Shrink Azure Data Disks Safely: Reduce Size Without Data Loss for Virtual Machines

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This article outlines the process of safely reducing the size of an Azure data disk without encountering data loss. Directly downsizing a disk is not a supported operation due to the inherent risks of data corruption or data loss. Instead, the recommended approach involves creating a new, smaller disk, securely copying all data from the original disk to the new disk, and subsequently deleting the original, larger disk.

It is important to note that this article does not cover the process of downsizing the operating system (OS) disk. The guidelines provided here are intended for general use and might require adjustments based on your specific requirements and configurations.

1 - Create a New Data Disk¶

The first crucial step in downsizing your Azure data disk is to create a new data disk of the desired smaller size. This new disk will serve as the destination for your data. You can accomplish this through the Azure portal by following the steps outlined below.

1. Access Virtual Machines in Azure Portal: Begin by navigating to the Azure portal. In the search bar at the top, type “Virtual machines” and select the “Virtual machines” service from the search results. This will take you to a list of all virtual machines within your current Azure subscription.

   

2. Select Target Virtual Machine: From the list of virtual machines displayed, identify and select the specific virtual machine to which you intend to attach the new, smaller data disk. Click on the Name of the virtual machine to open its management page.

   

3. Navigate to Disks Settings: Within the virtual machine’s navigation pane, locate the “Settings” section. Under “Settings,” click on “Disks”. This action will display the disks configuration page for the selected virtual machine, showing both OS disks and data disks currently attached.

   

4. Create and Attach a New Disk: On the VM disks page, find the “Data disks” section. Click on “+ Create and attach a new disk”. This option initiates the process of creating a new data disk and attaching it to your virtual machine.

   

5. Configure New Data Disk Properties: A new row will appear within the “Data disks” section, prompting you to configure the properties of the new disk. Carefully enter or select values for the following fields based on your needs:

   • LUN (Logical Unit Number): This number identifies the disk within the VM. Azure automatically suggests an available LUN. You can typically accept the default value unless you have specific LUN assignment requirements.
   • Disk name: Provide a descriptive name for your new data disk. This name will help you easily identify the disk in the Azure portal. Choose a name that reflects its purpose or size.
   • Storage type: Select the appropriate storage type for your disk based on performance and cost requirements. Options include Premium SSD, Standard SSD, and Standard HDD. Premium SSDs offer the highest performance but are also the most expensive. Standard HDDs are the least expensive but offer the lowest performance. Standard SSDs provide a balance between performance and cost.
   • Size (GB): Specify the desired size for your new data disk in Gigabytes (GB). Ensure this size is smaller than your original data disk but large enough to accommodate all the data you intend to copy. Carefully plan your size requirements to avoid running out of space later.
   • Max IOPS (Input/Output Operations Per Second) & Max throughput (MBps): These values are automatically determined based on the storage type and size you select. They indicate the performance capabilities of the disk.
   • Encryption: If required, configure encryption settings for the new disk. Azure Disk Encryption helps protect your data at rest.
   • Host caching: Configure host caching settings. Caching can improve performance for frequently accessed data. Options include Read-only, Read/Write, or None. Choose based on your workload characteristics.

   

6. Save Disk Configuration: After configuring all the necessary properties for the new data disk, click on Save at the top of the VM disks page. Azure will then proceed to create the new data disk and automatically attach it to your selected virtual machine. This process might take a few minutes depending on the disk size and storage type.

   

2 - Configure the Data Disk Within the VM¶

Once the new data disk is created and attached to your Azure virtual machine, the next step is to configure it within the virtual machine’s operating system. This involves bringing the disk online, initializing it, creating a volume or partition, and assigning a drive letter (for Windows) or mount point (for Linux). The exact steps vary slightly depending on whether your VM is running Windows or Linux. Refer to the specific documentation for your operating system for detailed instructions on disk management.

For Windows VMs, you would typically use Disk Management (diskmgmt.msc) to initialize the disk, create a new volume, and assign a drive letter.

For Linux VMs, you would typically use commands like fdisk, parted, or gdisk to partition the disk, and then format the partition using mkfs (e.g., mkfs.ext4, mkfs.xfs). Finally, you would create a mount point and mount the new file system.

It is crucial to properly format the new disk with a file system that is compatible with your operating system and application requirements (e.g., NTFS for Windows, ext4 or XFS for Linux). Ensure you choose the appropriate file system and configuration options during this step.

3 - Move Files from the Old Data Disk to the New Data Disk¶

After the new, smaller data disk is configured and accessible within your virtual machine, the essential step is to copy all the data from the original, larger data disk to this newly created disk. This data transfer process is critical, and its duration will depend significantly on the amount of data you need to migrate. It is advisable to perform this operation during off-peak hours to minimize any potential impact on application performance.

To perform the data transfer, you will need to use file copying utilities available within your operating system. The most common and robust tools for this purpose are robocopy for Windows and rsync for Linux. These tools are preferred over simple copy commands because they offer features like:

• Mirroring: Ensuring an exact copy, including directory structure.
• Resilience: Handling network interruptions and retrying failed transfers.
• Efficiency: Copying only changed files in subsequent runs (for incremental copies, though not directly relevant in this initial full copy scenario).
• Multi-threading: Utilizing multiple threads for faster transfer speeds.

Here are examples of how to use robocopy for Windows and rsync for Linux to copy data from the old disk to the new disk.

