Oracle DBA · Lesson 3
Single Instance vs RAC
Copy a Data Guard runbook onto a RAC cluster — or the reverse — and you stop the wrong thing. Topology is not trivia.
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Oracle DBA Lesson 3 — Single Instance vs RAC
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The one-database rule
A given instance is associated with exactly one database. That rule never flips.
Several databases on one host means several instances — each with its own SGA and background processes. They do not share an SGA.
Single-instance picture
One engine, one set of files. That is the lab default and most of this series.
When people say “start the database,” they usually mean start that one instance against those files.
RAC picture
RAC (Real Application Clusters): multiple instances on different hosts, one shared database on shared storage. Same files, many engines.
Why it exists: an instance failure should not equal application failure; you can scale compute without moving data files.
What it is not: not a backup strategy, not a substitute for redo off-host, and not “start the instance twice on one laptop.”
Operational tell: in RAC you start instances, not “the database,” and services pin work to nodes. Cluster start/stop often goes through srvctl — not used in a single-instance lab.
What this series assumes
Master single-instance CDB operations first. RAC-shaped commands appear only as signposts so you recognize them later.
Commands to try
SELECT parallel FROM v$instance;NO on a typical single-instance lab. YES when this instance is part of a RAC cluster.
SELECT inst_id, instance_name FROM gv$instance;gv$ is the RAC-shaped cousin of v$. On single-instance you usually still see one row (your local instance).
Gotchas
- One instance → one database always. RAC does not break that rule; it adds instances against the same shared files.
- Two instances on two servers cannot safely open the same data files without RAC (or an equivalent clustered product) — you risk corruption.
- RAC is not your backup. Redo still needs an off-host home.
Quick quiz
Choose one answer per question, then submit. You’ll see the correct answer and a short why.
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