Windows Fabric and Server Placement – Part II


Part I of this article talks about Windows Fabric basics. In part II, will describe how Lync Server 2013 and Skype for Business is tied together with Windows Fabric. Will call out few best practices which should be taken care while placing Front End Servers in virtualized environment.

In Lync Server 2013 & Skype for Business Server 2015, a pool can have maximum 12 Front End Servers. Lync Server 2013 works with Windows Fabric V1 while Skype for Business works with Windows Fabric V2/V3. As Lync Server 2013 and Skype for Business Server Front End pools uses a distributed systems model which is based on Windows Fabric. In this model, it keeps important users and conference data of each user on as many as three Front End Servers in a FE Pool. With this model a least number of FE servers must be running for the pool to function. There are two loss modes where quorum loss comes in picture for a FE Pool.

Routing group level quorum loss: Every user is assigned to a particular routing group with in a FE Pool and there are three servers in one Routing group where one is a primary replica and another two are secondary replicas. When enough replica servers of a particular routing group become unavailable then it results in routing group level quorum loss.

Total number of servers in the pool Number of servers that must be running for the pool to be started the first time
2 1
3 3
4 3
5 4
6 5
7 5
8 6
9 7
10 8
11 9
12 10

 

Pool level quorum loss: Same as Windows Server Cluster, FE pool needs minimum N/2+1 to make the pool state up and running. In the odd numbers it is automatically taken care while in the even numbers Primary SQL database plays a role of witness. When minimum numbers of server in a FE Pool becomes unavailable, it results in Pool level quorum loss.

Total number of Front End Servers in the pool Number of servers that must be running for pool to be functional
2 1
3-4 Any 2
5-6 Any 3
7 Any 4
8-9 Any 4 of the first 7 servers
10-12 Any 5 of the first 9 servers

 

If organization has plan to deploy Skype for Business Server in virtualized environment, FE Servers placement is a key to make sure minimum impact on FE services in case one host goes down. If you place more than one FE Server on any particular host in virtualized environment that can surely result in routing group level quorum loss or might be result in Pool level quorum loss as well if pool has only four FE Servers and SQL primary database is part of the host which is down.

The above example consider SfB deployment with two physical hosts where each host caters two FE Server and one SQL Back End Server. If only routing group goes down then it can be recovered via Reset-CsPoolRegistrarState -ResetType QuorumLossRecovery cmdlet. Please make sure if two FE Servers in a particular routing group go down, the users belongs to that routing group will be downgraded in limited functionality until FE Servers come back or pool registrar state has been reset.

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