VM Types and Sizes in Azure — Picking the right fit for your needs

One of the most-asked questions that I answer for my customers concerns the topic of what their VMs will look like in the cloud post-migration or failover. Zerto makes it very simple to select a VM type and size while creating a virtual protection group, but there are a dizzying amount of VMs that you can pick from!

“A simple problem to resolve,” an administrator may say. “I shall just pick a VM type that has the same hardware as what I have running on-premise in vSphere!” Sadly, this approach is often not possible. VMs in Azure are not built with individual virtual hardware components but rather via pre-defined templates. Thus, one may not be able to make a VM that has the same specifications as on premise. What is an administrator or architect to do?

In this article, we shall explore the topic of virtual machines in Azure. We shall talk about VM types, how to read the Microsoft Azure shorthand to identify those VMs, and how to pick the best VM type for the performance that you need at the price you are willing to pay.

VM Types

When you create a VM in Azure, you are greeted with a window that looks like this:

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Here, you will specify the name of the VM, the region it will run in, and other important details. What we want to focus on is the size. Clicking on “Change size” presents you with this:

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You are presented with a variety of different options, including the VM size, whether it’s a promo or a standard offering, the family type, and the hardware profile.

So what do the VM size letters and numbers mean? Here is a table of what you may see:

Type Purpose
A General purpose
B Burstable – discounted VMs capable of bursting when needed. Do not use for consistently high-CPU workloads.
D General purpose compute, ideal for enterprise applications
DC Security optimized. Built-in encryption capabilities and uses secure enclaves.
E Memory optimized. High memory-to-CPU core ratio.
F CPU optimized. High CPU core-to-memory ratio.
G Giant or “Godzilla”. Massive VMs that exceed the capabilities of the D-series family.
H High-Performance Computing. Extremely high memory bandwidth VMs, ideal for scientific simulations (ie fluid dynamics, quantum simulation, rendering, etc.)
Ls Storage-optimized. High throughput, low-latency, directly mapped NVMe storage. Ideal for databases and data warehousing.
M (Massive) memory optimized. The largest memory optimized VMs possible in Azure. Ideal for SAP HANA and other in-memory critical workloads.
N GPU-optimized. Ideal for graphics rendering, deep learning, gaming, etc.

You may also notice that your VM type may have additional letters appended to it. For example, you may see something like “DS3_v2” or “B8ms”. These additional letters refer to VM capabilities or a revision of a VM family type. So DS3_V2 refers to:

  • D = General purpose enterprise compute
  • S = SSD, machine capable of supporting Premium Managed disks
  • 3 = Size “3”, which is an indicator of how “big” the VM is in terms of CPU, memory, etc.
  • V2 = Version 2 of the VM type.

Similarly, B8ms refers to:

  • B = Burstable
  • 8 = Size “8”
  • m = Memory multiplier, indicating there is more memory available for this VM than usual for this family type
  • s = Premium storage options available (ie Premium managed disks).

There are other codes as well, such as “i” (isolated) and “r” (remote direct memory access). When in doubt, check the official Microsoft page to verify what features a VM may have at https://docs.microsoft.com/en-us/azure/virtual-machines/windows/sizes.

Performance, Cost, and Compatibility

Now that you understand what the VM name code means, you will want to compare the actual underlying performance of the VM relative to other options. One way to do this would be to look at how many Azure Compute Units (ACUs) that a VM has. ACUs are a way to compare CPU performance between different types of virtual machines in Azure. The benchmark starts at 100 for A1 through A4; a rough rule of thumb is that a 200 ACU \ vCPU value VM is twice as performant than a 100 ACU \ vCPU VM. Use the following link to show the ACU count for any particular VM SKU: https://docs.microsoft.com/en-us/azure/virtual-machines/windows/acu.

Next, you will want to consider the cost of running a VM in the cloud. To do this, navigate to https://azure.microsoft.com/en-us/pricing/details/virtual-machines/windows/ and find your instance type. You may notice that certain VM types are not available in the region of your choice (for example, at the time that this article was written, G-type VMs were not available in the East US region). Make sure you account for that when planning your deployment!

Now that we have determined the VM type that we want to run our migrated or failed over VMs on, and we have determined that the cost is acceptable for the performance we expect, we want to finish our planning by make sure that the VMs themselves are supported in the cloud. I often tell my customers that Zerto doesn’t care what you’re replicating, but Azure does. Microsoft maintains a supportability matrix for both Windows and Linux, so it is wise to ensure that you are failing over a VM that has a supported OS.

https://support.microsoft.com/en-us/help/2721672/microsoft-server-software-support-for-microsoft-azure-virtual-machines

https://docs.microsoft.com/en-us/azure/virtual-machines/linux/endorsed-distros

Example:

Your customer is running SQL Server 2018 on Windows Server 2016. The virtual machine consists of 4 vCPUS, 32GB of RAM, and 16 VMDKs. The customer wants to utilize premium managed disks in Azure. Which VM size will most closely fulfill the customer’s requirements?

  1. DS13-4_v2
  2. E4s_v3
  3. A4m_v2
  4. E4_v3

You can immediately discount 3 and 4, as neither of those instance types support premium managed disks. Looking at the VM size matrix, you can see that DS13-4_v2 will be enough, but it has more RAM and disks than is required. E4s_v3 however has 4 vCPUs, 32GB of memory, but only 8 data disks are supported, and you cannot exceed that number without moving onto a new VM type! Thus, out of the four options available, the best answer is 2.

Note that for the aforementioned example, an even better option exists in Azure: E8-4s_v3, which provides 4 vCPUs, 64GB of RAM, and 16 data disks, at a cost of $374.98 per month, versus the DS13-4_v2, which costs $551.30. Also note that the ACU for Es_v3 is 160-190 versus 210-250 for the DS13-4_v2. However the DS13-4_v2 VM is not hyperthreaded while the E4s_v3 is. Take into consideration these details before committing to a specific VM family type!

Conclusion

It is ultimately the end-user’s responsibility to choose the correct VM type and size for their workloads when moving or rearchitecting VMs to run in the cloud. However, as trusted advisors and solutions engineers, we can help guide our customers through the often-confusing journey to the cloud. Picking the right size VM requires an understanding of your customer’s data and applications, but by investing the time and effort to understanding their requirements, you will significantly improve their chances of success.

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