Extend an Eager Zeroed Thick Disk for MS Cluster

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In this blog, we will take a look at the options available for us to expand a disk on a Microsoft Cluster Server on a cluster-in-box config (ie: Both clustered VMs are running on the same ESXi host)

Disks participating in Microsoft Cluster will be configured in Thick Provisioning Eager Zeroed mode which means it allocates the space on the datastore and zeroed all the bits claimed.

The recommended procedure is to extend via CLI but you could do this via GUI as well and later do a Storage vMotion before attempting to power up the VM – CLI being recommended as always.

Expansion of an MS Clustered VM Disk via GUI

By default and the support configuration of MS Clusters is disks will be carved out Thick Provisioning Eager Zeroed mode so you could be hit with a major issue if you stick the UI based on extending a VMDK file. These disks once extended via the UI will covert themselves to Thick Provisioned Lazy Zeroed mode which could break the Microsoft cluster and probably will result in the VM not being able to power on.

The expansion of a Clustered VM Disk that typically will be configured in Thick Provisioning Eager Zeroed is done via the CLI and we will need to shut the VM’s down to do this work but here is what happens when you do the expansion from GUI.

Step 1 – Poweroff both the clustered VM’s

Step 2 – Extend the disk from GUI

As per this example, I’ve got two file servers configured with shared datastore disks being mapped across in physical mode in a Thick Provisioning Eager Zeroed mode configuration.

Step 3 – Disk extended

Step 3 – Disk changes state to Thick Provisioning Lazy Zeroed mode

The disk is now seen mapped across in physical mode in a Thick Provisioning Lazy Zeroed mode configuration. The Z suffixed to the VMDK would mean it’s a lazy zeroed disk which will prevent the VM from coming back online.

Step 4 – Perform a Storage vMotion and in that process convert the disk from Lazy Zeroed to Eager Zeroed if you got into this situation. Until that process is complete, we won’t be able to powerup the VM.

Step 5 – Powerup the VM

Expansion of an MS Clustered VM Disk via CLI

use vmkfstools -X <newsize> -d eagerzeroedthick <vmdkfile>

Here we will extend our disk from 4GB to 5GB

Verify the disk change and it shows its Thick Provisioning Eager Zeroed mode configuration.

Finally, power up the VMs.

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