VMware vSAN and Virtual Volumes specifics

In the last couple of sections we have discussed the ins and out of HA. All of it based on VMFS based or NFS based storage. With the introduction of VMware vSAN and Virtual Volumes also comes changes to some of the discussed concepts.

HA and vSAN

vSAN is VMware’s approach to Software Defined Storage. We are not going to explain the ins and outs of vSAN, but do want to provide a basic understanding for those who have never done anything with it. vSAN leverages host local storage and creates a shared data store out of it.

Figure 27 - Virtual SAN Cluster

vSAN requires a minimum of 3 hosts and each of those 3 hosts will need to have 1 SSD for caching and 1 capacity device (can be SSD or HDD). Only the capacity devices will contribute to the available capacity of the datastore. If you have 1TB worth of capacity devices per host then with three hosts the total size of your datastore will be 3TB.

Having that said, with vSAN 6.1 VMware introduced a "2-node" option. This 2-node option is actually 2 regular vSAN nodes with a third "witness" node.

The big differentiator between most storage systems and vSAN is that availability of the virtual machine’s is defined on a per virtual disk or per virtual machine basis. This is called “Failures To Tolerate” and can be configured to any value between 0 (zero) and 3. When configured to 0 then the virtual machine will have only 1 copy of its virtual disks which means that if a host fails where the virtual disks are stored the virtual machine is lost. As such all virtual machines are deployed by default with Failures To Tolerate(FTT) set to 1. A virtual disk is what vSAN refers to as an object. An object, when FTT is configured as 1 or higher, has multiple components. In the diagram below we demonstrate the FTT=1 scenario, and the virtual disk in this case has 2 "data components" and a "witness components". The witness is used as a "quorom" mechnanism.

Figure 28 - Virtual SAN Object model

As the diagram above depicts, a virtual machine can be running on the first host while its storage components are on the remaining hosts in the cluster. As you can imagine from an HA point of view this changes things as access to the network is not only critical for HA to function correctly but also for vSAN. When it comes to networking note that when vSAN is configured in a cluster HA will use the same network for its communications (heartbeating etc). On top of that, it is good to know that VMware highly recommends 10GbE to be used for vSAN.

Basic design principle: 10GbE is highly recommend for vSAN, as vSphere HA also leverages the vAN network and availability of VMs is dependent on network connectivity ensure that at a minimum two 10GbE ports are used and two physical switches for resiliency.

The reason that HA uses the same network as vSAN is simple, it is too avoid network partition scenarios where HA communications is separated from vSAN and the state of the cluster is unclear. Note that you will need to ensure that there is a pingable isolation address on the vSAN network and this isolation address will need to be configured as such through the use of the advanced setting “das.isolationAddress0”. We also recommend to disable the use of the default isolation address through the advanced setting “das.useDefaultIsolationAddress” (set to false).

When an isolation does occur the isolation response is triggered as explained in earlier chapters. For vSAN the recommendation is simple, configure the isolation response to “Power Off, then fail over”. This is the safest option. vSAN can be compared to the “converged network with IP based storage” example we provided. It is very easy to reach a situation where a host is isolated all virtual machines remain running but are restarted on another host because the connection to the vSAN datastore is lost.

Basic design principle: Configure your Isolation Address and your Isolation Policy accordingly. We recommend selecting “power off” as the Isolation Policy and a reliable pingable device as the isolation address. It is recommended to configure the Isolation Policy to “power off”.

What about things like heartbeat datastores and the folder structure that exists on a VMFS datastore, has any of that changed with vSAN. Yes it has. First of all, in a “vSAN” only environment the concept of Hearbeat Datastores is not used at all. The reason for this is straight forward, as HA and vSAN share the same network it is safe to assume that when the HA heartbeat is lost because of a network failure so is access to the vSAN datastore. Only in an environment where there is also traditional storage the heartbeat datastores will be configured, leveraging those traditional datastores as a heartbeat datastore. Note that we do not feel there is a reason to introduce traditional storage just to provide HA this functionality, HA and vSAN work perfectly fine without heartbeat datastores.

