How to Convert Thick Provision Lazy Zeroed Disk to Thin on VMware ESXi?

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Nowadays VMware supports several virtual disk provisioning formats. These include Thin Provision, Thick Provision Lazy Zeroed, and Thick Provision Eager Zeroed. While Thick disks reserve all storage capacity upfront, Thin Provision disks consume storage only as data is written. Because of this, it is a popular choice in modern VMware environments. You can convert Thick disks to Thin to reclaim datastore space and improve storage efficiency.

Types of Virtual Disk in VMware vSphere ESXi

VMware uses three types of disks that allow you to choose the disk format option of the resulting hard disk:

Thick Provision Lazy Zeroed

With a thick provisioned virtual disk that is lazy zeroed disks, all space of the virtual disk is allocated upon creation, while the blocks are not cleared of the data that was there earlier. When the virtual machine first accesses a new block, it is cleared.

With a thick provisioned disk configured as lazy zeroed, the first time a block is accessed on such a disk, I/O performance for the clear operation is lost. The VM then accessed such a data block next time, the performance is identical to Eager zeroed Thick. Historically, Thick Provision Lazy Zeroed was commonly used as the default disk format in many VMware environments. However, in modern vSphere deployments, folks may use different provisioning formats (depending on datastore type, storage policies, and deployment templates).

Thick Provision Eager Zeroed

With thick provision eager zeroed disks, all space of such disk is allocated at the time of creation, and all storage blocks are cleared of the data that was there earlier. The advantage of such disk is performance and security, the disadvantage is the long creation time;

Thin Provision

With a thin provisioned virtual disk, these are initially created with minimum size and grow as they fill up with data to the specified vmdk file size. When a new block is filled up with the data, it is cleared beforehand. Keep in mind that Thin provisioning allows virtual disks to consume storage on demand. In current VMware vSphere 7/8 environments, thin and thick disks provide comparable performance in steady-state operations.

You may observe performance impact only during initial block allocation and metadata expansion events (it depends on underlying storage capabilities and features such as VAAI).

Note that the VM will stop working if the disk space on the datastore is exhausted and the thin disk cannot be expanded.

Ways for converting Thick to Thin Provision Disk

There are three different ways for converting Thick to Thin Provision Disk:

• Using Storage vMotion feature of a VMware vSphere;
• Using CLI (the only one way to convert disk on a standalone VMware ESXi host);
• Using VMware Converter.

Hint. Using PowerCLI, you can find out used and provisioned disk size for a specific virtual machine:

Get-VM -Name VMName1 | Select Name,@{N='Used (GB)';E={[math]::Round($_.UsedSpaceGB,2)}},@{N='Provisioned (GB)';E={[math]::Round($_.ProvisionedSpaceGB,2)}} | Format-List

VMware vSphere 8 Considerations

Important. In case the VM has active snapshots, you should consolidate/remove them before converting the disk. Keep in mind that snapshot chains can significantly complicate VMDK migration and conversion procedures.

While the process of converting Thick Provision disks to Thin Provision remains largely unchanged in VMware vSphere 8, you should consider several important factors before performing disk conversion.

Keep in mind that in modern vSphere environments, Storage vMotion is the preferred method for changing a virtual disk provisioning format. Note that this method is fully supported by VMware: it does not require VM downtime, and minimizes the risk of config/data consistency problems.

Before converting a virtual disk, you should check the following:

• No active VM snapshots exist;
• The datastore has sufficient free space to complete the conversion;
• A recent backup of the VM is available;
• VM Encryption or vSAN Encryption is not preventing disk migration or cloning operations;
• Any Storage Policies assigned to the VM are compatible with the target provisioning format.

For production workloads, we recommend using Storage vMotion whenever possible (instead of manually manipulating VMDK files from the ESXi command line).

Changing the Thick to Thin Provisioning of a Virtual Disk Using Storage VMotion

Using the Storage vMotion feature of a VMware vSphere, you can relocate any virtual machine with a disk to a different VMFS datastore (different LUN) without interruption of services and applications in a running VM. During a migration of VM disks between storages, you can also change the type of the disk.

To get the current disk provisioning type for the specific VM, you have to open its settings in vSphere Client and check the value of the disk type field. In our case, the type of virtual disk is Thick provision lazy zeroed.

To change the disk type, you should move VM files to another datastore.

Open the Virtual Machine action menu and select Migrate.

Select the migration type Change storage only.

Why Storage vMotion is the Recommended Method?

Keep in mind that Storage vMotion remains the safest and VMware-supported method for changing a virtual disk provisioning type in production environments.

Here are the key advantages of this method:

• No VM downtime is required;
• No manual VMDK manipulation is necessary;
• Existing VM configuration is preserved;
• Snapshot handling is performed automatically;
• It is fully supported in VMware vSphere 7/vSphere 8 environments.

