but cannot exceed  62 TB which is the maximum File size supported by vSAN Datastore.

 

 

What if I have 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.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Additional Information

Object Space Reservation Definition

Define a Storage Policy for vSAN Using vSphere Client