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Yes—but the supported method depends on the source platform. For VMware, physical servers, and Azure-to-Azure replication, you can increase a protected source disk before failover without disabling ASR. Hyper-V does not support changing a disk while replication is active: disable replication, resize the disk, then enable replication again. In-place shrinking is not supported as an ASR resize operation.
ASR normally carries the source disk configuration into the replica. It does not provide a general “make the target disk this size” option before failover. To enlarge a disk safely, resize the virtual disk, expand its guest partition and filesystem, wait for a post-change recovery point, and then fail over. You can also enlarge the Azure managed disk after failover when the target needs a different size.
Choose the correct ASR path first
| Source | Increase while protected? | Decrease while protected? | Procedure |
|---|---|---|---|
| VMware to Azure | Yes | No | Expand the source disk, expand the guest volume, allow replication to catch up, then fail over. |
| Physical server to Azure | Yes, under the VMware/physical support rules | No | Expand the source disk and wait for a recovery point containing the change. |
| Azure-to-Azure | Yes | No | Increase the source managed disk before failover; replication need not be disabled and re-enabled. |
| Hyper-V to Azure | No, not during active replication | No | Disable replication, resize the disk, re-enable replication, and complete a new initial synchronization. |
Check the applicable VMware/physical support matrix, Azure-to-Azure support matrix, or Hyper-V support matrix for your ASR architecture and Mobility Service version.
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Understand the three disk-size layers
- Virtual disk: the provisioned VMDK, VHD/VHDX, or Azure managed disk capacity.
- Partition: the operating-system partition that must be extended into the new unallocated space.
- Filesystem: NTFS, XFS, ext4, or another filesystem that must be grown before applications can use the capacity.
Increasing only the virtual disk does not necessarily increase usable space in Windows or Linux. ASR replicates the source disk state; it does not automatically perform every partition or filesystem operation.
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Safe sequence for supported online expansion
- Confirm that ASR is healthy and record the current recovery point.
- Expand the source virtual or managed disk upward.
- Rescan storage in the guest and expand the partition and filesystem.
- Verify the guest operating system reports the intended capacity.
- Wait for ASR to synchronize the changed disk metadata and blocks.
- Confirm a recovery point created after the resize.
- Perform a test failover for an important workload.
- Fail over using that later recovery point, then validate the disk and applications in Azure.
Do not select an older recovery point and expect it to contain a later resize. For Azure-to-Azure, changes made to the VM after failover are not captured through the normal protected-source path, which can affect failback planning.
VMware or physical server to Azure
Microsoft supports increasing a source disk during VMware replication to Azure; ASR does not need to be disabled and re-enabled. Shrinking is unsupported. VMware documentation also states that the target disk size is based on the source disk size, so there is no general ASR control for assigning an arbitrary larger target before failover.
- In vCenter (or your authorized VMware management tool), record the VM, disk/controller location, current size, target size, and whether the disk is for the OS or data.
- Expand the VMDK.
- Rescan the disk inside Windows or Linux.
- Extend the partition and filesystem using the procedures below.
- In the Azure portal, open Recovery Services vault > Replicated items > the VM and monitor replication health.
- Wait for a recovery point made after the expansion before test or production failover.
For VMware replication, recovery-point disks can appear with the asrseeddisk naming pattern. If source churn exceeds replica capacity, follow Microsoft’s replication troubleshooting guidance rather than repeatedly resizing disks.
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Hyper-V to Azure
A replicated Hyper-V disk must not be resized while replication is active. Disabling and recreating protection introduces synchronization time and operational risk, so record the existing configuration first:
- Recovery Services vault and replication policy
- Target region, network mappings, and target disk type
- Replication groups and consistency settings
- Disable replication for the VM.
- Expand the VHD or VHDX in Hyper-V.
- Expand the partition and filesystem inside the guest.
- Re-enable replication with the recorded settings.
- Monitor the new initial synchronization until the protected item is healthy.
- Perform a test failover before production cutover.
Azure-to-Azure replication
Increase the source Azure managed disk before failover. Depending on the disk and VM type, Azure may require the VM to be stopped or deallocated. Then expand the guest partition and filesystem, confirm replication health, and wait for a recovery point containing the new configuration. ASR normally creates replica managed disks using the source disk configuration and storage type; consult the Azure-to-Azure enablement documentation for current limitations.
