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Kgah
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vSAN stretched cluster 2+2+1

Trying to put 1+1+1 scenario vs 2+2+1 but google says that 2+2+1 basically doesn’t give any advantage and says 3+3+1 is the way to go. I don’t really have a need for 6 nodes compute wise. If I understand it correctly you want 3 nodes in a site because you also want a local witness besides the external one. Does that mean that node nr 3 in a site doesn’t need to be a full compute node with disks but only a witness?

Or does the local witness in a 3node scenario also hold a copy of the raid1 data and need the same amount and size of disks?

Basically what I would like from a 2+2+1 that I wouldn’t  get from a 1+1+1 is raid1 also between the local nodes and not only between the sites. Is this possible with 4 compute nodes in total across 2 sites?

The only arguments in favor of 1+1+1 is basically cost, licensing and no switches etc.

 

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depping
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that is not how vsan works. all hosts within a location are expected to be similar as components of objects are distributed across the three hosts. for a virtual disk with RAID-1 you will have (usually at minimum) 2 data components and a witness component. When you have multiple objects (a disk is an object, swap is an object etc) the components of that object are distributed. So within a location you do not have a dedicated witness host, all hosts can contain witness components.

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CyberNils
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Kgah
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Ok, but that link doesn’t really answer what the configuration have to be for node nr 3 at each site. Can that just be a witness appliance without the disk hardware and I will still have raid 1 site redundancy with just 2 fully spacced out nodes at each site?

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TheBobkin
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@Kgah , No you can't have a Witness on each site of a 3+3+1 (e.g. 3 Witnesses total), these need to be physical nodes.


The benefit of 3+3+1 over a smaller configuration is that local protection to Objects can be applied if needed (e.g. a RAID-1 on each site mirrored across sites or just local-only RAID1 pinned to just one site) which isn't possible in a smaller cluster as minimum number of nodes to do RAID1 (either as a normal 3-node cluster or if you want to use local RAID1 protection).

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Kgah
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Man this is confusing 🙂 ok let’s keep server 3 as a physical node. But does it need to be speced the same as the other two with regards to cpu, memory and disk? I mean it won’t take over any vm load?

Also will server nr3 require any licenses or can this be an older server running esxi for example?

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depping
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Just to be clear:

1+1+1 is supported for a stretched cluster, and you can do this even with a Standard License of vSphere. If you want a local R1 in each location you will need 3+3+1.

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depping
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And when we talk about 1+1+1 or 3+3+1 the last "+1" refers to the witness. The witness can be a physical host, or a Witness VM. The Witness VM will still need to run on something of course, and it needs to be in a 3rd location!

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Kgah
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Yes, I’m with you guys so far. I’m trying to understand the requirements on the 3 nodes in a site in the 3+3+1. If they need to be configured identically? Do all three nodes hold data in a RAID1-scenario.  

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Kgah
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@TheBobkin

As I mentioned above do all 3 nodes in one site need to be fully equipped with disk etc or is one of the three just considered a local witness and can have less hardware?

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depping
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that is not how vsan works. all hosts within a location are expected to be similar as components of objects are distributed across the three hosts. for a virtual disk with RAID-1 you will have (usually at minimum) 2 data components and a witness component. When you have multiple objects (a disk is an object, swap is an object etc) the components of that object are distributed. So within a location you do not have a dedicated witness host, all hosts can contain witness components.

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