Types of Hyper-V virtual storage

In the previous section, we discovered what virtual storage is and how it contributes to the server virtualization architecture. With all the information we got from the previous section about virtual storage, let’s now move ahead and see the virtual machine storage options that Hyper-V offers us.

In this section, we will go through different types of virtual machine storage options, such as VHD, VHDX, fixed disk, dynamic disk, differencing disk, and pass-through disk. We will discuss each of them in detail so that you understand all their ins and outs for better planning and sizing.

Each of these virtual storage options has a different set of properties than the others, and the administrator must choose the correct virtual storage that best fits the server operating systems and application needs.

Let’s now start by discussing each of these virtual storage options for virtual machines based on Hyper-V.

Virtual disk formats

First we must select the virtual disk format before we go ahead and create the virtual hard disk file. There are two possible virtual disk formats available with Windows Server 2012 Hyper-V; they are VHD and VHDX. Let’s first make a short comparison between the two virtual hard disk formats, which will provide us with some quick guidance on making the appropriate selection according to the usage.

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Virtual hard disk (VHD)

After the release of Hyper-V, Microsoft provided its customers with a native hypervisor for their server virtualization needs. But this newly released product had to face challenges for being called an enterprise virtualization platform. And virtual hard disk (VHD) limitation was one of the caveats that customers faced.

There are a few limitations to the VHD format; one is the limited size of the virtual hard disk (which we faced before) and another is the possibility of data inconsistency due to power failures. Virtual hard disk (VHD) is a file-based storage for your virtual machine that is based on Hyper-V; this is a default and basic level of storage functionality for a virtual machine. An administrator can create a virtual hard disk (VHD) file for a virtual machine within Hyper-V for a specific size, where defining the size is mandatory. This VHD file can have a different set of properties based on its type.

A virtual hard disk (VHD) file or file extension is similar to the VMKD file format, which is a VMware virtual machine hard disk extension. VHD files also existed in the earlier versions of server and desktop virtualization software from Microsoft, for example, Virtual PC and Virtual Server.

Virtual hard disk (VHDX)

As we saw, the two main limitations of VHD format based virtual hard disk are size and data inconsistency due to power failure; Microsoft addressed these two main limitations of the VHD file format and introduced a new virtual hard disk format called VHDX. This virtual hard disk format allowed customers to create virtual hard disks of up to 64 TB, where earlier the virtual hard disk format (VHD) only allowed virtual hard disks up to the size of 2 TB. Also, as this new format has a resilient architecture, the possibility of data corruption due to power failure also reduced.

Virtual disk types

The virtual hard disk format decides the maximum size of a virtual hard disk, while the virtual hard disk type decides the functionality and features a virtual hard disk will provide. Microsoft Windows Server 2012 Hyper-V provides four types of virtual hard disks for virtual machines based on Hyper-V. These four virtual hard disk types are as follows:

• Dynamic disk

• Fixed disk

• Differencing disk

• Pass-through disk

You should choose the virtual hard disk type based on your server and application requirements. Each type of virtual hard disk provides different set of disk performance and functionalities, so proper planning is highly important to ensure that you select the right virtual storage for your workload.

Let’s now discover each of these storage types in detail.

Dynamic disk

When you create a new virtual machine, and create a new virtual hard disk from New Virtual Machine Wizard, the wizard chooses the dynamic disk as the virtual hard disk type for you. Dynamic disks, as they sound, are dynamic; this means they get changed over time or due to the occurrence of certain events. Dynamic disks are the best choice for economic usage of the server’s storage. With whatever size of dynamic disk you create, it won’t immediately deduct the same amount of disk space from the physical storage of the Hyper-V server but instead will get created with a very small size, and over a period of time, keep growing as you put data and content on this disk. This dynamic growth of a disk is the actual concept behind this type of virtual hard disk.

Since dynamic disks are not of a fixed size and are actually small in size, they cannot deliver a good disk I/O for storage-intensive applications.

Real-world example

Over the years, I have seen many cases where a production workload (VM) has had performance bottlenecks, especially for the disk subsystem of a virtual machine. And among these, in the case of performance problems related to the virtual machine disk subsystem, the majority of times I saw people using dynamic virtual hard disks for their production workloads. And since dynamic disks do not have a fixed size storage for virtual machines, they are not a good choice for disk-intensive applications and server roles. A dynamic disk has another problem of not being able to provide good results for disk fragmentation or other similar activities due to its design. Dynamic disks are good for the testing and research and development types of virtual machines where the performance factor is not very important.

