SCSI pass-through disks allow you to connect a Hyper-V virtual machine to a physical storage resource (rather than relying on virtual hard disks). However, the use of relay discs has some restrictions that are good to know.
Before launching Windows Server 2012, relay drives were a popular option. At the time, the VHD (Virtual Hard Disk) format used by Hyper-V was limited to 2 TB, a capacity that was unsuitable for certain virtual machines (VMs). Relay disks have gained popularity as a way to bypass the 2TB storage limit.
However, on Windows Server 2012, Microsoft launched the VHDX virtual hard disk format, which is not subject to the limit of 2 TB.
In many cases, the use of VHDX virtual hard disks thus makes it possible to avoid the use of relay disks. If you still plan to use relay disks, you must understand that they are linked to the host server. As such, they can complicate dynamic migrations (even if they remain possible if the relay disks are configured as VM-dependent cluster disks).
What’s more, Hyper-V is not able to capture Snapshots from relay drives.
Some professionals appreciate SCSI disks because they make (a little) Storage Area Network (SAN) management easier. For those who prefer to connect VMs directly to SAN storage, rather than creating virtual disks, I would recommend using a virtual FC (Fiber Channel) link or establishing an iSCSI connection from the VM.
Both techniques have the advantage of minimizing the involvement of the host operating system. This gives the VM more efficient access to storage while improving portability. You can freely move a VM with iSCSI connectivity as long as there is a network path to the storage.
A VM that uses a virtual FC connection can also be migrated dynamically, provided that the target host has the necessary HBA and is properly configured to support this type of migration.