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Using small magnetic disks for vSAN might result in VM failures (2080503)

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Details

  • When the physical magnetic disks used for vSAN are smaller than 255 GB, virtual machines that run on vSAN datastores might experience disk space issues.
  • You see 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 magnetic disks that are smaller than 255 GB, these storage components might end up to be larger than actual underlying magnetic disks. As a result, virtual machines associated with these storage components experience unpredictable behavior and might fail.

Solution

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 Manage tab and click Settings.
  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 magnetic disk that you use in the vSAN cluster.

    For example, if the smallest magnetic disk in 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 180GB (which is 90% 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 2TB 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 is 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 vSAN datastore. However, future deployments that do not use object space reservation policy are thin provisioned and their objects utilize maximum component size defined by the VSAN.ClomMaxComponentSizeGB option.

Update History

07/04/2016 - Added vSAN 6.2 to Products. 04/18/2017 - Added vSAN 6.6 to Products

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