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 Oracle 11g Shared Cluster Architecture
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Oracle 11g Grid & Real Application Clusters by Rampant TechPress is written by four of the top Oracle database experts (Steve Karam, Bryan Jones, Mike Ault and Madhu Tumma).  The following is an excerpt from the book.

Shared Cluster Architecture

Unlike the ‘shared nothing’ Google search platform, Oracle RAC is a shared everything database.  The following items are shared by all RAC instances. 

  • data files

  • control files


  • redo log files (shared during recovery)

  • interconnect

Since Oracle RAC is a shared everything system, the data files, control files, SPFILEs and redo log files must reside on cluster aware shared storage. 

Storage Options

The shared storage options for Oracle 11g RAC are ASM, OCFS for Windows, OCFS2 for Linux, NFS (now with Direct NFS in 11g), certified third-party cluster-aware volume manager, certified network file system, and raw devices.  Oracle requires that RAC implementations using Network Attached Storage (NAS) use their own private network, which is separate from the interconnect.


Metalink Note 578455.1 indicates Oracle’s support of raw devices is going away in Oracle 12g.

Virtual IP Address

Unlike a single instance database, an Oracle RAC instance can be connected to a client through the server’s IP address and through VIP addresses.  The VIP address or corresponding virtual host name is the logical way to connect to the RAC database, because of the built-in failover ability.


The interconnect is a critical component of the Oracle RAC architecture. 


From Oracle:  “The interconnect network is a private network that connects all of the servers in the cluster. The interconnect network uses a switch (or multiple switches) that only the nodes in the cluster can access. Configure User Datagram Protocol (UDP) on a Gigabit you can configure Oracle Clusterware to use either the UDP or Reliable Data Socket (RDS) protocols. Windows clusters use the TCP protocol. Crossover cables are not supported for use with Oracle Clusterware interconnects.”


Using 10 Gigabit Ethernet as the interconnect is not mentioned above by Oracle, but is a good choice because it can move 1,250 MB/s versus Gigabit Ethernet’s 125 MB/s. 

Cache Fusion

Cache Fusion is a core part of Oracle RAC.  The System Global Area (SGA) consists of multiple components, one of which is the buffer cache .  In RAC, cache fusion combines the buffer cache, allowing the buffer cache size to be much larger.  Cache fusion traffic and other inter-instance communication takes place over the RAC interconnect private network.


A data block initially resides on disk.  As queries are run, data blocks are copied from disk to the requesting instance’s buffer cache.  If another instance needs any of the data blocks that are now in the first instance’s buffer cache, the second instance will obtain the blocks from the first instance, preventing a more expensive trip to disk.  The Global Cache Service (GCS) and the Global Enqueue Service (GES) processes keep track of the status of each cached block using a memory resident data structure called the Global Resource Directory (GRD). 


There are a number of RAC specific processes which will be explained in more detail in other chapters.  The processes are ACMS, GTX0-j, LMON, LMD, LMS, LCK0, RMSn and RSMN.


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