Oracle7 Server Concepts Manual

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Data Blocks, Extents, and Segments

• The Relationships Between Data Blocks, Extents, and Segments
• Data Blocks
• Extents
• Segments
• Temporary Segments

Edmund Burke: On Pitt's first speech

This chapter describes the nature of and relationships between logical storage structures in the Oracle Server. It includes:

• The Relationship Between Data Blocks, Extents, and Segments

• Data Blocks

• Extents

• Segments

• Temporary Segments

The Relationships Between Data Blocks, Extents, and Segments

Figure 3 - 1. The Relationship Among Segments, Extents, and Data Blocks

Data Blocks

In contrast, all data at the physical, operating system level is stored in bytes. Each operating system has what is called a block size. Oracle requests data in multiples of Oracle blocks, not operating system blocks. Therefore, you should set the Oracle block size to a multiple of the operating system block size to avoid unnecessary I/O.

Extents

Segments

Oracle allocates space for segments in extents. Therefore, when the existing extents of a segment are full, Oracle allocates another extent for that segment. Because extents are allocated as needed, the extents of a segment may or may not be contiguous on disk. The segments also can span files, but the individual extents cannot.

Data Blocks

Data Block Format

Figure 3 - 2. Data Block Format

Header (Common and Variable)

Table Directory

Row Directory

Once the space has been allocated in the row directory of a block's header, this space is not reclaimed when the row is deleted. Therefore, a block that is currently empty but had up to 50 rows at one time continues to have 100 bytes allocated in the header for the row directory. Oracle only reuses this space as new rows are inserted in the block.

Row Data

Free Space

Space Used for Transaction Entries

An Introduction to PCTFREE, PCTUSED, and Row Chaining

The PCTFREE Parameter

PCTFREE 20 

This states that 20% of each data block used for this table's data segment will be kept free and available for possible updates to the existing rows already within each block. Figure 3 - 3 illustrates PCTFREE.

Figure 3 - 3. PCTFREE

Notice that before the block reaches PCTFREE, the free space of the data block is filled by both the insertion of new rows and by the growth of the data block header.

The PCTUSED Parameter

PCTUSED 40 

In this case, a data block used for this table's data segment is not considered for the insertion of any new rows until the amount of used space in the block falls to 39% or less (assuming that the block's used space has previously reached PCTFREE). Figure 3 - 4 illustrates this.

Figure 3 - 4. PCTUSED

How PCTFREE and PCTUSED Work Together

Figure 3 - 5. Maintaining the Free Space of Data Blocks with PCTFREE and PCTUSED

How Oracle Uses PCTFREE and PCTUSED

For each data and index segment, Oracle maintains one or more free lists; a free list is a list of data blocks that have been allocated for that segment's extents and have free space greater than PCTFREE; these blocks are available for inserts. When you issue an INSERT statement, Oracle checks a free list of the table for the first available block and uses it if possible; if the free space in that block is not large enough to accommodate the INSERT statement, and it is at least PCTUSED, Oracle takes the block off the free list. Multiple free lists per segment can reduce contention for free lists when concurrent inserts take place.

After you issue DELETE and UPDATE statements, Oracle checks to see if the space being used in the block is less than PCTUSED; if it is, the block goes to the beginning of the free list, and it is the first of the available blocks to be used.

Availability and Compression of Free Space in a Data Block

• If the INSERT statement is in the same transaction and subsequent to the statement that frees space, the INSERT statement can use the space made available.

• If the INSERT statement is in one transaction and the statement that frees space is in a second transaction (perhaps being executed by another user), the INSERT statement can only use the space made available after the second transaction commits, and only if the space is needed.

Row Chaining across Data Blocks

Note: The format of a row and a row piece are described in "Row Format and Size" .

If a table contains a column of datatype LONG, which can hold up to two gigabytes of information, the data for a row may need to be chained to one or more data blocks. Nothing can be done to avoid this type of row chaining.

If a row in a data block is updated so that the overall row length increases and the block's free space has been completely filled, the data for the entire row is migrated to a new data block, assuming the entire row can fit in a new block. Oracle preserves the original row piece of a migrated row to point to the new block containing the migrated row; the ROWID of a migrated row does not change.

When a row is chained or migrated, I/O performance associated with this row decreases because Oracle must scan more than one data block to retrieve the information for the row. For information about reducing migrated rows and improving I/O performance, see Oracle7 Server Tuning.

Extents

This section describes how extents are allocated for segments.

When Extents Are Allocated for Segments

Note: Rollback segments always have at least two extents.

