A VMware vSAN Stretched Cluster configuration runs across three different geographical locations -- two data sites...
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and one site that hosts a special witness component. There are different ways to configure a vSAN Stretched Cluster depending on how you want to protect the workloads that run on each side.
A stretched cluster configuration allows you to access a single cluster with a single vSAN data store from both a main office and a remote office. A single vSAN data store means that this data store -- this storage -- spans across both sites and hosts and that sites have access to the shared data store. In order to establish a connection between these two data sites, you need at least 10 Gigabit Ethernet (GbE) network speed and 5 millisecond roundtrip time.
In the event of a failure at the main office, VMs are started via vSphere High Availability on the remaining hosts within the cluster located at the remote site. Some downtime is to be expected, but the restart is automatic.
The third host -- the witness host -- is a dedicated ESXi host or vSAN witness appliance, whose purpose is to host the witness component of VM objects. This witness host runs at another geographical location. The connection between this third location and the other two data sites is less restrictive and does not require 10 GbE links.
You can define a primary level of failure to tolerate, which protects data through cross-site protection -- a RAID 1 with two copies of data only, one on each site. You can define the secondary level of protection locally within each site and activate not only RAID 1 but also RAID 5 or RAID 6 protection if you have enough hosts within the site. RAID 5 protection requires a minimum of four hosts; RAID 6 requires a minimum of six. Note that the secondary level of failures to tolerate is available only when vSAN is configured as a stretched cluster.
So far, vSAN Stretched Cluster scenarios have only been used for stretched cluster configurations, but they can also be used for remote office/branch office (ROBO) scenarios. In the past, ROBO sites could only be protected with RAID 1, but they can now use a secondary level of protection in conjunction with RAID 5 protection to locally run workloads.
VMware vSAN 6.6 is more flexible than previous versions due to the additional configuration options that ensure critical protection even on workloads that run at remote offices. However, the third witness component, which runs at a third site, is necessary for stretched cluster scenarios.
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