VMware ESXi and vSphere Cluster Management
How to Expand a VMFS Datastore in VMware ESXi
Learn how to expand an existing VMFS datastore into unused space on its backing LUN using the vSphere Client, then validate the result and troubleshoot common issues.
Expanding a VMFS datastore increases the storage container available to ESXi hosts and virtual machines. This procedure uses unallocated, contiguous capacity on the datastore's existing backing device or LUN. The operation can normally be performed while the datastore is online, but it should still follow your organization's maintenance, backup, and change-control procedures.
This lesson covers expansion into free space on the current LUN. It does not cover adding a separate datastore extent, shrinking a datastore, increasing a virtual disk, or extending a filesystem inside a guest operating system.
What VMFS Datastore Expansion Does
VMFS is the VMware Virtual Machine File System used by ESXi hosts. A VMFS datastore is a storage container formatted with VMFS that stores virtual machine files, templates, ISO images, and related data.
During an expansion, ESXi extends the existing VMFS partition and filesystem into unused capacity located after the current partition on the same backing device. The underlying LUN must already be larger, or the device must already contain suitable unallocated space.
A LUN is a logical storage device presented to ESXi, commonly by a SAN or storage array. The backing device is the physical or logical device on which the datastore resides. Unallocated space is capacity on that device that is not assigned to the current VMFS partition.
Prerequisites and Conditions
- Confirm that the target storage is a VMFS datastore, not an NFS datastore or another storage type.
- Identify the datastore's backing device, LUN identifier, and current capacity.
- Confirm that the LUN was enlarged on the SAN or storage array, or that usable unallocated space already exists beyond the VMFS partition.
- Ensure the additional space is contiguous and available for extending the existing partition. Free capacity elsewhere on a device may not be usable by the wizard.
- If the LUN was enlarged, rescan the ESXi storage adapters so the host detects the new device size.
- Verify the new capacity is visible to the host before opening the datastore expansion wizard.
- Have permissions to manage datastore capacity. If necessary, review assigning permissions.
- Check storage-array health, path availability, multipathing, and LUN presentation.
- Confirm a recent backup or a tested recovery plan before changing the storage layout.
Pre-expansion checklist
| Check | Why it matters | How to verify |
|---|---|---|
| Underlying LUN has been expanded or has unallocated space | The datastore can grow only into capacity available on its backing device. | Check the storage array and the device details in vSphere. |
| ESXi detects the new device capacity | The wizard cannot use capacity that ESXi has not discovered. | Rescan storage adapters and inspect the device size. |
| Correct datastore and backing device identified | Similar datastore or LUN names can lead to an incorrect change. | Compare the datastore name, UUID, device identifier, and capacity. |
| Storage health and multipath status are normal | Stable paths reduce the risk of a storage interruption during the change. | Review path state, connectivity, and array alerts. |
| Expected capacity increase documented | It provides a baseline for validating the completed task. | Record the current and expected capacities before starting. |
Rescan Storage After Increasing a LUN
When a storage team enlarges a SAN LUN, ESXi may continue to display the old device size until it performs a storage rescan. A storage rescan redetects storage devices, paths, and capacity changes.
- Select the relevant ESXi host in the vSphere Client.
- Open the host's storage view and start a rescan of the storage adapters or storage devices.
- Wait for the task to finish, then inspect the backing device and confirm that its capacity reflects the enlarged LUN.
- For a shared datastore, repeat the verification on every host that mounts the datastore. Rescan affected hosts when their displayed device size remains unchanged.
The exact labels can vary between vSphere releases. The important result is that the correct host sees the larger backing device before you start datastore expansion.
Locate the VMFS Datastore in the vSphere Client
- Select the relevant ESXi host in the inventory.
- Open the host's Related Objects view.
- Select Datastores.
- Select the VMFS datastore whose capacity you want to increase.
- Open the action to Increase datastore capacity.
The navigation path is: Select ESXi host > Related Objects > Datastores > select VMFS datastore > Increase datastore capacity.
Use the Increase Datastore Capacity Wizard
- Start the Increase Datastore Capacity workflow for the selected datastore.
- Confirm that the wizard selected the intended datastore.
- Review the backing device or LUN shown by the wizard. Compare its identifier and capacity with your pre-change notes.
- Continue to Partition Configuration.
- Choose the option that uses the displayed free space on the backing device. This option may be described as using available space or expanding into free space, depending on the vSphere version.
- Review the amount of capacity to be added. The exact value varies by environment and represents usable unallocated LUN capacity available to the partition.
- Finish the wizard and wait for the task to complete. ESXi extends the VMFS partition and filesystem into the selected space.
