RAC - Parallel Server

(If have used version 7.3 and 8)

RAC is useful for high availability and applications that can be scaled.

You can tune each instance differently, so some instances can be tuned for heavy transaction processing while others tuned for batch processing.

Version 8

set PARALLEL_SERVER=TRUE

Need raw disk for database files (more difficult to manage)

Each instance has extra processes LCKn (these manage PCM locks), DLM (distributed lock manager, this is integrated into Oracle in version 8, in version 7.3.4 it was provided by hardware vendor).

Assignment of PCM locks is critical, too many and too much memory is used, too few results in pinging.
Pinging : One instance needs a PCM lock held by another instance. Before releasing the lock, the owning instance wakes up its DBWR process to inspect the blocks held by the lock, if they are dirty they are pushed back to disk. The more blocks allocated per PCM lock the more likely a ping.

PCM locks are controlled via GC_ init.ora parameters.

You need an additional rollback segment per new instance.