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Hyper-V VM IDE VS SCSI | What’s The Difference?
Microsoft Hyper-V offers the IDE and SCSI virtual storage controllers. But do you know what’s the difference between Hyper-V VM IDE vs SCSI controllers? This article will introduce them for you.
By Crystal / Updated on March 8, 2023
Case: Hyper-V VM IDE vs SCSI Controller？
When looking at the settings for a VM in Hyper-V manager, you have an option for adding IDE Controllers and another option for adding SCSI Controllers. You can only add a VHD through IDE settings (from what I understand and have tested), when the VM is off. However, using SCSI Settings, you are able to add a VHD to the VM at any time - even when on...
So the question is, other than the C drive VHD, why would you use IDE for any other disk over SCSI in VM Management? Does a VM have a performance hit using SCSI over IDE or something?
- Question from community.spiceworks.com
--community.spiceworks.com
Hyper-V, a type-1 hypervisor developed by Microsoft, offers three types of virtual storage controllers: IDE, SCSI, and Virtual host bus adapters (HBAs), which are used to connect the virtual machine to a VHD/VHDX virtual hard disk or a passthrough disk.
Since Windows Server 2012, a generation 1 virtual machine has had two IDE controllers and one SCSI controller. But Hyper-V VM IDE vs SCSI controller, how to choose from them? This article will cover these Hyper-V controllers’ definitions and differences to help you make an appropriate choice.
What is IDE controller
IDE controllers expose IDE disks to the virtual machine. The IDE controller is emulated, and it is the only controller that is available for guest VMs running older version of Windows without the Virtual Machine Integration Services.
You can have up to two IDE controllers with two disks on each controller. The boot disk must be attached to one of the IDE devices. The startup disk can be either a virtual hard disk or a physical disk.
Although a virtual machine must use an IDE device as the startup disk to start the guest operating system, there are more options to choose from when selecting the physical device that will provide the storage for the IDE device.
Disk I/O that is performed by using the IDE filter driver that is provided with the Virtual Machine Integration Services is significantly better than the disk I/O performance that is provided with the emulated IDE controller. It is recommended by Microsoft that only use IDE disks for the operating system disks because they have performance limitations due to the maximum I/O size that can be issued to these devices.
What is SCSI controller
SCSI controllers expose SCSI disks to the virtual machine. SCSI path is not emulated, which makes it the preferred controller for any disk other than the operating system disk. In fact, introduced in Windows Server 2012 R2 and is reported as SAS to support shared VHDX, SCSI controller it is also the only type of controller possible for Generation 2 VMs.
SCSI controllers use a type of device developed specifically for use with virtual machines and use the virtual machine bus to communicate. The virtual machine bus must be available when the guest operating system is started. Therefore, virtual hard disks attached to SCSI controllers cannot be used as startup disks.
For best performance, it is recommended that you attach multiple disks to a single virtual SCSI controller, and create additional controllers only if you need to expand the number of disks attached to a virtual machine. Each virtual SCSI controller can support up to 64 devices, and each virtual machine supports up to four SCSI controllers, or 256 SCSI disks.
Hyper-V VM IDE vs SCSI controller
Every virtual machine on Hyper-V supports IDE and SCSI disks, and each one having its own set of benefits and limits. For example, Hyper-V IDE vs SCSI controller, the Generation 1 virtual machine can only be booted from the IDE, and not possible to boot from a SCSI disk.
One of the benefits of a Generation 2 virtual machine over a Generation 1 VM is that Generation 1 VMs allow a VM to boot from a SCSI virtual hard disk. However, Generation 2 VMs do not support the original VHD virtual hard disk format. So, there is a trade-off that needs to be taken into consideration, when designing your VMs storage. Or you can also convert generation 1 to generation 2.
Except for their limitations, users are more concerned about the performance difference between IDE and SCSI disks. Although the I/O performance of physical SCSI and IDE devices can differ significantly, this is not true for the virtualized SCSI and IDE devices in Hyper-V. In fact, Hyper-V IDE and SCSI devices both offer equally fast I/O performance when integration services are installed in the guest operating system.
How to choose from IDE and SCSI
By default, every virtual machine has two IDE controllers and one SCSI controller that can be used to add devices like virtual hard disks or DVD drives. It is important to identify when they are necessary, what their limitations are, and hence to decide how to add and manage them.
You can mix IDE and SCSI controllers for the same VM. A good starting point when deciding your virtual machine's storage layout is to use the IDE controllers for the boot disk, and the attached drive will contain the operating system, and use SCSI controllers for additional data disks, with the benefit of being able to add more VHDX virtual disks on the fly.
It is recommended you create Generation 1 VM which the guest OS installed on a virtual hard disk on IDE controller 0, Location 0, the paging file in a disk on IDE controller 0, location 1, and one or more data disks located on a virtual SCSI controller. The benefit of this is that you get the extra functionality that VHDX or SCSI controllers can offer, such as live resizing of VHDX files.
Creating a Generation 2 virtual machine will be much simpler, since the SCSI disk can also be used as a boot disk. And all the added features of SCSI controllers are available to this boot disk.
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Summary
On Hyper-V, all the virtual machines by default come with 2 IDE controllers and 1 SCSI controller. You can add or remove Hyper-V controller based on the limits that each one has. However, before doing this, you may need to know what they are and how they differ in advance. This article introduced the virtual IDE controller and SCSI controller respectively, and analyzed the difference of Hyper-V VM IDE vs SCSI controller.
For Generation 1 virtual machines, the IDE disk has to be used by default, as it is the only supported method to boot an operating system. But if you have a virtual machine that requires more than four disks, the best option may be the SCSI controller that can be added when the virtual machine is running.
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