Normally HA metadata is stored in the root of the datastore, for vSAN this is different as the metadata is stored in the VMs namespace object. The protectedlist is held in memory and updated automatically when VMs are powered on or off.

Now you may wonder, what happens when there is an isolation? How does HA know where to start the VM that is impacted? Lets take a look at a partition scenario.

Figure 29 - VSAN Partition scenario

In this scenario there a network problem has caused a cluster partition. Where a VM is restarted is determined by which partition owns the virtual machine files. Within a vSAN cluster this is fairly straight forward. There are two partitions, one of which is running the VM with its VMDK and the other partition has a VMDK replica and a witness. Guess what happens? Right, vSAN uses the witness to see which partition has quorum and based on that result, one of the two partitions will win. In this case, Partition 2 has more than 50% of the components of this object and as such is the winner. This means that the VM will be restarted on either “esxi-03″ or “esxi-04″ by vSphere HA. Note that the VM in Partition 1 will be powered off only if you have configured the isolation response to do so. We would like to stress that this is highly recommended! (Isolation response –> power off)

One final thing which is different for vSAN is how a partition is handled in a stretched cluster configuration. In a regular stretched cluster configuration using VMFS/NFS based storage VMs impacted by APD or PDL will be killed by HA. With vSAN this is slightly different. HA VMCP in 6.0 is not supported with vSAN unfortunately. vSAN has its own mechanism. vSAN recognizes when a VM running on a group of hosts has no access to any of the components. When this is the case vSAN will simply kill the impacted VM. You can disable this behavior, although we do not recommend doing this, by setting the advanced host setting called VSAN.AutoTerminateGhostVm to 0.

HA and Virtual Volumes

Let us start with first describing what Virtual Volumes is and what value it brings for an administrator. Virtual Volumes was developed to make your life (vSphere admin) and that of the storage administrator easier. This is done by providing a framework that enables the vSphere administrator to assign policies to virtual machines or virtual disks. In these policies capabilities of the storage array can be defined. These capabilities can be things like snapshotting, deduplication, raid-level, thin / thick provisioning etc. What is offered to the vSphere administrator is up to the Storage administrator, and of course up to what the storage system can offer to begin with. In the below screenshot we show an example for instance of some of the capabilities Nimble exposes through policy.

Figure 30 - Capabilities exposed by Nimble array

When a virtual machine is deployed and a policy is assigned then the storage system will enable certain functionality of the array based on what was specified in the policy. So no longer a need to assign capabilities to a LUN which holds many VMs, but rather a per VM or even per VMDK level control. So how does this work? Well lets take a look at an architectural diagram first.

Figure 31 - Virtual Volumes Architecture

The diagram shows a couple of components which are important in the VVol architecture. Lets list them out:

  • Protocol Endpoints aka PE
  • Virtual Datastore and a Storage Container
  • Vendor Provider / VASA
  • Policies
  • Virtual Volumes

Lets take a look at all of these three in the above order. Protocol Endpoints, what are they?

Protocol Endpoints are literally the access point to your storage system. All IO to virtual volumes is proxied through a Protocol Endpoint and you can have 1 or more of these per storage system, if your storage system supports having multiple of course. (Implementations of different vendors will vary.) PEs are compatible with different protocols (FC, FCoE, iSCSI, NFS) and if you ask me that whole discussion with Virtual Volumes will come to an end. You could see a Protocol Endpoint as a “mount point” or a device, and yes they will count towards your maximum number of devices per host (256). (Virtual Volumes it self won’t count towards that!)

Next up is the Storage Container. This is the place where you store your virtual machines, or better said where your virtual volumes end up. The Storage Container is a storage system logical construct and is represented within vSphere as a “virtual datastore”. You need 1 per storage system, but you can have many when desired. To this Storage Container you can apply capabilities. So if you like your virtual volumes to be able to use array based snapshots then the storage administrator will need to assign that capability to the storage container. Note that a storage administrator can grow a storage container without even informing you. A storage container isn’t formatted with VMFS or anything like that, so you don’t need to increase the volume in order to use the space.

