August 28, 2023

vSAN Prescriptive Disk Claim for the ESA in vSAN 8 U2

The new vSAN ESA Prescriptive Disk Claim capability offers new ways for customers and partners to configure quickly and consistently.  Read on to learn more!

The assignment of storage devices for use by vSAN is a necessary step in the initial configuration of vSAN.  The term "disk claiming" as it is known in the product represents this task.  While claiming devices for use by vSAN is relatively easy, limitations in hardware, software, and the unique desires to customize which devices were claimed made it challenging to provide a way to claim devices that met the requirements of simple, repeatable, and flexible.  

The Express Storage Architecture (ESA) in vSAN 8 U2 introduces a new option for claiming storage devices that will help customers and partners standardize the disk claiming process to account for a variety of configuration scenarios.  Let's look at the new vSAN ESA Prescriptive Disk Claim capability in more detail.

A Brief History of Disk Claiming in vSAN

The original storage architecture (OSA) in vSAN provided multiple ways to claim disks.  A fully automated approach was in the early versions of vSAN, but technical limitations in the hardware and the software made this difficult to produce a predictable outcome.  Accurate identification of devices intended as cache devices versus capacity devices, their association with their attached disk controllers, and a user's preference on the quantity of disk groups all made automatic claiming in the OSA difficult at best.  It was removed as an option in favor of a manual disk claiming method.  The manual process guaranteed the ability for a user to claim devices as desired, but many found it a tedious process for new and existing clusters and was still prone to human error and lacked repeatability at scale.

With the introduction of the ESA in vSAN, the manual claiming method remained, but an optional automated disk claiming capability was reintroduced in the form of "vSAN managed disk claim."  This type of disk claiming is inherently much simpler in the ESA because there was no longer the concepts of disk groups and cache devices.  When enabled, it checks the Hardware Compatibility List (HCL) for vSAN, and will claim all eligible devices on the host for use by vSAN. It has been a significant leap forward for many of our customers who wanted this process to be easy and predictable.

ESA Managed Disk Claim

Figure 1.  vSAN managed disk claiming in the ESA.

While the vSAN managed disk claim option is a good one for a large majority of our customers, there remained a need to accommodate a few more uses cases that could not be achieved with the "claim everything eligible" approach used with the vSAN managed disk claim toggle enabled.  Some customers and solution providers needed more customization than the vSAN managed disk claim for ESA could offer.

vSAN ESA Prescriptive Disk Claim

vSAN 8 U2 introduces a new prescriptive disk claim capability for the ESA.  This third option of claiming storage devices in vSAN combines the automation of the managed disk claim option with the customization of a manual claiming approach.  It uses a declarative type of approach that prescribes a desired outcome for both initial Day-0 deployment efforts, and ongoing Day-2 activities such as capacity expansion efforts.  It will apply and continuously check for this desired state across all hosts in a vSAN cluster.  It is a cluster-level mechanism that establishes vCenter Server to be the source of truth for the claiming of disks in hosts that comprise a cluster, to a desired outcome. 

This capability is available to clusters using vSAN ESA, and vSAN Max, which is built using the ESA.

From Desired State to Applied State

This new approach uses a cluster-specific definition residing on the vCenter Server that manages the cluster.  It is where one can set a variety of attributes that will denote device characteristics such as a disk vendor, disk capacity, and the number of disks per hosts.  Many of the attributes are an optional specification to provide as much flexibility as possible.

For example, let’s suppose the ESA prescriptive disk claim for this cluster specifies that 6 devices in each host should be claimed for vSAN even though there are 8 eligible devices in the host.  vSAN will claim no more than 6 eligible devices in each host across the cluster.  If a host is added to the cluster, it will apply this same desired state to the new host.  If additional storage devices are added to each host in increase capacity, no additional devices are claimed until the desired state configuration is adjusted to say otherwise.  Any type of non-compliance such as a change in definition, or a change in a host configuration will trigger a "vSAN Managed disk claim" health finding to identify configuration drift. 

ESA Prescriptive Disk Claim

Figure 2. vSAN ESA Prescriptive Disk Claim process.

There are also safety mechanisms in place that will ensure device handling is treated in a very conservative way when detection of changes is identified.  This will prevent the ESA prescriptive disk claim process from leaving the host in a non-ideal state, or changing a previously claimed device as an unclaimed device.

Who's it for?

The new ESA prescriptive disk claim capability is initially targeted for scenarios that can't be easily or reliably addressed using the automated "managed disk claim" or manual claiming process.  Here are some examples.

  • Third party manufacturers.  Some engineered solutions using vSAN, such as Dell VxRail have custom workflows that assist with configurations.  These custom workflows can now accommodate the intelligence of ESA Prescriptive Disk Claim via API.
  • VMware Cloud Service Providers.  Cloud Service Providers powered by vSAN may have a combination of unique hardware paired with operating procedures for hardware failures and cluster expansion that are different than on-premises customers.  This will allow them to provision hosts consistently and quickly with a desired-state model of storage device claiming.
  • VMware Cloud Foundation.  VMware Cloud Foundation configurations using vSAN may have a need to only consume a subset of eligible disks for use by vSAN.  APIs will be available for VCF to adhere to these configurations.

The ESA prescriptive disk claim capability is initially available via API and CLI only, offering a programmatic way to define a desired outcome of devices to be claimed.  It also can only be used if the vSAN managed disk claim for ESA toggle shown in Figure 1 is turned off. The ESA prescriptive disk claim capability does require vCenter Server to be updated to vCenter 8 U2, as well as all of the hosts in the cluster to be upgraded to vSAN 8 U2.


The new vSAN ESA prescriptive disk claim capability in vSAN 8 U2 can make the management of claimed devices across hosts in a cluster fast, consistent, and repeatable. 


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