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kevinE1979
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vSAN for Remote Office

I set up vSAN with 2 hosts and a witness to do some testing. If it works, I will deploy this to my remote sites that have 2 physical servers. Because of this, I will need to have a witness which is fine. Can I set this up so that there isn't a fault domain since both hosts are in the same location? I want to be able to run on both hosts and not have any performance degradation if I move a VM between hosts.

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elerium
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short answer, yes, witness only type setup will use stretched cluster/fault domains.

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zdickinson
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Good morning, maybe someone with more experience with fault domains can chime in, but it seems like there is some confusion with them.  In your case, I don't think you would setup any fault domains.  As an example, a fault domain would be used if you had 4 hosts in 3 racks.  Rack 1 with 2 hosts and then 2 and 3 with 1 host each.  You would setup 3 fault domains, that way you could lose any 1 rack and still have all VMs accessible.  vSAN would make sure the witness and the two copies of the data are all in separate domains.

Hope that helps.  Thank you, Zach.

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If you are running 2 physical host + remote witness (aka VSAN 2 node ROBO), you will need to setup fault domains as described in the stretched cluster guide http://www.vmware.com/files/pdf/products/vsan/VMware-Virtual-SAN-6.2-Stretched-Cluster-Guide.pdf.  VSAN 2 node ROBO setup is actually a stretched cluster setup as a 1+1+1 environment (read more here:A brief overview of the new Virtual SAN 6.1 - CormacHogan.com). Otherwise you will need to use 3 servers at your remote office (2 hosts + 1 witness) in your remote sites with no fault domains (regular VSAN 3 node setup).


For 2 node ROBO setup, you can disable site read locality to improve performance since the 2 hosts are in the same location, probably connected to the same switch. This would allow round-robin reads/default VSAN read algorithm instead of stretched cluster read algorithm.

Page 45 of stretched cluster guide http://www.vmware.com/files/pdf/products/vsan/VMware-Virtual-SAN-6.2-Stretched-Cluster-Guide.pdf

Disable VSAN site locality in low latency stretched cluster - Yellow Bricks


I just got my first VSAN 2 node ROBO working a week ago (2 hosts in remote office, witness host VM back in HQ datacenter). Getting L3 routing and adding static routes to allow hosts/witness to communicate was the part that took us the most time get working properly. Hopefully future versions will make this easier with GUI/menus instead of esxcfg commands.

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kevinE1979
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Thanks. Would it be possible to set up a witness at the site that does nothing but be a witness? Would this still be like a ROBO setup or could you do one without fault domains?

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If the witness only performs witness functions (i.e. doesn't store any data), then it would work like the ROBO setup requiring fault domains. You would need another machine at the site to serve as the witness or serve a witness VM.

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kevinE1979
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If the witness only performs witness functions (i.e. doesn't store any data), then it would work like the ROBO setup requiring fault domains. You would need another machine at the site to serve as the witness or serve a witness VM.

I'm confused. You mentioned a witness in both scenarios. No matter what, if I have a witness that acts like a witness only and doesn't host VM's then it will work like a ROBO setup and require fault domains, correct?

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zdickinson
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Good morning, there is a ROBO scenario where you have two physical hosts that are licensed and run the VMs and then a witness appliance that is a VM.  Does that meet your needs?  Basically, 2 node vSAN.  Thank you, Zach.

A closer look at the VSAN witness appliance - CormacHogan.com

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short answer, yes, witness only type setup will use stretched cluster/fault domains.

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kevinE1979
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Yes, but it needs fault domains, right?

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