Now let’s see how we can create a dynamic disk for a virtual machine.

As we have said previously, when you create a virtual machine via New Virtual Machine Wizard, it also gives you the option to create a virtual hard disk for the virtual machine by default. This wizard for creating a new virtual machine, along with the creation of the virtual hard disk, provides a dynamic disk by default, and so the disk type option is not provided as a selection option.

In the following brief steps, we will see how to create a dynamic disk:

1. Open Hyper-V Manager from Administrative Tools.

2. From the Hyper-V Manager snap-in, find the New button on the right action pane and click on Hard Disk.

3. New Hard Disk Wizard will open; it will first ask you to select the hard disk format, which could be either VHD or VHDX, depending on the size of your hard disk.

4. Then you will be prompted to select the disk type; here we will select Dynamic Disk.

5. The next section of New Hard Disk Wizard will ask you the name of the virtual hard disk and the location where you want this virtual hard disk to be created and stored.

6. Now the last section of this wizard will ask you the size of the disk you want to create. This section also gives you the functionality to copy content from a physical disk of the server or any other virtual disk that has already been created.

Fixed disk

A fixed disk is like a static disk that has a fixed size and doesn’t grow over time if we go on adding content to it. Fixed disks provide better performance as compared to dynamic disks, because when we create a fixed disk of 100 GB, Hyper-V creates a VHD or VHDX file of 100 GB. It should be noted here that creating this 100 GB fixed disk will take a long time as it has to create a VHD/VHDX file of 100 GB, and the larger disk you create, the longer time it will take. A fixed disk allocates a fixed size from the physical storage of the Hyper-V server, and so this big chunk of allocated disk space allows the virtual machine to receive better I/O performance from this type of virtual hard disk.

Fixed disks are always recommended for production workloads because their better performance allows administrators to perform faster read/write operations on virtual disks. Fixed disks are mainly created for virtual machines that run disk-intensive applications, where a high disk I/O is required for virtual machine storage, for example, the virtual hard disk you will create if you are going to virtualize a file sever. Here you will store all the files to the hard disk and so it should be a fixed disk, but at the same time the operating system disk of the file server can be kept as a dynamic disk because there will not be much disk activity on it.

To create a fixed disk, you need to perform the following steps:

1. Open Hyper-V Manager from Administrative Tools.

2. From the Hyper-V Manager snap-in, find the New button on the right action pane and click on Hard Disk.

3. New Hard Disk Wizard will open; it will first ask you to select the hard disk format, which could be either VHD or VHDX, depending on the size of your hard disk.

4. You will then be prompted to select the disk type; here we will select Fixed Disk.

5. The next section of New Hard Disk Wizard will ask you for the name of your virtual hard disk and the location where you want this virtual hard disk to be created and stored.

6. Now the last section of this wizard will ask you the size of the disk you want to create. This section also gives you the functionality to copy content from a physical disk of the server or any other virtual disk that has already been created.

Differencing disk

A differencing disk has a parent-child model associated with its architecture. Mainly, it comes into use when an administrator takes a snapshot of a virtual machine, where after creating the snapshot, Hyper-V leaves the first parent VHD intact and creates a new child disk that gets linked to the parent virtual hard disk. Both parent and child disks always have the same disk format; this means that if the parent disk is created as VHD, the child disk cannot be VHDX.

A differencing disk is usually never recommended for production workloads because if you to create a snapshot of a production workload, you will stop writing to the production virtual hard disk. Differencing disks are the same in nature as dynamic disks, where the disk size grows over a period of time as we go on adding more data to the disk; this nature of the disk may not give you good performance for the disk subsystem of the production workload.

Another problem with the differencing disk is that when we create a snapshot of the virtual machine, from that point in time, all the data gets written on to the differencing disk and your parent VHD/VHDX becomes idle and isolated from the new data changes. And in the case of multiple snapshots you will have data written on multiple differencing disks. So if any of the differencing disks (snapshots) get misplaced or deleted, you will lose all the data that was written on it at that particular period of time. So in a nutshell, it is highly recommended not to create a snapshot of a production virtual machine; but if you have taken it already, make sure that you restore it to its parent VHD/VHDX as early as possible.

