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Don Burleson Blog 







Oracle Storage Parameter Exceptions

Oracle Database Tips by Donald BurlesonOctober 11, 2015

Oracle uses various parameters to define how data blocks are handled within the database.  These storage parameters are all explained below, giving a brief overview of what they do and how they operate, as well as when the parameter is handled differently by Oracle.  It is important to note that with Automated Segment Space Management enabled, and in some cases with Locally Managed Tablespaces, certain parameters are disregarded.  These storage parameters are pctfree, pctused, minimum extent, maxextents, initial, next, pctincrease, freelist groups and freelists

pctfree is a block storage parameter which specifies the amount of free space Oracle should leave in a database block for future updates. Oracle will continue adding new rows to the block until the remaining space is equal to the pctfree parameter. The default value for the pctfree parameter is pctfree=10. Any value between 0 and 99 may be used as long as the sum of pctfree and pctused is not greater than 100.

When talking about pctfree, it must be noted that indexes are a special case. pctfree may be specified for an index, but only when the index is initially created.

pctused is a storage parameter in Oracle which specifies when a database block is empty enough for Oracle to add it to the free list. When the percentage of used space in a block is greater than the pctused parameter, Oracle will not add new rows to the block. The default settings for all Oracle tables are pctused=40.

pctused is disregarded for all objects created in locally managed tablespaces with Automated Segment Space Management enabled.

In a dictionary-managed tablespace, the minimum extent parameter defines the total number of extents to be allocated when a segment is created. minimum extent is used for locally managed tablespaces, but only to compute the initial amount of space to be allocated.

maxextents, for dictionary-managed tablespaces, is the maximum number of extents that can ever be allocated for the segment.

Because Oracle manages the extents under a locally managed tablespace, maxextents will be ignored.

The initial parameter defines the size of the first extent of an object. The default value for initial is the size of 5 data blocks. In a dictionary-managed tablespace, if the minimum extent parameter was set at the time the tablespace was created, then Oracle rounds the value of initial up to the specified minimum extent size, but only if necessary.

In a locally managed tablespace, Oracle uses the value of initial together with the size of extents specified for the tablespace to calculate the size of the first extent of the object. It must also be noted that initial cannot be set by using an ALTER statement.

next represents, for dictionary-managed tablespaces, the size of the next incremental extent to be allocated for a segment. The default value for next is always the size of 5 data blocks. The default value for the second extent is always equal to the original setting for next. For each new extent, the next is set to the previous size of next multiplied by (1 + pctincrease/100). If you manually set the value of the next parameter, the next allocated extent will have the specified size regardless of the size of the most recently allocated extent and the pctincrease parameter.

next is not valid for locally managed tablespaces, because the database automatically manages extents.

pctincrease is the storage parameter, for dictionary-managed tablespaces, which specifies the percentage by which each incremental extent grows over the previous extent allocated for a segment.

The pctincrease parameter, like the next parameter, is not valid for locally managed tablespaces.

The freelist groups parameter specifies the number of groups of free lists for the database object you are creating. The default, and minimum, is 1 which causes all free lists of a segment to be available to all instances.

The freelist groups parameter is disregarded for objects created in locally managed tablespaces with segment space management set to AUTO.

The freelists parameter specifies the number of free lists available for each of the free list groups for the object. This parameter is not used for tablespaces. The default and minimum value for freelists is 1, and the maximum value depends on block size. Attempting to specify freelists to a value greater than allowed will generate an error message informing you of the maximum value.

The freelists parameter is disregarded for objects created in locally managed tablespaces.

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