Practical Expansion Examples
SAN-backed datastore after a LUN increase
- A VMFS datastore currently has 500 GB on a SAN LUN.
- The storage team expands that LUN from 500 GB to 1 TB.
- The administrator rescans storage and confirms that ESXi sees the larger device.
- The administrator opens the existing datastore's capacity wizard and selects the available free space in Partition Configuration.
- After the task completes, the datastore capacity is approximately 1 TB, subject to storage and filesystem measurement differences.
Datastore with pre-existing unused space
A backing device may already contain unused space beyond the current VMFS partition. If the increase-capacity wizard offers that space, selecting the free-space option extends the existing datastore without creating a new datastore.
Virtual machine reports low disk space
Suppose a virtual machine reports low space in its guest filesystem, but the VMFS datastore still has ample free capacity. Expanding the datastore is unnecessary. The virtual disk may need to be increased, followed by an appropriate filesystem expansion inside Windows or Linux. Conversely, if the datastore itself is nearly full, VMFS expansion may be appropriate, but it still will not change the guest's disk or filesystem size.
Validate the Expansion
- Refresh the vSphere Client or close and reopen the datastore details.
- Confirm that datastore capacity increased by the expected amount.
- Check the remaining datastore free space and compare it with the planned result.
- Verify that existing virtual machines, virtual disks, templates, and other files remain accessible.
- If the datastore is shared, inspect it from the other hosts that mount it. Confirm consistent capacity reporting and investigate any host that still shows the old size.
- Review recent tasks and events for warnings or errors.
Datastore Expansion Versus Other Storage Operations
| Operation | What it changes | When to use it |
|---|---|---|
| Expand VMFS datastore | Extends the existing VMFS partition and filesystem on its current LUN. | The LUN has suitable unused capacity and the datastore should become larger. |
| Increase underlying LUN capacity | Changes the size of the storage device presented by the array or SAN. | The storage device itself must be enlarged before ESXi can use the new space. |
| Increase virtual disk size | Increases a VM's virtual disk capacity. | The guest needs more disk capacity, regardless of datastore size. |
| Extend guest operating system filesystem | Expands the filesystem recognized inside Windows or Linux. | A virtual disk has been enlarged and the guest must use the additional space. |
| Create a new datastore | Creates a separate storage container, usually on a separate device or partition. | The existing datastore cannot or should not be expanded. |
Operational Cautions and Limitations
- Online expansion is generally supported, but storage changes should still be scheduled and governed by change control.
- Expansion consumes available space. Treat the completed change as permanent; the same wizard does not provide a datastore shrink operation.
- Adding a separate datastore extent is different from extending the existing partition. An extent is a storage-device partition added to a VMFS datastore. This lesson focuses on adjacent free space on the current LUN, not extent management.
- Do not assume that a larger LUN is visible to every host. Shared-storage presentation, zoning, masking, and paths must be consistent.
- Do not confuse datastore free space with guest filesystem free space.
Troubleshooting
The free-space option does not appear
Common causes include an unexpanded LUN, a missing storage rescan, inconsistent visibility across storage paths, or free space that is not suitable for extending the existing VMFS partition.
- Confirm the LUN size on the storage system.
- Rescan ESXi storage adapters and verify the device capacity.
- Check path status, LUN masking, zoning, and storage presentation.
- Review the device's partition layout before making further changes.
Capacity does not change after the wizard completes
The task may have failed, the interface may show stale inventory data, or the wrong datastore or device may have been selected.
- Review recent tasks and events for errors.
- Refresh datastore details and verify the backing-device size.
- Confirm the datastore name, UUID, device identifier, and LUN before retrying.
One host sees the new size but another does not
The affected host may not have rescanned storage, the expanded LUN may not be presented consistently, or a path or connectivity problem may exist.
- Rescan storage on the affected host.
- Verify LUN masking, zoning, and presentation on every host.
- Check multipathing and connectivity to the storage array.
The virtual machine still reports low disk space
The shortage is probably inside the guest operating system, or the virtual disk has not been increased. Check datastore free space separately from guest filesystem usage. If required, increase the virtual disk and then extend the guest filesystem using the appropriate Windows or Linux procedure.
Exam-Relevant Notes
- VMFS expansion uses unallocated capacity on the existing backing device or LUN.
- A LUN expansion is a storage-array operation; an ESXi rescan makes the changed device capacity visible to the host.
- The vSphere workflow uses Partition Configuration to select available free space.
- Datastore expansion is not the same as increasing a virtual disk or extending a guest filesystem.
- An extent is separate from extending the existing VMFS partition into adjacent free space.
For broader background, see the VMware ESXi online course and basic ESXi configuration topics.