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Part V: Enterprise Data Management
■ (^) No requirement for a witness server to achieve high-availability/automatic failover
for your databases.
■ (^) Multiple availability modes, including Asynchronous-commit mode and
Synchronous-commit mode. Asynchronous is a disaster-recovery solution recom-
mended when secondary replicas are distributed over large distances (for example
across a WAN). In Asynchronous mode, the primary does not wait for any of the
secondary replicas to harden their logs; instead, after writing the log record locally
to its log fi le, it then sends the transaction confi rmation back to the client.
In Synchronous-commit mode, the primary and secondary are always fully syn-
chronized. This means that the primary replica must wait until all the secondary
replicas harden their logs to disk before returning a transaction confi rmation back
to the client.
■ (^) Multiple failover modes, including automatic failover (without data loss), planned
manual failover (without data loss), and forced manual failover (with possible data
loss).
■ Availability group listeners, which are virtual network names (VNN) for the avail-
ability group, which applications can connect to. For those familiar with clustering,
this is the same as using a cluster name rather than using individual node names.
This provides applications a way to quickly failover seamlessly after an availability
group fails over. Listeners are also the vehicles that provide support for multisub-
net failover.
■ Automatic page repair. If a page is marked as suspect due to corruption, SQL Server
automatically attempts to recover the page from a database mirroring partner
(principal or mirror) or an availability replica (primary or secondary).
■ (^) By default, data is compressed and encrypted between primary and secondary
replicas.
■ (^) Ability to force Windows Server Failover Cluster (WSFC) quorum.
Requirements and Prerequisites
To set up an AlwaysOn Availability Group, you must take many considerations into
account, including the host servers, Windows Server Failover Cluster (WSFC) cluster, server
instances, and availability group confi gurations. The following list gives you some of the
high-level prerequisites to take into account when setting up AlwaysOn Availability Groups.
For a fully detailed list see http://msdn.microsoft.com/en-us/library/ff878487.
■ (^) The systems that are members of the availability group are not domain controllers.
Availability groups are not supported on domain controllers.
■ (^) Each computer in the availability group must run either x86 (non-WOW64) or x64
Windows Server 2008 or later. In addition, it must use the Enterprise Edition of the
Windows Server software.
■ Each computer in the availability group must be joined to the same domain.
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