Exchange 2010 Stretch Cluster – 2
Posted by Brajesh Panda on May 11, 2010
In my previous article we discussed about 3 Node (ODD Member) DAG (2 in primary site & 1 in secondary site) along with Node Set Majority & File Share Witness Quorum Model.
In this article lets talk about EVEN number based stretched DAG & which has more than 2 member nodes of DAG. So lets consider 4 Node based DAG. Where Primary & Secondary site has 2 nodes each. As this model is based on even number we can’t consider Node Set Majority Quorum. We have to consider a extra vote in quorum model. As DAG concept is based on shared nothing architecture, Disk based Quorum goes out of consideration, so we have to use File Share Based Witness.
If active mailbox databases are only in primary site, we can include all these 4 nodes into a single DAG and create 3 Data Base Copies; Keeping one copy at secondary site for failover.
But in case both sites are going to have active mailboxes we will be needing TWO Stretch DAG between sites with two different file share witness. Well it can be created using 1 node from each site with a file share witness at the primary site. But by doing this we will only have 2 Database Copies & each failure will be resulted as site fail over. So better to increase number of nodes in each site to at least 3 ; DAG1= 2 Node from Primary + 1 Node from Secondary, DAG2= 2 Node from Secondary + 1 Node from Primary. Fail-over & fail-back process for these DAGs will be equal to 1st scenario we discussed in previous article.
If our Active Directory site is streched then we have to use a single CAS Array FQDN for both side CAS servers but it may generate cross site MAPI traffics & complex load balancing issues. But if we have two different AD Sites we can have two different CAS Array Objects in two different site & users in that site can use the same for Outlook connectivity. While using two Active\Active sites better to use two different namespace for client access.