The cited support matrix lists a 4,095-GiB maximum OS disk for Azure-to-Azure. It also states that caching is not supported for disks of 4 TB or larger. Treat these as architecture-specific limits and verify current Azure disk guidance for your disk type.
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Expand the guest volume
Windows
After the virtual disk is larger, open Disk Management, rescan disks if necessary, right-click the intended partition, and choose Extend Volume. The unallocated space must be contiguous and on the same disk. For automation:
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list volume
select volume <volume-number>
extend
exitReplace the placeholder with the correct volume. A recovery or other partition between the volume and free space can prevent extension through the built-in tools.
Linux
Identify the actual device and partition before changing anything:
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lsblk
sudo fdisk -lFor a common partition layout using growpart:
sudo growpart /dev/sdX <partition-number>Then grow the filesystem:
# XFS
sudo xfs_growfs <mount-point>
# ext4
sudo resize2fs /dev/sdX<partition-number>Do not copy /dev/sdX literally. LVM, RAID, encryption, and clustered storage add layers that must be expanded in the correct order. Take a backup or snapshot and verify the filesystem type first.
Resize the Azure managed disk after failover
Use post-failover resizing when the target should differ from the source, the source must remain unchanged, Hyper-V protection cannot be conveniently rebuilt, or the VM has already been failed over.
- Fail over and confirm the Azure VM is stopped or deallocated if required.
- Open Virtual machines > Disks.
- Select the OS or data managed disk.
- Choose Size + performance, select a larger supported size, and save.
- Rescan storage and expand the guest partition and filesystem.
- Start the VM and verify usable capacity, mounts, drive letters, and applications.
Azure managed disks can generally be expanded but not shrunk in place. Down-sizing normally requires creating a smaller disk and copying or restoring data into it. A disk enlarged after failover should be treated as a separate failback design decision, not something ASR will automatically mirror back.
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Verify that ASR captured the resize
- Replication health is Healthy, with no critical synchronization error.
- The replicated-item disk list shows the expected disk.
- A recovery point timestamp is later than the resize.
- Test failover creates the intended disk size.
- The guest sees and can use the additional capacity.
- Boot volumes, drive letters, mount points, database paths, and application services work normally.
Troubleshooting
The failed-over disk still has its old size
Check the source virtual disk size, guest-visible size, ASR health, and selected recovery-point timestamp. If the recovery point predates the resize, wait for a new one and test again. If the source type does not support online resizing, use its documented disable/re-enable workflow. You can enlarge the Azure managed disk after failover if necessary.
Replication becomes unhealthy
Stop making storage changes and inspect the affected disk and error. A resize can coincide with high write churn, throughput pressure, or an offline disk change that triggers full resynchronization. Follow Microsoft’s churn and resynchronization guidance; increase a replica disk only when that documentation identifies it as necessary.
The filesystem will not expand
Inspect the complete storage stack with lsblk, blkid, pvs, vgs, and lvs on Linux, or Disk Management and diskpart on Windows. The free space may not be adjacent, the wrong partition may have been selected, or LVM/RAID/Storage Spaces/encryption may require additional steps. Never force a filesystem command that does not match the actual filesystem.
Resize, add a disk, or choose another migration method?
- Resize before failover when preserving the source layout is important and there is time for synchronization.
- Resize after failover when the target needs a different capacity or the source must remain unchanged.
- Add a new Azure data disk when only extra data capacity is needed; this can be safer than altering an OS or database volume, but applications may need reconfiguration.
- Use Azure Migrate or a data-copy/backup-restore approach when you need estate assessment or a clean one-time migration rather than continuous ASR replication. The workflows and supported scenarios differ.
A larger disk does not automatically provide more IOPS or throughput. Check disk tier, VM limits, caching, attachment count, regional availability, and workload requirements together. Capacity and tier can also increase cost; use the Azure pricing calculator for your region and configuration.
Quick Recap
Production cutover checklist
- Identify the source platform and confirm its resize rule.
- Back up or snapshot before partition changes.
- Expand the virtual disk, then the partition and filesystem.
- Confirm the guest reports the new capacity.
- Wait for healthy replication and a post-resize recovery point.
- Test failover and validate boot, mounts, applications, and performance.
- Fail over using the verified recovery point.
- If required, resize the Azure managed disk and update the failback plan.
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