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Oracle Internals: An Introduction by Steve Adams

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Other Instance Locks

There are a number of other instance locks used for controlling certain operations in parallel server databases that have no counterpart in single-instance Oracle. For example, the SM (SMON) instance lock is used to ensure that the SMON processes of multiple instances cannot be simultaneously active. This is not necessary in single-instance Oracle. Similarly, the DR (distributed recovery) instance lock is used to ensure that only one RECO process can be active at any one time.

The DF (data file) instance locks are another group of locks that are not needed in single-instance Oracle. There is one DF resource for each data file, and each DBWn process holds a shared mode instance lock on each resource. If a data file is taken offline in one instance, then the remote DBWn processes are notified to no longer attempt to write to that data file by converting the mode of the instance lock on that resource.

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