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To access the contents, click the chapter and section titles.
Oracle Performance Tuning and Optimization
PCTUSED Once PCTFREE is reached, no new rows can be inserted into the data block until the space in the block has fallen below PCTUSED. Another feature of PCTUSED is that Oracle tries to keep a data block at least PCTUSED full before using new blocks. A high PCTUSED value has the following effects:
A lower value for PCTUSED has the following effects:
Just as with PCTFREE, it is worth the effort to look at each table individually and determine a value for PCTUSED that more accurately fits your application. Doing so can improve your systems performance. A Review of the PCTFREE and PCTUSED Options As with most tuning efforts, there is a tradeoff between space usage and performance. Here are a few things to keep in mind when you are adjusting the values for PCTFREE and PCTUSED:
Here are a few guidelines to follow when adjusting PCTFREE and PCTUSED:
A Review of I/O Reduction TechniquesReduction of I/O is important for performance because I/O is one of the slowest operations in the computer system. Caching data is many times faster than an access from disk. In fact, a read from memory can take place over 100,000 times faster than a read from disk. You have seen that you can speed up I/O by separating the sequential and the random I/Os and separating data from indexes to get maximum concurrency. You have also seen how to speed up I/Os by avoiding them altogethersuch as by reducing dynamic extensions and by avoiding migrated and chained rows. As you see in later chapters, the speed of I/O from disk is a fixed value that you can work around only by caching, reducing contention, and avoiding I/Os if possible. Tuning Rollback SegmentsAnother area of contention may be in the rollback segments. Rollback segments are constantly used during transaction processing; any delays caused by contention on rollback segments affect performance. Rollback segments record transactional information that may be used in the event that the transaction should be rolled back. Rollback segments are also used to provide read consistency and are used for database recovery. Read consistency allows a long-running transaction to always obtain the same data within the query. During the transaction, the data is consistent to a single point in time and does not change. Even though the data may have changed, and perhaps the DBWR may even have written it out, other transactions do not see those changes until a COMMIT has occurred. In fact, only transactions that start after this transaction has been committed see those changes. Rollback segments must be carefully watched and can be tuned in several ways. It is important not only to size the rollback segments correctly but also to create the proper number of rollback segments and properly distribute them according to the number of user processes that require them. The next section discusses how the rollback segments operate.
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