Windows: Using robocopy¶

Open Command Prompt as an administrator and use the robocopy command.

robocopy <source_drive_letter>:\ <destination_drive_letter>:\ /mir /z /w:5 /r:2 /mt

• Replace <source_drive_letter> with the drive letter of your old, larger data disk (e.g., x:).
• Replace <destination_drive_letter> with the drive letter of your new, smaller data disk (e.g., y:).
• /mir: Mirrors a directory tree. This option will copy all files and directories, including subdirectories, and ensure that the destination exactly matches the source. It also deletes files in the destination that are not present in the source.
• /z: Copies files in restartable mode. This is helpful for large file transfers over networks, as it allows transfers to be resumed if interrupted.
• /w:5: Specifies the wait time between retries in seconds (5 seconds in this case).
• /r:2: Specifies the number of retries on failed copies (2 retries in this case).
• /mt: Enables multi-threading for copying files, which can significantly speed up the transfer process, especially for a large number of small files. robocopy will use 8 threads by default with this parameter.

Example: If your old disk is drive D: and your new disk is drive E:, the command would be:

robocopy D:\ E:\ /mir /z /w:5 /r:2 /mt

Linux: Using rsync¶

Open a terminal and use the rsync command.

rsync -avz /<source>/<directory>/ /<destination>/<directory>/

• Replace /<source>/<directory>/ with the mount point of your old, larger data disk. Ensure you include the trailing slash / after the directory name. For example, if your old disk is mounted at /mnt/olddata, use /mnt/olddata/.
• Replace /<destination>/<directory>/ with the mount point of your new, smaller data disk. Similarly, include the trailing slash /. For example, if your new disk is mounted at /mnt/newdata, use /mnt/newdata/.
• -a: Archive mode; this is a combination of several options that are useful for mirroring directories, including recursive copy, preserving timestamps, permissions, ownership, etc.
• -v: Verbose mode; increases the amount of information you see during the transfer, showing file names as they are being copied.
• -z: Compresses file data during the transfer. This can be beneficial if you are transferring data over a network, especially if the network bandwidth is limited.

Example: If your old disk is mounted at /mnt/olddata and your new disk is mounted at /mnt/newdata, the command would be:

rsync -avz /mnt/olddata/ /mnt/newdata/

Important Considerations for Data Transfer:

• Time Estimate: Be prepared for a potentially lengthy transfer process. The time required depends on the amount of data, disk I/O performance, and network bandwidth if data is being transferred over a network.
• Verification: After the copy process completes, it is absolutely critical to verify that all data has been successfully transferred to the new disk. Manually compare directory structures, file counts, and, if possible, checksums of critical files to ensure data integrity and completeness. Do not proceed to the next steps until you are absolutely confident that the data transfer was successful and without errors. Data loss at this stage can be irreversible if the old disk is deleted prematurely.
• Application Downtime (If Applicable): If the data disk being downsized hosts data for a running application or service, you will likely need to plan for a maintenance window or downtime to perform this operation. Ensure proper application shutdown and restart procedures are in place.

4 - Detach and Delete the Old Data Disk¶

Once you have meticulously verified that all data has been successfully copied from the old, larger data disk to the new, smaller data disk, you can proceed with detaching and then deleting the old disk. This is a two-part process. Detaching the disk must be done before deletion because a disk cannot be deleted if it is still attached to a virtual machine.

Part 1: Detach the Old Disk¶

1. Access Virtual Machines in Azure Portal: Navigate to the Azure portal and go to the “Virtual machines” service, as you did in Step 1.

   

2. Select Target Virtual Machine: Select the virtual machine from which you want to detach the old data disk by clicking on its Name.

   

3. Navigate to Disks Settings: In the VM’s navigation pane, go to “Settings” and then “Disks”.

   

4. Locate Old Data Disk to Detach: In the “Data disks” section of the VM disks page, locate the row corresponding to the old, larger data disk that you want to remove. Identify it by its name or LUN.

   

5. Detach the Disk: At the end of the row for the old data disk, you will see a Detach icon, usually represented by an “X” symbol. Click on this “Detach” icon.

   

6. Save Configuration to Detach: After clicking “Detach”, click Save at the top of the VM disks page. Azure will now detach the selected data disk from the virtual machine. This operation might take a short while to complete. After detachment, the disk will no longer be associated with the VM but will still exist as a resource in your Azure subscription until you explicitly delete it.

   

Part 2: Delete the Old Disk¶

After successfully detaching the old data disk, you can now proceed to delete it. Deleting the disk will permanently remove it and its associated storage costs from your Azure account.

1. Find Unattached Disks: In the Azure portal, search for and select “Disks”. This will take you to a list of all disks in your subscription.

   

2. Identify the Old Data Disk for Deletion: From the list of disks, locate the old data disk that you just detached. You can usually identify it by its name. Since it’s detached, its “Attached to virtual machine” column should be empty or indicate “No”.

   

3. Delete the Disk: Select the old data disk by checking the box next to its name. Then, click on the Delete button at the top of the disks list.

   

4. Confirm Deletion: Azure will prompt you to confirm the deletion. This is a permanent action, and deleted disks cannot be recovered. Carefully review the disk name to ensure you are deleting the correct old disk. Type “Yes” or confirm as prompted to proceed with the deletion.

   

5. Wait for Deletion to Complete: Azure will initiate the disk deletion process. It might take a few minutes for the disk to be completely deleted. You can monitor the progress in the Azure portal notifications. Once deleted, the old, larger data disk is removed, and you will only be billed for the new, smaller data disk.

Conclusion¶

By diligently following these steps, you can effectively downsize an Azure data disk without risking any data loss. The key to a successful and safe disk downsizing operation lies in meticulous planning, careful execution of each step, and, most importantly, thorough verification of data transfer before deleting the original disk. Always prioritize data integrity and ensure you have a robust backup strategy in place as a general best practice for data management.

Do you have any questions or experiences with shrinking Azure data disks? Share your thoughts and comments below!