stefan_varga
Contributor
Contributor

fault domains in stretched cluster

Is there a concept of fault domains also available in stretched cluster configurations?

i other words can I have fault domains configured in conjuction with PFTT and SFTT especially to tell VSAN to store VM components in separate fault domains?

thank you

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jameseydoyle
VMware Employee
VMware Employee

Yes, in stretched clusters there are 3 Fault Domains. The Witness Host constitutes one fault domain, and each group of hosts in each physical location of the stretched cluster form the other 2 fault domains.

It is for stretched clusters, in fact, that this concept of Primary and secondary fault domains was devised.

So typically, a VM object would be recreated within a cluster half (i.e. one physical location) to ensure data locality is honoured where possible.

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TheBobkin
VMware Employee
VMware Employee

Hello Stefan,

Stretched-clusters are designed around the concept of Fault Domains (FDs), these are the only vSAN clusters in which these must be configured.

vSAN will by-design place data-components distributed across Fault Domains - e.g. Assuming 3 FDs configured, an FTT=1 vSAN Object with 3 components (2 data,  1 witness) will place 1 data component on each data-site and the witness component on the Witness Appliance.

vSAN 6.6 has the capability to choose site-affinity for Object creation when using FTT=0 Objects and Local Protection (PFTT/SFTT):

http://cormachogan.com/2017/04/11/whats-new-vsan-6-6/

More resources about FDs and stretched clusters:

storagehub.vmware.com/#!/vmware-vsan/vmware-vsan-6-6-technical-overview-1/fault-domains-10

https://storagehub.vmware.com/export_to_pdf/vsan-stretched-cluster-2-node-guide

yellow-bricks.com/2017/03/29/vsan-needs-3-fault-domains/

Bob

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stefan_varga
Contributor
Contributor

thank you

assuming I set PFTT=1 (cross site mirror) and SFTT=1(in site mirror), my question is can I specify additional  fault granularity per site basis? for example I have 4 nodes per site, 2 and 2 are located in separate racks, how can I know that VSAN will not place the data into 2 nodes in the same rack? I understand that It does not have impact on overall VM availability.

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TheBobkin
VMware Employee
VMware Employee

Hello Stefan,

As per Cormac's article I referenced there - No, it cannot ensure rack-awareness using this feature.

Bob

depping
Leadership
Leadership

We do not offer that yet, we have however filed feature requests for this. If and when it will make a release is something I cannot comment on however.

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