Fast Commits, Group Commits, and Deferred Writes

Fast commits, group commits, and deferred writes operate on each instance in Oracle and work the same whether in exclusive or shared mode. Oracle only reads data blocks from disk if they are not already in the buffer caches of one of the instances. Because data block writes are deferred, they often contain modifications from multiple transactions. Optimally, Oracle writes modified data blocks to disk only when necessary:

• When the blocks have not been used recently and new data requires buffer cache space (in shared or exclusive mode) • During checkpoints (in shared or exclusive mount mode) • When another instance needs the blocks (only in shared mode) • Cache copies of dirty blocks (changed blocks) across the interconnect (write/write consistency).

Row Locking and Multi version Read Consistency

Oracle’s row locking feature allows multiple transactions from separate nodes to lock and update different rows of the same data block. This is done without any of the transactions waiting for the others to commit. If a row has been modified but not yet committed, then the original row values are available to all instances for read access. This is called multi version read consistency.

Online Backups and Archiving

Real Application Clusters supports all Oracle backup features that are available in exclusive mode, including both online and offline backups of either an entire database or individual table spaces. If you operate Oracle in ARCHIVELOG mode, then each online redo log file is made into an archive (ARCH) file before it is overwritten. In Real Application Clusters, each instance can automatically archive its own redo log files or one or more instances can manually archive the redo log files for all instances.

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