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Traditional Oracle Storage Parameters
and Performance


Oracle Tips by Burleson
 

Let’s begin this chapter by introducing the relationship between object storage parameters and performance. Poor object performance within Oracle is experienced in several areas:

  •  Slow inserts Insert operations run slowly and have excessive I/O. This happens when blocks on the freelist only have room for a few rows before Oracle is forced to grab another free block.

  • Slow selects Select statements have excessive I/O because of chained rows. This occurs when rows “chain” and fragment onto several data blocks, causing additional I/O to fetch the blocks.

  • Slow updates Update statements run very slowly with double the amount of I/O. This happens when update operations expand a VARCHAR or BLOB column and Oracle is forced to chain the row contents onto additional data blocks.

  • Slow deletes Large delete statements can run slowly and cause segment header contention. This happens when rows are deleted and Oracle must relink the data block onto the freelist for the table.

As we see, the storage parameters for Oracle tables and indexes can have an important effect on the performance of the database. Let’s begin our discussion of object tuning by reviewing the common storage parameters that affect Oracle performance.

The pctfree Storage Parameter

The purpose of pctfree is to tell Oracle when to remove a block from the object’s freelist. Since the Oracle default is pctfree=10, blocks remain on the freelist while they are less than 90 percent full. As shown in Figure 10-5, once an insert makes the block grow beyond 90 percent full, it is removed from the freelist, leaving 10 percent of the block for row expansion. Furthermore, the data block will remain off the freelist even after the space drops below 90 percent. Only after subsequent delete operations cause the space to fall below the pctused threshold of 40 percent will Oracle put the block back onto the freelist.

Figure 10-83: The pctfree threshold

The pctused Storage Parameter

The pctused parameter tells Oracle when to add a previously full block onto the freelist. As rows are deleted from a table, the database blocks become eligible to accept new rows. This happens when the amount of space in a database block falls below pctused, and a freelist relink operation is triggered, as shown in Figure 10-6.

Figure 10-84: The pctused threshold

For example, with pctused=60, all database blocks that have less than 60 percent will be on the freelist, as well as other blocks that dropped below pctused and have not yet grown to pctfree. Once a block deletes a row and becomes less than 60 percent full, the block goes back on the freelist. When rows are deleted, data blocks become available when a block’s free space drops below the value of pctused for the table, and Oracle relinks the data block onto the freelist chain. As the table has rows inserted into it, it will grow until the space on the block exceeds the threshold pctfree, at which time the block is unlinked from the freelist.

The freelists Storage Parameter

The freelists parameter tells Oracle how many segment header blocks to create for a table or index. Multiple freelists are used to prevent segment header contention when several tasks compete to INSERT, UPDATE, or DELETE from the table. The freelists parameter should be set to the maximum number of concurrent update operations.

Prior to Oracle8i, you must reorganize the table to change the freelists storage parameter. In Oracle8i, you can dynamically add freelists to any table or index with the alter table command. In Oracle8i, adding a freelist reserves a new block in the table to hold the control structures. To use this feature, you must set the compatible parameter to 8.1.6 or greater.

The freelist groups Storage Parameter for OPS

The freelist groups parameter is used in Oracle Parallel Server (Real Application Clusters). When multiple instances access a table, separate freelist groups are allocated in the segment header. The freelist groups parameter should be set the number of instances that access the table. For details on segment internals with multiple freelist groups, see Chapter 13.

NOTE: The variables are called pctfree and pctused in the create table and alter table syntax, but they are called PCT_FREE and PCT_USED in the dba_tables view in the Oracle dictionary. The programmer responsible for this mix-up was promoted to senior vice president in recognition of his contribution to the complexity of the Oracle software.

Summary of Storage Parameter Rules

The following rules govern the settings for the storage parameters freelists, freelist groups, pctfree, and pctused. As you know, the value of pctused and pctfree can easily be changed at any time with the alter table command, and the observant DBA should be able to develop a methodology for deciding the optimal settings for these parameters. For now, accept these rules, and we will be discussing them in detail later in this chapter.

There is a direct trade-off between effective space utilization and high performance, and the table storage parameters control this trade-off:

  • For efficient space reuse A high value for pctused will effectively reuse space on data blocks, but at the expense of additional I/O. A high pctused means that relatively full blocks are placed on the freelist. Hence, these blocks will be able to accept only a few rows before becoming full again, leading to more I/O.

  • For high performance A low value for pctused means that Oracle will not place a data block onto the freelist until it is nearly empty. The block will be able to accept many rows until it becomes full, thereby reducing I/O at insert time. Remember that it is always faster for Oracle to extend into new blocks than to reuse existing blocks. It takes fewer resources for Oracle to extend a table than to manage freelists.

While we will go into the justification for these rules later in this chapter, let’s review the general guidelines for setting of object storage parameters:

  • Always set pctused to allow enough room to accept a new row. We never want to have a free block that does not have enough room to accept a row. If we do, this will cause a slowdown since Oracle will attempt to read five “dead” free blocks before extending the table to get an empty block.

  • The presence of chained rows in a table means that pctfree is too low or that db_block_size is too small. In most cases within Oracle, RAW and LONG RAW columns make huge rows that exceed the maximum block size for Oracle, making chained rows unavoidable.

  • If a table has simultaneous insert SQL processes, it needs to have simultaneous delete processes. Running a single purge job will place all of the free blocks on only one freelist, and none of the other freelists will contain any free blocks from the purge.

  • The freelist parameter should be set to the high-water mark of updates to a table. For example, if the customer table has up to 20 end users performing insert operations at any time, the customer table should have freelists=20.

  • The freelist groups parameter should be set the number of Real Application Clusters instances (Oracle Parallel Server in Oracle8i) that access the table.


This is an excerpt from "Oracle High Performance tuning with STATSPACK" by Oracle Press.


If you like Oracle tuning, you may enjoy the new book "Oracle Tuning: The Definitive Reference", over 900 pages of BC's favorite tuning tips & scripts. 

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