Azure Failover Cluster

Shared Storage Options in Azure: Part 1 – Azure Shared Disks

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In an IaaS world, shared storage between virtual machines is a common ask. “What is the best way to configure shared storage?”, “What options do we have for sharing storage between these VMs?”, both are questions I’ve answered several times, so let’s go ahead and blog some of the options! The first part in this blog series titled “Shared Storage Options in Azure”, will cover Azure Shared Disks.

As I write subsequent posts in this series, I will update this post with the links to each of them.

 

When shared disks were announced in July of 2020, there was quite a bit of excitement in the community. There are so many applications that still leverage shared storage for things like Windows Server Failover Clustering, on which many applications are built like SQL Server Failover Cluster Instances. Also, while I highly recommend using a Cloud Witness, many customers migration workloads to Azure still rely on a shared disk for quorum as well. Additionally, many Linux applications leverage shared storage that were previously configured to use a shared virtual disk, or even RAW LUN mappings, for applications such as GFS2 or OCFS2.

Additional sample workloads for Azure Shared Disks can be found here: Shared Disk Sample Workloads.

There are a few limitations of shared disks, the list of which is constantly getting smaller. For now, though, let’s just go ahead and jump into it and see how to deploy them. After which, we’ll do a quick “Pros” and “Cons” list before moving on to the other shared storage options. I deployed Shared Disks in my lab using the portal first (screenshots below), but also created a Github Repository (https://github.com/matthansen0/azure-shared-storage-options) with the Azure PowerShell script and an ARM template to deploy a similar environment – feel free to use those if you’d like!

As a prerequisite (not pictured below) I created the following resources:

  • A Resource Group in the West US region
  • A Virtual Network with a single subnet
  • 2x D2s v3, Windows Server 2016 Virtual Machines (VM001, VM002) each with a single OS disk

Now that those are created, I deployed a Managed Disk (named “sharedDisk001”) just like you would if you were deploying a typical data disk.

On the “advanced” tab you will see the ability to configure the managed disk as a “shared disk”, here is where you set the max shares which specifies the maximum number of VMs that can attach that particular disk type.

After the disk is finished deploying, we head over to the first VM and attach an existing disk. You’ll note that the disk shows up as a “shared disk” and shows the number of shares left available on that disk. Since this is the first time it’s being mounted it shows 0.

After attaching the disk to the first VM, we head over and do the same thing on VM002. You’ll note that the number of shares has increased by 1 since we have now mounted the disk on VM001.

Great, now the disk is attached to both VMs! Heading over to the managed disk itself you’ll notice that the overview page looks a bit different from typical managed disks, showing information like “Managed by” and “Max Shares”.

In the properties of the disk, we can see the VM owners of that specific disk, which is exactly what we wanted to see after mounting it on each of the VMs.

Although I setup this configuration using Windows machines, you’ll notice I didn’t go into the OS. This is to say that the process, from an Azure perspective, is the same with Linux as it is with Windows VMs. Of course, it will be different within the OS, but there is nothing Azure-specific from that aspect.

Okay, here we go the Pros and Cons:

Pros:

  • Azure Shared disks allows for the use of what is considered to be “legacy clustering technology” in Azure.
  • Can be leveraged by familiar tools such as Windows Failover Cluster Manager, Scale-out File Server, and Linux Pacemaker/Corosync.
  • Premium and Ultra Disks are supported so performance shouldn’t be an issue in most cases.
  • Supports SCSI Persistent Reservations.
  • Fairly simple to setup.

Cons:

  • Does not scale well, similar to what would be expected with a SAN mapping.
  • Only certain disk types are supported.
  • ReadOnly host caching is not available for Premium SSDs with maxShares >1.
  • When using Availability Sets and Virtual Machine Scale sets, storage fault domain alignment with the VMs are not enforced on the shared data disk.
  • Azure Backup not yet supported.
  • Azure Site Recovery not yet supported.

Alright, that’s it for Azure Shared Disks! Go take a look at my Github Repository and give shared disks a shot!

Please reach out to me in the comments, LinkedIn, or Twitter with any questions or comments about this blog post or this series.  

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