To create a differencing disk, you may perform the following steps:

• Previously, we saw the steps for creating other disk types; follow those same steps until we reach the step where we need to select the disk type.

• When we are prompted to select the disk type, select Differencing Disk.

• The next section of New Hard Disk Wizard will ask you the name of the virtual hard disk and the location where you want this virtual hard disk to be created and stored.

• Now the last section of this wizard will ask you the size of the disk you want to create. This section also gives you the functionality to copy content from a physical disk of the server or any other virtual disk that has already been created.

Pass-through disk

A pass-through disk is a storage type in which an administrator presents a physical hard disk, which is associated or attached to the Hyper-V host server, to the virtual machine as a raw disk. This type of virtual machine storage is called a pass-through or raw disk; in this type of storage, the physical disk or LUN passes through the hypervisor and later to the virtual machine guest system. This physical disk that is associated or attached to the Hyper-V server could be a SAN LUN storage bound to the Hyper-V server, or it could be a locally installed physical hard disk.

To mitigate all these aforementioned risks, administrators prefer to use a pass-through disk as a local disk for highly critical virtual workloads and keep the virtual machine storage on this pass-through disk. As a first step, the pass-through disk is attached or made available to the Hyper-V server; once the disk is available to the server, you have to bring the disk offline before you pass the disk through to a virtual machine available on the same Hyper-V box. We cannot make a partition on the disk available to a virtual machine; it is only the local disk of the Hyper-V server that can be made available as a pass-through disk.

In many cases, admins prefer to use pass-through disks instead of using VHDs, especially if they want their virtual machine to boot from a SAN LUN, or in the case of databases, where an Exchange Server mailbox database can be placed on pass-through SAN LUN disks. Pass-through disks are also suitable for environments where the application’s high availability methodology is placed on a build blocks level. In these types of high availability requirements for the application, an administrator needs to bind the same data disks (pass-through LUNs) to another virtual machine, and you are all good to go.

It should also be noted here that pass-through disks don’t get included in a backup based on a VSS snapshot. This means that if you have taken a backup of a virtual machine using a VSS-based backup solution, all the pass-through disks of the virtual machine will not be backed up and only the VHD-based/VHDX-based disks will be included in the VSS snapshot backup.

The following steps need to be carried out to provide a pass-through disk to a virtual machine within the Hyper-V server:

1. Open Hyper-V Manager from Administrative Tools.

2. Take the virtual machine settings that you want to configure and add a pass-through disk.

3. Add SCSI Controller from the Add Hardware section of the virtual machine settings, or if you wish, you can also add a physical disk to an IDE controller. Then select the hard drive and click on the Add button.

4. Once you click on the Add button at the controller level, you will be prompted to select the physical hard drive that you want to connect to the virtual machine.

5. After selecting the appropriate disk, you need to connect it to the virtual machine. First click on Apply and then click on the OK button to commit the changes.

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Virtual Fibre Channel SAN

With the release of Windows Server 2012, Hyper-V now offers virtual FC SAN connectivity to virtual machines, to allow virtual machines to connect to a virtual SAN. The administrator first connects a virtual SAN network setup on the Hyper-V server, just like what we would do to create a virtual switch for different network segments. Once the virtual fibre SAN network gets set up, the Fibre Channel adapter can be added to the virtual machine that needs to be associated with the Fibre Channel SAN network.

Hyper-V allows the Hyper-V administrator to configure WWNs and other settings related to Fibre Channel SAN; all these settings can be customized from the virtual machine Fibre Channel adapter or as global settings from the Fibre Channel network of the virtual SAN manager.

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The preceding screenshot describes the connectivity of a standalone Hyper-V server to the FC SAN. In the first phase of connectivity, our Hyper-V server gets connected to the FC SAN switch through a Fibre Channel medium. Then in the second phase of connectivity, we configure the virtual Fibre Channel switch on the Hyper-V server. Once the virtual fabric switch is configured, we can simply add the virtual Fibre Channel adapter to the virtual machine and connect the adapter to the virtual Fibre Channel switch.