This document provides an overview of high availability and site resilience features in Microsoft Exchange Server 2013, focusing on Database Availability Groups (DAGs) and their components including the DAG replication service, active manager, and cluster service. It discusses the role of witness servers in maintaining quorum during failovers, their placement recommendations based on different deployment scenarios, and outlines the enhanced functionalities introduced in Exchange Server 2013. Additionally, the document emphasizes specific considerations for witness server deployment across multiple datacenters and advises against the use of infrastructure as a service (IaaS) for this purpose.

2. Exchange Server 2013
High Availability and Site Resilience
(1/2)
Scott Schnoll
Senior Content Developer
Microsoft Corporation
scott.schnoll@microsoft.com
http://aka.ms/Schnoll
Twitter: @Schnoll
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3. Agenda – Part 1
• Database Availability Group Internals
• Witness Server
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4. Agenda – Part 2 (16:30-17:15, salle 253)
• Dynamic quorum
• DAG member maintenance
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5. DATABASE AVAILABILITY GROUP
INTERNALS
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6. DAG Internals
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Microsoft Exchange Replication service
Active Manager Client component
Microsoft DAG Management service
Cluster service and components
Windows Crimson Channel
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7. DAG Replication Service
• Introduced in Exchange 2007 RTM
– Microsoft Exchange Replication service |
MSExchangeRepl
– MSExchangeRepl.exe
– Required on all Mailbox servers (not just DAG
members)
– Communicates with Active Directory and other DAG
members
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8. DAG Replication Service
• Includes 16 components
Active Directory lookup
Copy status lookup
Replay core manager
Seed manager
Autoreseed manager
• Disk reclaimer manager
• Replay RPC server wrapper
• Remote data provider
wrapper
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• Active Manager
• Active Manager RPC server
wrapper
• Failure item manager
• TPR API manager
• Support API manager
• Server locator manager
• Health state tracker
• VSS Writer
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9. Active Manager Client Component
• Runs inside client access and transport
services
– Microsoft Exchange RPC Client Access MSExchangeRPC
– Microsoft Exchange Transport –
MSExchangeTransport
– Microsoft Exchange Frontend Transport –
MSExchangeFrontendTransport
– Client Access Front End (CAFÉ) components
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10. Active Manager Client Component
• When connecting clients or routing
messages, client access and transport
services query Active Directory and Active
Manager to find out location of the active
copy of a mailbox
– DAG members with Standby Active Manager (SAM)
role respond to these queries
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11. DAG Management Service
• Introduced in Exchange 2013 CU2
– Microsoft Exchange DAG Management service | MSExchangeDagMgmt
– MSExchangeDagMgmt.exe
– Runs on all Mailbox servers (not just DAG members)
– Communicates with Active Directory and other DAG
members
• Includes 4 components
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Active Directory lookup
Copy status lookup
Monitoring
Tracer instance
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12. DAG Management Service
• Created for two primary reasons:
– so the Replication service can have more focused
functionality
– so Managed Availability actions can kill lower-priority
activities
• Logs events in same place as Replication
service
• Other functions will move to this service
– AutoReseed, Disk reclaimer, Dynamic replay lag playdown
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– Future AutoDAG copy layout and mobility features

13. Cluster service
• Introduced in NT Server Enterprise Edition
(1997)
– Cluster Service | ClusSvc
– Clussvc.exe
• Exchange DAGs use several cluster
components
– Membership and node management
– Networks and heartbeating
– Quorum
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14. Cluster service
• Quorum is required in order to mount databases
• Quorum is based on votes, not membership
• Voting can be rigged
– Votes can be taken away manually or dynamically
• Exchange manages quorum model, not quorum
– Exchange management of quorum model based on nodes, not
votes
– Removing votes requires manual configuration of quorum model
– Exchange will make incorrect quorum model management
decisions if votes are manually removed at the cluster level
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15. Cluster registry
• Active Manager stores database / server information in the cluster
registry for DAG members
– Registry changes are replicated immediately to all DAG members
• Stored information is used as part of BCSS
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16. Cluster registry
• ActiveServer
– Name of server where database is currently mounted or
expected to be mounted when mount operation completes
• LastMountServer
– Name of server where database was last successfully
mounted
• LastMountedTime
– Date and time stamp of the last time database was mounted
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17. Cluster registry
• MountStatus
– Current mount status for database (mounted /
dismounted)
• IsAdminDismounted
– Designates whether current dismounted status is the
result of administrator action (true / false)
• IsAutomaticActionsAllowed
– Designates whether the database can be automatically
activated (true / false)
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18. Crimson Channel
• Applications and Services logs
– Area of Windows Server event log used by applications for logging
and internal communication
– These logs store events from a single application or component
rather than events that might have system-wide impact
– This is referred to as an application's crimson channel
• Exchange 2013 has a crimson channel with multiple
