Currently, MTV migrates VMs and defaults the disk controller to virtio.

We have a customer who would like the migration toolkit to provide the ability to specify the disk controller on each disk, so that after migration, it is possible for the disks have virtio-scsi controller.

Mixing virtio-block and virtio-scsi in the same virtualized environment can introduce some operational challenges:

1. Management Complexity:

Different device names: Keeping track of which disks are /dev/vdX (virtio-block) and /dev/sdX (virtio-scsi) can become confusing, especially with many VMs and disks. This can complicate scripting, automation, and troubleshooting.
Varying feature support: Remembering which disks support advanced features like TRIM/DISCARD (virtio-scsi) and which don't (virtio-block) adds another layer of complexity to storage management.
Performance considerations: Understanding the performance characteristics of each type and how they might interact with different workloads can be challenging. You might encounter scenarios where some disks perform better than others due to the different drivers and underlying architecture.
2. Guest OS Compatibility:

Driver support: While most modern guest operating systems support both virtio-block and virtio-scsi, older or specialized OSes might have limitations. Ensure your chosen guest OSes have the necessary drivers for both types.
Configuration differences: Configuring storage within the guest OS might differ slightly depending on the device type. This could involve using different tools or command-line options for partitioning, formatting, and mounting.
3. Monitoring and Troubleshooting:

Performance analysis: Analyzing storage performance can be more complicated when dealing with a mix of device types. You'll need to consider the specific characteristics of each type when interpreting performance metrics.
Troubleshooting issues: Identifying the root cause of storage-related problems can be trickier when different devices are involved. You'll need to be mindful of the potential differences in driver behavior, error reporting, and logging.
4. Potential for Inconsistency:

Backup and restore: Using different device types might introduce inconsistencies in how backups and restores are handled, potentially leading to complications or errors.
Live migration: Migrating VMs with a mix of virtio-block and virtio-scsi devices might require additional configuration or introduce compatibility issues with certain hypervisors.