How to Perform KVM Storage Migration on AlmaLinux

Introduction

Managing virtualized environments efficiently often requires moving virtual machine (VM) storage from one location to another. This process, known as storage migration, is invaluable for optimizing storage utilization, performing maintenance, or upgrading storage hardware. On AlmaLinux, an enterprise-grade Linux distribution, KVM (Kernel-based Virtual Machine) offers robust support for storage migration, ensuring minimal disruption to VMs during the process.

This detailed guide walks you through the process of performing KVM storage migration on AlmaLinux. From prerequisites to troubleshooting tips, we’ll cover everything you need to know to successfully migrate VM storage.

What is KVM Storage Migration?

KVM storage migration allows you to move the storage of a running or stopped virtual machine from one disk or storage pool to another. Common scenarios for storage migration include:

• Storage Maintenance: Replacing or upgrading storage systems without VM downtime.
• Load Balancing: Redistributing storage loads across multiple storage devices or pools.
• Disaster Recovery: Moving storage to a safer location or a remote backup.

KVM supports two primary types of storage migration:

1. Cold Migration: Migrating the storage of a stopped VM.
2. Live Storage Migration: Moving the storage of a running VM with minimal downtime.

Prerequisites

Before performing storage migration, ensure the following prerequisites are met:

1. Host System:

   • AlmaLinux 8 or newer installed.
   • KVM, QEMU, and libvirt configured and operational.

2. Storage:

   • Source and destination storage pools configured and accessible.
   • Sufficient disk space on the target storage pool.

3. Network:

   • For remote storage migration, ensure reliable network connectivity.

4. Permissions:

   • Administrative privileges to execute migration commands.

5. VM State:

   • The VM can be running or stopped, depending on the type of migration.

Step-by-Step Guide to KVM Storage Migration on AlmaLinux

Step 1: Verify KVM and Libvirt Setup

Ensure the necessary KVM and libvirt packages are installed:

sudo dnf install qemu-kvm libvirt virt-manager -y

Start and enable the libvirt service:

sudo systemctl enable --now libvirtd

Verify that KVM is functional:

virsh version

Step 2: Check VM and Storage Details

List the running VMs to confirm the target VM’s status:

virsh list --all

Check the VM’s current disk and storage pool details:

virsh domblklist <vm-name>

This command displays the source location of the VM’s storage disk(s).

Step 3: Add or Configure the Target Storage Pool

If the destination storage pool is not yet created, configure it using virsh or virt-manager.

• Creating a Storage Pool:

  1. Define the new storage pool:

     virsh pool-define-as <pool-name> dir --target <path-to-storage>
     

  2. Build and start the pool:

     virsh pool-build <pool-name>
     virsh pool-start <pool-name>
     

  3. Make it persistent:

     virsh pool-autostart <pool-name>
     

• Verify Storage Pools:

  virsh pool-list --all
  

Step 4: Perform Cold Storage Migration

If the VM is stopped, you can perform cold migration using the virsh command:

virsh dumpxml <vm-name> > <vm-name>.xml
virsh shutdown <vm-name>
virsh migrate-storage <vm-name> <destination-pool-name>

Once completed, start the VM to verify its functionality:

virsh start <vm-name>

Step 5: Perform Live Storage Migration

Live migration allows you to move the storage of a running VM with minimal downtime.

1. Command for Live Storage Migration:

   virsh blockcopy <vm-name> <disk-target> --dest <new-path> --format qcow2 --wait --verbose
   

   • <disk-target>: The name of the disk as shown in virsh domblklist.
   • <new-path>: The destination storage path.

2. Monitor Migration Progress:

   virsh blockjob <vm-name> <disk-target> --info
   

3. Commit Changes: After the migration completes, commit the changes:

   virsh blockcommit <vm-name> <disk-target>
   

Step 6: Verify the Migration

After the migration, verify the VM’s storage configuration:

virsh domblklist <vm-name>

Ensure the disk is now located in the destination storage pool.

Troubleshooting Tips

1. Insufficient Space:

   • Verify available disk space on the destination storage pool.
   • Use tools like df -h to check storage usage.

2. Slow Migration:

   • Optimize network bandwidth for remote migrations.
   • Consider compressing disk images to reduce transfer time.

3. Storage Pool Not Accessible:

   • Ensure the storage pool is mounted and started:

     virsh pool-start <pool-name>
     

   • Verify permissions for the storage directory.

4. Migration Fails Midway:

   • Restart the libvirtd service:

     sudo systemctl restart libvirtd
     

5. VM Boot Issues Post-Migration:

   • Verify that the disk path is updated in the VM’s XML configuration:

     virsh edit <vm-name>
     

Best Practices for KVM Storage Migration

• Plan Downtime for Cold Migration: Schedule migrations during off-peak hours to minimize impact.
• Use Fast Storage Systems: High-speed storage (e.g., SSDs) can significantly improve migration performance.
• Test Before Migration: Perform a test migration on a non-critical VM to ensure compatibility.
• Backup Data: Always backup VM storage before migration to prevent data loss.
• Monitor Resource Usage: Keep an eye on CPU, memory, and network usage during migration to prevent bottlenecks.

Conclusion

KVM storage migration on AlmaLinux is an essential skill for system administrators managing virtualized environments. Whether upgrading storage, balancing loads, or ensuring disaster recovery, the ability to migrate VM storage efficiently ensures a robust and adaptable infrastructure.

By following this step-by-step guide, you’ve learned how to perform both cold and live storage migrations using KVM on AlmaLinux. With careful planning, proper configuration, and adherence to best practices, you can seamlessly manage storage resources while minimizing disruptions to running VMs.