Oracle Extended Distance Clusters (Oracle 擴充套件距離叢集 - 異地RAC)

Oracle Extended Distance Clusters指的是RAC的節點分別放在不同的地方，這些節點可以放在不同的機房，也可以放在不同的城市

1. 適合那些昂貴的高速光纖已經架設，且距離小於100KM，大多數案例是租用裸光纖或DWDM。
2. 不適合full Disaster Recovery requirements(全量災難恢復)，只適合停電，洪水，空難等事故。
3. Oracle 推薦使用本地RAC+dataguard  。  

Oracle RAC on Extended Distance Clusters

Oracle RAC on Extended Distance Clusters is an architecture where servers in the cluster reside in
locations that are physically separate. Oracle RAC on Extended Distance Clusters provides extremely
fast recovery from a site failure and allows for all servers, in all sites, to actively process transactions as
part of a single database cluster. While this architecture creates great interest and has been successfully
implemented, it is critical to understand where this architecture best fits especially in regards to distance,
latency, and degree of protection it provides.

The high impact of latency, and therefore distance, creates some practical limitations as to where this
architecture can be deployed. This architecture fits best where the 2 datacenters are located relatively
close (< 100km) and where the extremely expensive costs of setting up direct cables with dedicated
channels between the sites have already been taken.
 

Oracle RAC on Extended Distance Clusters provides greater availability than local Oracle RAC but it
may not fit the full Disaster Recovery requirements of every organization. Feasible separation is great
protection for some disasters (local power outage, airplane crash, server room flooding) but not all.

Disasters such as earthquakes, hurricanes, and regional floods may affect a greater area. One should
analyze their situation in order to determine if both sites are likely to be affected by the same disaster.

For comprehensive protection against disasters including protection against corruptions and regional
disasters, Oracle recommends the use of Oracle Data Guard in combination with Oracle RAC as
described in the Oracle Maximum Availability Architecture guidelines. Oracle Data Guard also provides
additional  benefits such as support for rolling upgrades across Oracle versions.

Configuring an extended distance cluster is more complex than configuring a local cluster. Specific
focus needs to go into node layout, voting disks, and data disk placement. Implemented properly, this
architecture can provide greater availability than a local Oracle RAC database, but should be considered
with Oracle’s Maximum Availability Architecture in mind.