But what if there is a catastrophic event? Fire, flood,
earthquake
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apps fail over to a separate physical location Servers in
separate locations in the same cluster
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Recovery Time Objective (RTO)Recovery Point Objective
(RPO)
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WAN Different datacenters (usually) equates to different
subnets Longer distance means greater network latency
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PropertyDefaultRecommendedDescription SameSubnetDelay
11Frequency heartbeats (HB) sent SameSubnetThreshold 510Missed HB
before interface considered down CrossSubnetDelay 11Frequency HB
sent to nodes on dissimilar subnets CrossSubnetThreshold 520 Missed
HB before interface considered down to nodes on dissimilar subnets
PowerShell: (Get-Cluster).SameSubnetThreshold = 10
(Get-Cluster).CrossSubnetThreshold = 20
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Dependencies in Cluster Validation Report Network Name Resource
IP Address Resource A IP Address Resource B OR
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10.10.10.10 DNS Replication Record Created Record Updated
Record Obtained 20.20.20.20 DNS Client access point fails across
subnets Client needs new address Nodes in dissimilar subnets
Adjust intra-node heartbeat thresholds Understand NetName
Resource Configuration Optimize Client Reconnection on CAP Failover
Encrypt intra-node communication over unsecure WANs
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Each node can have 1 vote Witness can only have 1 vote
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Vote 5 1 2 3 4 Site 2 Down!!! Site 1 can reach Cloud Witness!
Cluster Survives!
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Azure Witness
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Cloud WitnessFile Share Witness Share the same arbitration
logic Do not keep copy of cluster database
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Cluster Site 1 Site 2
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1 2 3 4 Vote Loss of Primary Site: Start-ClusterNode
-ForceQuorum Recovery of Primary Site: Start-ClusterNode
-PreventQuorum
Recommended to use Cloud Witness When no access to Azure use
File Share Witness in a 3 rd site Automatic failover Keep number of
nodes on primary and secondary sites equal Manual failover Remove
votes of nodes on secondary site
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Chicago (you are here) NYC Can you hear me now?
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Replication Block-level, volume-based Synchronous &
asynchronous SMB 3.1.1 transport Flexibility Any Windows volume Any
fixed disk storage Any storage fabric Managemen t Failover Cluster
Manager Windows PowerShell WMI End to end MS Storage Stack
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Cluster Site1 Site2
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Applications (local or remote) Source Server Node (SR) Data Log
1 t 2 Destination Server Node (SR) Data Log t1t1 3 2 5 4