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The Global Cache Service controls data exchange between nodes using the Cache Fusion technology. Cache Fusion synchronizes the memory cache in each node using high-speed communications. This allows any node to access any data in the database.
Shared storage consists of data and index files, as well as control files.
This architecture makes RAC systems highly available. For example, if Node 2 in Figure 9-9 fails or requires maintenance, the remaining nodes will keep the database available.
This activity is transparent to the user or application and as long as at least one node is active, all data is available. RAC architecture also allows near-linear scalability and offers increased performance benefits. New nodes can easily be added to the cluster when needed to boost performance.
Administering and maintaining data files on both RAC and single-node systems have always required a good deal of effort, especially when data partitioning is involved. Oracle's solution to reducing this effort is Automatic Storage Management, which we will discuss next.
In previous versions of Oracle and with most other databases, management of data files for large databases consumes a good portion of the DBA's time and effort. The number of data files in large databases can easily be in the hundreds or even thousands. The DBA must coordinate and provide names for these files and then optimize the storage location of files on the disks. The new Automatic Storage Management(ASM) feature in Oracle Database 10g Enterprise Edition addresses these issues.
ASM simplifies the management of disks and data files by creating logical groupings of disks into disk groups. The DBA need only refer to the groups, not the underlying data files. Data files are automatically named and distributed evenly (striped) throughout the disks in the group for optimal throughput. As disks are added or removed from the disk group, ASM redistributes the files among the available disks, automatically, while the database is still running. ASM can also mirror data for redundancy.
When ASM is implemented, each node in the database (clustered or nonclustered) has an ASM instance and a database instance, with a communication link between
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