![]() In case you need those hardware resources for your VM’s data, instead of using a dedicated physical ESXi host, you can deploy a vSAN witness appliance. Keep in mind that the physical witness host doesn’t store any of the VM’s data. The general recommendation is to place the vSAN Witness Host in a different datacenter.įig.4. vSAN Witness Host in a separate datacenterĪnother golden rule says that the witness host should be the same version and build as the vSAN data nodes because although the vSAN Witness Host is not part of the vSphere Cluster, it is contributing disks to the vSAN Cluster. For example, each witness appliance or witness host should be deployed outside of the Stretched cluster or the 2-Node cluster, but it should be part of the same vCenter. Now that we know how crucial the role of the witness host is, we can look at some of the most important recommendations concerning its configuration and deployment. Witness host or appliance recommendations vSAN removes this possibility by enabling the witness host to provide additional votes to form a quorum with the remaining site and elect the healthy site where the VM should be vMotioned and restarted. This event may result in an application being active in both sites and might break it. What happens in case of a site failure and no witness host?Ī split-brain scenario may happen when an actual fault occurs within one of the sites, and the sites cannot decide independently which one is the surviving one. ![]() So, let’s get back to the main question – Why do we need a witness host? As we mentioned, it will serve as a tiebreaker in a split-brain scenario, where the witness host will determine which site is the authoritative one. When a policy consisting of Site Disaster Tolerance of Dual Site Mirroring is assigned to an object, vSAN commits any writes from one object synchronously to both, the preferred and non-preferred sites, while reading locally from whichever site the VM resides within. Availability levels in a stretched cluster storage policy The user has the following options for local protection: for a Stretched cluster – 1-3 failures/ RAID-1,5,6, and for a 2-node cluster – 1 failure / RAID-1. Then, depending on the number of Failures to tolerate, each object might have additional replicas or parity components placed on different hosts/devices inside each site. Each VM object should have at least one complete replica on each site to satisfy a policy consisting of “Site disaster tolerance =1”, which is the default policy for this type of cluster. Stretched cluster implements the same model by extending it on a per site level and introducing the witness host as a tiebreaker in case of a failure scenario. The Fault domain concept represents a specific vSAN approach where it treats each host as an independent failure domain, and it makes sure that each component of a given VM object is placed onto a separate host for redundancy. vSAN Stretched cluster resolves the site availability issue by building on top of the concept of a fault domain. Whenever there’s a risk of a failure of an entire site, vSAN Stretched cluster is the recommended configuration to alleviate this type of temporary workload interruption. Why do we need a witness host and how does it work? This witness appliance VM is treated as an ESXi host and will be represented in blue in the vSphere user interface. The witness appliance can be deployed using an OVA template and does not require an additional vSphere license. Each vSAN witness host can be deployed as a witness appliance. The vSAN witness host is basically an ESXi host that does not store VM data but only VM metadata, or in other terms, it stores the witness components for each VM object from a Stretched cluster or a 2-node cluster configuration. Additionally, we’ll cover some more insights around the witness host configuration, witness lifecycle, what are the options for deployment, and are there any specifics when deployed on VMC on AWS or VMware Cloud Foundation (VCF). Let’s dive into the details. What is a vSAN witness host? Why do we need it? How does it work? What would have happened in the case of a vSAN stretched cluster failure scenario without a witness host? We will give answers to these frequently asked questions in the following paragraphs.
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