Which design should you identify?

HOTSPOT
Your company has two offices. The offices are configured as shown in the following table.

The offices connect to each other by using a WAN link that has a latency of more than 700 ms. You
plan to deploy an Exchange Server 2013 organization to meet the following requirements:
• Ensure that users can access their mailbox if the WAN link fails.
• Ensure that users can access their mailbox if a single server fails.
• Ensure that users can access their mailbox if a single database fails.
You recommend deploying one or more database availability groups (DAGs) and mailbox database
copies. You need to identify which design meets the requirements for the planned deployment.
Which design should you identify?
To answer, select the appropriate design in the answer area.

Answer:

Explanation:

File Share Witness
The file share witness is used to establish a majority node set. This is done by create a share on a
server that gets a little file place into it automatically.
The server hosting the cluster resource (which in the DAG I think is the Primary Activation Manager
server) keeps an open file lock on this file.
The other servers see this open file lock and interpret this as meaning another cluster node is online,
healthy, and available.
A file share witness is used when the DAG contains an even number of servers within it.
When you initially create the DAG you must specify the server and file location that will act as the
file share witness regardless of how many servers are in the DAG (0 to start) to ensure that if you do
add an even number of DAG members the FSW will be properly used.
Database Availability Group
A database availability group (DAG) is a set of up to 16 Microsoft Exchange Server 2013 Mailbox
servers that provide automatic database-level recovery from a database, server, or network failure.
When a Mailbox server is added to a DAG, it works with the other servers in the DAG to provide
automatic, database-level recovery from database, server, and network failures.
DAGs use continuous replication and a subset of Windows failover clustering technologies to provide
high availability and site resilience.
Mailbox servers in a DAG monitor each other for failures. When a Mailbox server is added to a DAG,
it works with the other servers in the DAG to provide automatic, database-level recovery from
database failures.

When you create a DAG, it’s initially empty, and a directory object is created in Active Directory that
represents the DAG. The directory object is used to store relevant information about the DAG, such
as server membership information. When you add the first server to a DAG, a failover cluster is
automatically created for the DAG. In addition, the infrastructure that monitors the servers for
network or server failures is initiated. The failover cluster heartbeat mechanism and cluster database
are then used to track and manage information about the DAG that can change quickly, such as
database mount status, replication status, and last mounted location.
Witness server and witness directory The witness server is a server outside the DAG that acts as a
quorum voter when the DAG contains an even number of members. The witness directory is a
directory created and shared on the witness server for use by the system in maintaining a quorum.
Lagged copy of a mailbox database
A Lagged Mailbox Database Copy is a mailbox database copy configured with a replay lag time value
greater than 0.
A lagged database copy is one that is not updated by replaying transactions as they become
available.
Instead, the transaction logs are kept for a certain period and are then replayed.
The lagged database copy is therefore maintained at a certain remove to the active database and
the other non-lagged database copies. If you are planning to have more than two passive database
copies of a database, think about a lagged copy also as an additional protection against unpredicted
situations Lagged copies aren’t considered highly available copies. Instead, they are designed for
disaster recovery purposes, to protect against store logical corruption.
The greater the replay lag time set, the longer the database recovery process. Depending on the
number of log files that need to replayed during recovery, and the speed at which your hardware
can replay them, it may take several hours or more to recover a database.

The above configuration provides a symmetrical design. All four servers have the same four
databases all hosted on a single disk per server.
The key is that the number of copies of each database that you have should be equal to the number
of database copies per disk.

In the above example, there are four copies of each database: one active copy, two passive copies,
and one lagged copy. Because there are four copies of each database, the proper configuration is
one that has four copies per volume. In addition, activation preference is configured so that it’s
balanced across the DAG and across each server.
For example, the active copy will have an activation preference value of 1, the first passive copy will
have an activation preference value of 2, the second passive copy will have an activation preference
value of 3, and the lagged copy will have an activation preference value of 4.
Lagged mailbox database copy
A passive mailbox database copy that has a log replay lag time greater than zero.
Crossed Lines
DAG Replication 1 and 2
Circled Areas
Site 1 and Site 2 (or DataCenter1 and DataCenter2)
WAN LINK between Site1 and Site2
High Availability with Site Resiliency Exchange 2010 Example
http://jaworskiblog.com/2011/05/17/exchange-2010-design-principles-for-high-availability-and-siteresiliency/
FSW is the File Share Witness

ASIDE
Windows NLB is not supported across sites.
It is not recommended to use an HLB to load balance across sites.
PICTURE1 OFFERS THE BEST DESIGN IN ORDER TO MEET THE SPECIFIED CRITERIA.
MORE FAULTS WITH THE OTHERS.
PICTURE1 HOWEVER DOES NOT OFFER SITE RESILIENCY.
– Ensure that users can access their mailbox if the WAN link fails.
– Ensure that users can access their mailbox if a single server fails.
– Ensure that users can access their mailbox if a single database fails.
Picture1
The DAG is NOT extended across multiple data centers in a site resilience configuration.
The design offers high availability within each site.
However if a node fails or the wan link fails the respective file share witness for each DAG is still
available unlike the other 3 configurations.
Picture2
The DAG is extended across multiple data centers in a site resilience configuration.
No high availability within each site.
If the wan link is unavailable the file share witness for Site2 would be unavailable
This is a split brain scenario, both sites believe that they are the rightful owner of the database, and
thus would mount their respective DB’s. This would cause a divergence in data.
Email could be sent to either database leaving to a difference between the databases on the
respective mailbox servers.
Picture3
The DAG is extended across multiple data centers in a site resilience configuration.
FSW on Site2 in the event of a wan failure means that the servers cannot contact a FSW
Even number of nodes on the respective site with an inability to contact the FSW.
This is a split brain scenario, both sites believe that they are the rightful owner of the database, and
thus would mount their respective DB’s. This would cause a divergence in data.27

Email could be sent to either database leaving to a difference between the databases on the
respective mailbox servers.
A file share witness is used when the DAG contains an even number of servers within it.
A Node Majority quorum model is used for DAGs with an odd number of members.
A Node and File Share Majority quorum is used for DAGs with an even number of members.
The DAG needs to be able to make Quorum.
When 1 node fails in Site1 and the wan link is down,1 out of 2 nodes left is not a majority. Need to
be able to connect to the file share witness to obtain a majority.
Picture4
1 DAG across both sites provides site resiliency but FSW on Site1 in the event of a wan failure means
that the servers cannot contact a FSW A file share witness is used when the DAG contains an even
number of servers within it.
Even number of nodes on the respective site with an inability to contact the FSW.
This is a split brain scenario, both sites believe that they are the rightful owner of the database, and
thus would mount their respective DB’s. This would cause a divergence in data.
Email could be sent to either database leaving to a difference between the databases on the
respective mailbox servers.
A file share witness is used when the DAG contains an even number of servers within it.
A Node Majority quorum model is used for DAGs with an odd number of members.
A Node and File Share Majority quorum is used for DAGs with an even number of members.