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Oracle Parallel Processing by Sanjay Mishra, Tushar Mahapatra

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PCM Lock Allocation Guidelines

So far in this chapter we’ve discussed various types of PCM locks, their characteristics, and the initialization parameters used to allocate the locks. This section discusses key criteria that you should consider when deciding on PCM lock allocation in your environment and guides you in deciding what types of locks to use in different situations.

Strategy for PCM Lock Allocation

There are a number of strategies that you should keep in mind when deciding on PCM lock allocation. These include:

  • Allocating PCM locks based on blocks in cache, not on disk

  • Planning for more datafiles

  • Treating each datafile separately

  • Giving special treatment to the SYSTEM tablespace

  • Separating read-only and modifiable data

  • Separating indexes from tables

  • Adding locks when you add a datafile

The following sections discuss each of these strategies.

Allocate PCM locks based on blocks in cache, not on disk

Data on the disk does not contribute to the problem of maintaining cache coherency. Data in the cache does. In addition, however, clean data (data that has not been modified) in the cache and locally accessed data in the cache also do not impact cache coherency. Consequently, you should base the number of instance locks on the amount of data in the cache that is being modified globally, not the total amount on disk.

For example, let’s assume that an OPS database has one billion data blocks distributed among several datafiles. Allocating PCM locks for all of these blocks may not be practical. ...

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