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InfoQ Homepage News Microsoft Ships Preview of Cluster-Friendly Cloud Disks

Microsoft Ships Preview of Cluster-Friendly Cloud Disks

Storage is one of the more mature services in the public cloud, but rarely supports traditional clustered systems. To attract those on-premises workloads, the Microsoft Azure team released a preview of Azure Shared Disks, a block storage option for attaching managed disks to multiple virtual machines.

In a blog post about this preview release, Microsoft corporate vice president Tad Brockway called out categories and Windows-related technologies that can take advantage of Azure Shared Disks.

Azure Shared Disks enables the next wave of block storage workloads migrating to the cloud including the most demanding enterprise applications, currently running on-premises on Storage Area Networks (SANs). These include clustered databases, parallel file systems, persistent containers, and machine learning applications.


With Azure Shared Disks, customers now have the flexibility to migrate clustered environments running on Windows Server, including Windows Server 2008 (which has reached End-of-Support), to Azure. This capability is designed to support SQL Server Failover Cluster Instances (FCI)Scale-out File Servers (SoFS)Remote Desktop Servers (RDS), and SAP ASCS/SCS running on Windows Server.

In general, Azure Disk Storage is Microsoft's service for persistent storage on Azure Virtual Machines. Today, there are four types of disks offered:

  • Standard HDD are the cheapest option and use mechanical disks with up to 32TB of storage and 2000 IOPS per disk.
  • Standard SDD gets recommended for web applications and dev/test environments. It also offers 32TB disks, with up to 6000 IOPS.
  • Premium SSD targets "performance sensitive workloads" and delivers up to 32TB and 20,000 IOPS per disk.
  • Ulta disks are also SSD and designed for "IO-intensive" worklaods like SAP HANA or high performing databases. It maxes out at 70TB and 160,000 IOPS.

Azure Shared Disks are currently only available with Premium SSDs—they say that Ultra Disk support is coming soon—and only as data disks, not operating system disks. 

A shared disk, as Microsoft explains, is exposed as a logical unit number (LUN) to the target virtual machine. To that virtual machine, these "LUNs look like direct-attached storage." All this is possible because these disks support SCSI Persistent Reservations (SCSI PR).

SCSI PR is a well-known industry standard leveraged by applications running on Storage Area Network (SAN) on-premises. Enabling SCSI PR on a managed disk allows you to migrate these applications to Azure as-is.

Microsoft makes clear that Azure Shared Disks aren't a substitute for other storage services, namely Azure Files. Azure Files serves up a managed file share using Server Message Block (SMB) protocol and isn't a replacement for a SAN. Likewise, Azure Blob Storage is an object store not designed for the heavy I/O that workloads demand of attached disks.

While announcing this preview release, Microsoft's Brockway claimed that Azure Shard Disks were an "industry first" solution. Analyst Janakiram MSV pointed out that "GCP had Shared Persistent Disks for [Google Compute Engine] since 2014." He noted, however, that GCE only supports a single VM for read-write access, while Azure Shared Disks support read-write on any VM in the cluster. The day after the Azure announcement, AWS also announced support for multi-attached block storage volumes. It appears that AWS supports read-write permission for any attached virtual machine.

Microsoft encourages those interested in signing up for a preview of the service to fill out this form for early access.

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