Note that whenever Storage vMotion is available, you should prefer it over manual VMDK conversion methods. The ESXi CLI approach that we described later in this article is primarily intended for standalone ESXi hosts/environments where Storage vMotion is not available.

Now in the Select virtual disk format field, choose which format you want to convert the vmdk during Storage vMotion. Select Thin Provision as a virtual machine disk format and choose one of the available VMFS/NFS datastore on which you want to move the VM files. Click Next.

The new Relocate virtual machine task will appear in the vSphere Resent task pane. Wait until the virtual machine files will be moved to the new storage under the configured VM folder, including the virtual machine base disk and any snapshots.

Now make sure that the type of the disk is changed to Thin provision.

Tip. When migrating a virtual machine to any storage other than the VMFS file system, virtual disks are always converted to Thick format and will take up their maximum size.

Check for VM Snapshots Before Conversion

We already mentioned that you should verify if the VM does not have active snapshots before converting a virtual disk using ESXi CLI tools.

Note that when snapshots exist, VMware creates additional delta disk files (for example, diskname-000001.vmdk) and stores new writes in the snapshot chain rather than in the base VMDK. In such case, when you converting/copying only the base disk, it may result in an incomplete VM state and potential data loss.

In order to check for snapshots in vSphere Client:

1. Right-click the virtual machine;
2. Select Snapshots > Manage Snapshots;
3. Check that no active snapshots are present.

In case snapshots exist, you need to remove/consolidate them before proceeding with disk conversion.

For PowerCLI environments, you can check snapshot status with the following command:

Get-Snapshot -VM MyVM

You should continue with the conversion only after all snapshots have been removed/consolidated successfully.

How to Change Disk Provisioning Type using ESXi CLI?

The way of changing VMware virtual disk provisioning type using CLI involves copying the current vmdk file to a new thin-provisioned disk, and removing the old thick disk.

At first, you should power off your VM. To connect to the ESXi host CLI over SSH, you have to enable SSH service in the ESXi settings and allow incoming connection on TCP/22 port (open on the ESXi host: Configuration > Services). Select the SSH service and press Start.

Connect to ESXi server as root using a favorite SSH client (for example, Putty), and change directory to the vmfs/volumes:

# cd /vmfs/volumes

List the available datastores:

# ls -lh

And go to the folder, containing your VM files.

List the files containing in the directory:

# ls

After checking that the VM is powered off and no active snapshots exist, you need to convert the disk from Thick to Thin using the following command:

# vmkfstools -i w10rtm-test.vmdk -d thin w10rtm-test-thin.vmdk

Note. Before starting the conversion, you need to ensure that the datastore has enough free space to temporarily store both the original and converted VMDK files during the cloning process.

Hint. Several examples of creating disks of a certain type from ESXi CLI using the built-in vmkfstools tool:

# vmkfstools –c 40G –d thick thick.vmdk

# vmkfstools –c 40G –d zeroedthick zeroedthick.vmdk

# vmkfstools –c 40G –d eagerzeroedthick eagerzeroedthick.vmdk

Important. Note that you should not remove the original VMDK until you have verified that the converted disk is valid and it can be successfully attached to the VM.

After conversion completes, delete the original Thick disk file w10rtm-test.vmdk:

# rm w10rtm-test.vmdk

And rename the resulting Thin disk:

# mv w10rtm-test-thin.vmdk w10rtm-test.vmdk

Now using vSphere Client or vSphere Web client, open the VM settings and delete the old disk (Select Remove from virtual machine). After that add new disk: Add new device > Existing Hard Disk > Use an existing virtual disk > select file w10rtm-test.vmdk on your datastore > Next > Ok.

Now, you can power on your VM.

Legacy Method: Converting Thick to Thin Disk Using VMware Converter

Although VMware Converter is still available for certain migration cases, we recommend you using Storage vMotion and vmkfstools approaches as preferred in most modern VMware environments.

VMware Converter Standalone is a free application that can be installed on Windows to convert physical machines (P2V) or virtual machines (V2V) to VMware format.

Using VMware Converter, you can convert a VM to a new VM on the same ESXi host, but with a new disk format (you can even reduce the provisioned size of disks, which is not supported in vSphere Client).

1. Download and install VMware Converter Standalone on any Windows computer. Run the conversion wizard;
2. Specify that vSphere Virtual Machine needs to be converted;
3. Specify vCenter/ESXi hostname and credentials to access it;
4. At the Source Data step, you can specify the new size of the virtual disks and the disk provisioning type. Choose Thin;
5. The converter will create a new VM with disks of the required size. Copy the data and reduce the size of the guest OS file system partition.