areas
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ActiveMonitoring
HighAvailability
MailboxDatabaseFailureItems
ManagedAvailability
PushNotifications
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19. #mstechdays
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20. WITNESS SERVER AND WITNESS
SERVER PLACEMENT
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21. Witness Server
• A server that participates in a failover
cluster with an even number of members
– Is not a member of the cluster/DAG
– Does not contain a copy of quorum data
• File share on this server is represented by
File Share Witness resource in cluster core
resource group
– Uses IsAlive check for availability
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22. Witness Server
• File Share Witness Resource Behavior
– If server or share are not available, cluster resources
are failed and moved to another node
– If FSW resource does not come back online, it remains
in a Failed state, with restart attempts every 60
minutes
– If witness server needed for quorum, and resource
cannot be brought online, quorum will be lost
• Single restart attempt for FSW resource in Failed
state
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23. Witness Server
• When witness server is needed to maintain
quorum, one of the nodes locks the
witness.log on witness server
– Node that locks witness.log file is called the locking
node
– If enough nodes are in contact with the locking node to
constitute a majority, quorum is maintained
– Nodes that can’t communicate with locking node lose
quorum
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24. Witness Server
• Attempts to lock witness.log file occur in a
specific order
– Node that owns cluster core resource group tries
immediately
– Nodes not owning cluster core resources wait 6
seconds before trying
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25. Witness Server
•Cluster Core Resources
•Sequence #: 20
22
21
•Lock witness.log
•Sequence #: 21
•Sequence #: 20
22
Challenging node
attempts witness lock.
Lock already exists –
sequence # higher,
challenge not successful.
Cluster state change –
node owning cluster
core resources locks FSW
– updates sequence
number
0
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12
All nodes available.
FSW lock released.
Changes replicated,
sequence numbers in
sync.
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26. Witness Server
•Sequence #: 22
•Cluster Core Resources
•Sequence #: 20
•Lock witness.log
•Sequence #: 21
•Sequence #: 20
•Cluster Core Resources
•Sequence #: 21
22
Cluster state change –
node owning cluster
core resources
unavailable.
0
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1
2
3
4
5
Challenging node
attempts witness lock.
No lock exists, lock
successful, sequence
number updated.
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All nodes available.
FSW lock released.
Changes replicated,
sequence numbers in
sync.
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14
15
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27. Witness Server Placement
• Exchange 2010 guidance
– “We recommend that you use a Hub Transport server running
on Exchange Server 2010 in the Active Directory site
containing the DAG. This allows the witness server/directory to
remain under the control and visibility of an Exchange
administrator.”
– “If your DAG is extended to multiple datacenters, we
recommend deploying the witness server in the datacenter
considered to be the primary datacenter.”
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28. Witness Server Placement
• Exchange 2013 guidance more complicated
due to options introduced by architectural
changes
• Exchange 2013 includes support for new DAG
configuration options
– A third location, such as a third physical datacenter or
branch office
• Ultimately, the placement of a DAG’s witness
server depends on your business
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29. Witness Server Placement
Deployment scenario
Placement Recommendation
Single DAG deployed in one
datacenter
Locate witness server in the same datacenter as DAG members
Single DAG deployed across two
datacenters; no additional locations
available
Locate witness server in primary datacenter
Multiple DAGs deployed in one
datacenter
Locate witness server in the same datacenter as DAG members.
Additional options include:
• Using the same witness server for multiple DAGs
• Using a DAG member to act as a witness server for a different DAG
Multiple DAGs deployed across two
datacenters
Locate witness server in the same datacenter as DAG members.
Additional options include:
• Using the same witness server for multiple DAGs
• Using a DAG member to act as a witness server for a different DAG
Single or Multiple DAGs deployed
across more than two datacenters
Locate the witness server in the datacenter where you want the majority
of quorum votes to exist
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30. Witness Server Placement
• If your organization has a 3rd location, a
witness server can be deployed there for
automatic database failover between two
other sites
– The witness server location must have network
infrastructure and connectivity that is isolated from network
failures that affect the two datacenters with DAG members
• For all DAGs, the availability of the witness
server should be on the Exchange
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31. Witness Server Placement
• IaaS providers and cloud providers are not
supported for use as a witness server
– This includes Windows Azure, which does not yet
support the required underlying network configuration
to allow an Azure file server VM to act as a witness
server in a multi-datacenter deployment
– More info at http://aka.ms/DAGAzure
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32. Related Content
• Exchange 2013 Haute disponibilité et
tolérance aux sinistres (Session 2/2
première partie) – 12/02/14 - 16:30-17:15,
salle 253
• Exchange 2013 Dimensionnement et
Performance – 12/02/14 – 17:45-18:30,
salle 252B
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33. QUESTIONS?
Thank You!
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