But how does vSphere know which container is capable of doing what? In order to discover a storage container and its capabilities we need to be able to talk to the storage system first. This is done through the vSphere APIs for Storage Awareness. You simply point vSphere to the Vendor Provider and the vendor provider will report to vSphere what’s available, this includes both the storage containers as well as the capabilities they possess. Note that a single Vendor Provider can be managing multiple storage systems which in its turn can have multiple storage containers with many capabilities. These vendor providers can also come in different flavours, for some storage systems it is part of their software but for others it will come as a virtual appliance that sits on top of vSphere.

Now that vSphere knows which systems there are, what containers are available with which capabilities you can start creating policies. These policies can be a combination of capabilities and will ultimately be assigned to virtual machines or virtual disks even. You can imagine that in some cases you would like Quality of Service enabled to ensure performance for a VM while in other cases it isn’t as relevant but you need to have a snapshot every hour. All of this is enabled through these policies. No longer will you be maintaining that spreadsheet with all your LUNs and which data service were enabled and what not, no you simply assign a policy. (Yes, a proper naming scheme will be helpful when defining policies.) When requirements change for a VM you don’t move the VM around, no you change the policy and the storage system will do what is required in order to make the VM (and its disks) compliant again with the policy. Not the VM really, but the Virtual Volumes.

Okay, those are the basics, now what about Virtual Volumes and vSphere HA. What changes when you are running Virtual Volumes, what do you need to keep in mind when running Virtual Volumes when it comes to HA?

First of all, let me mention this, in some cases storage vendors have designed a solution where the "vendor provider" isn't designed in an HA fashion (VMware allows for Active/Active, Active/Standby or just "Active" as in a single instance). Make sure to validate what kind of implementation your storage vendor has, as the Vendor Provider needs to be available when powering on VMs. The following quote explains why:

When a Virtual Volume is created, it is not immediately accessible for IO. To Access Virtual Volumes, vSphere needs to issue a “Bind” operation to a VASA Provider (VP), which creates IO access point for a Virtual Volume on a Protocol Endpoint (PE) chosen by a VP. A single PE can be the IO access point for multiple Virtual Volumes. “Unbind” Operation will remove this IO access point for a given Virtual Volume.

That is the "Virtual Volumes" implementation aspect, but of course things have also changed from a vSphere HA point of view. No longer do we have VMFS or NFS datastores to store files on or use for heartbeating. What changes from that perspective. First of all a VM is carved up in different Virtual Volumes:

  • VM Configuration
  • Virtual Machine Disk's
  • Swap File
  • Snapshot (if there are any)

Besides these different types of objects, when vSphere HA is enabled there also is a volume used by vSphere HA and this volume will contain all the metadata which is normally stored under "/<root of datastore>/.vSphere-HA/<cluster-specific-directory>/" on regular VMFS. For each Fault Domain a seperate folder will be created in this VVol as shown in the screenshot below.

Figure 32 - Single FDM cluster on VVol array

All VM related HA files which normally would be under the VM folder, like for instance the power-on file, heartbeat files and the protectedlist, are now stored in the VM Configuration VVol object. Conceptually speaking similar to regular VMFS, implementation wise however completely different.

Figure 33 - VVol FDM files

The power-off file however, which is used to indicate that a VM has been powered-off due to an isolation event, is not stored under the .vSphere-HA folder any longer, but is stored in the VM config VVol (in the UI exposed as the VVol VM folder) as shown in the screenshot below. The same applies for vSAN, where it is now stored in the VM Namespace object, and for traditional storage (NFS or VMFS) it is stored in the VM folder. This change was made when Virtual Volumes was introduced and done to keep the experience consistent across storage platforms.

Figure 34 - VVol based poweroff file

And that explains the differences between traditional storage systems using VMFS / NFS and new storage systems leveraging Virtual Volumes or even a full vSAN based solution.

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