vSAN 6.7: Reclaim Disk Space by Converting Thick-Provisioned VMs to Thin on vSAN

First we need to know that objects on vSAN are all thinly provisioned by default and there are not thick disks. Instead, vSAN has the concept of Space Reservation, defined by the Storage-Policy ruleset Object Space Reservation (OSR).

In my case after migration of a virtual machine the space was totally reserved even if the Default Storage-Policy ruleset has been set on thin.

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And also after migrating the virtual machines show the disks as thick provisioning.

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And if you see the VM Storage Policy there is still space to reclaim.

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Then, we need to force OSR rule to Thin. Despite the attached storage policy has already been configured. The only method to force Thin and convert the disk to Thin is to reapply a different storage policy, this method triggers vSAN to reapply the OSR ruleset.

Follow the steps below to force and apply the OSR to Thin.

1 – Clone the current VM Storage policy (vSAN Raid5) and validate that OSR is set to Thin.

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2 – Apply the Storage policy to the Virtual machine.

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You should notice Object Reserved disappeared in vSAN Capacity.

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After about 10 – 15 minutes you see that the Storage Usage decreased.

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3 – Finally when you configured the VM Storage Policy back and there is no space to reclaim.

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Hoping you have enjoyed this post, see you next time.


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