For example, when you create a table, its data segment contains an initial extent of a specified number of data blocks. Although no rows have been inserted yet, the Oracle data blocks that correspond to the initial extent are reserved for that table's rows.

If the data blocks of a segment's initial extent become full and more space is required to hold new data, Oracle automatically allocates an incremental extent for that segment. An incremental extent is a subsequent extent of the same or greater size than the previous extent in that segment. The next section explains the factors controlling the size of incremental extents.

For maintenance purposes, each segment in a database contains a segment header block that describes the characteristics of that segment and a directory (list) of the extents in that segment.

Extents and the Parallel Query Option

Serial operations require the object to have at least one extent. Parallel creations require that non-clustered tables or indexes have at least as many extents as there are query servers that create the object.

When you create a table or index with the parallel query option, it is possible to create "pockets" of free space. This occurs when you specify more query servers than there are datafiles in the tablespace. Oracle cannot coalesce this free space with other free space, so this space is available only for subsequent inserts into that table.

For example, if you specify a degree of parallelism of three for a CREATE TABLE ... AS <subquery> statement but there is only one datafile in the tablespace, the situation illustrated in Figure 3 - 6 can arise. Oracle can only coalesce the free space in the last extent of a table or index in each datafile, so all "pockets" of free space within internal table extents of a datafile cannot be coalesced with other free space and allocated as extents.

Figure 3 - 6. Unusable Free Space

To alleviate the free space problem, set the degree of parallelism to less than or equal to the number of datafiles in the tablespace that you are placing the non-clustered table or index. Oracle assigns query servers to datafiles in a round-robin fashion, so specifying fewer query servers than datafiles ensures that all free space can be used later by all tables in the tablespace for subsequent extent allocation.

For more information about the parallel query option and creating non-clustered tables and indexes in parallel, see Oracle7 Server Tuning.

For more information about datafiles and tablespaces, see Chapter 4, "Tablespaces and Datafiles".

How Extents Are Allocated for Segments

1. Oracle searches through the free space (in the tablespace that contains the segment) for the first free, contiguous set of data blocks of an incremental extent's size or larger. Oracle finds the free space for the new extent by using the following algorithm:

1. 1.1 Oracle searches for a contiguous set of data blocks that matches the size of new extent, then adds one block to reduce internal fragmentation. For example, if a new extent requires 19 data blocks, Oracle searches for exactly 20 contiguous data blocks. However, if the new extent is 5 or fewer blocks, Oracle does not add an extra block to the request.

1. 1.2 If an exact match is not found, Oracle then searches for a set of contiguous data blocks equal to or greater than the amount needed. If Oracle finds a group of contiguous blocks that is at least five blocks greater than the size of the extent that is needed, it splits the group of blocks into separate extents, one of which is the size it needs; if Oracle finds a group of blocks that is larger than the size it needs, but less than five blocks larger, it allocates all the contiguous blocks.

Continuing with the example, if Oracle does not find a set of exactly 20 contiguous data blocks, Oracle then searches for a set of contiguous data blocks greater than 20. If the first set that Oracle finds contains 25 or more blocks, it breaks the blocks up and allocates twenty of them to the new extent. Otherwise, it allocates all of the blocks (between 21 and 24) to the new extent.

1. 1.3 If Oracle does not find a larger set of contiguous data blocks, Oracle then coalesces any free, adjacent data blocks in the corresponding tablespace so that larger sets of contiguous data blocks are formed. (The SMON background process also periodically coalesces adjacent free space.) After coalescing a tablespace's data blocks, Oracle performs the searches described in 1.1. and 1.2.. again. If an extent cannot be allocated after the second search, Oracle returns an error.

2. Once Oracle finds the necessary free space in the tablespace, Oracle allocates a portion of the free space that corresponds to the size of the incremental extent. If Oracle had found a larger amount of free space than was required for the extent, Oracle leaves the remainder as free space (no smaller than five contiguous blocks).

3. Oracle updates the segment header and data dictionary to show that a new extent has been allocated and that the allocated space is no longer free.

When Extents Are Deallocated

• The owner of a table or cluster, or a user with the DELETE ANY privilege, can truncate the table or cluster with a TRUNCATE...DROP STORAGE statement.

• Periodically, Oracle may deallocate one or more extents of a rollback segment.

• A DBA can deallocate unused extents using the following SQL syntax:

		ALTER TABLE table_name DEALLOCATE UNUSED

For More Information on Deallocating Extents

When extents are freed, Oracle updates the data dictionary to reflect the regained extents as available space. All data in the blocks of freed extents is inaccessible, and Oracle clears out the data when the blocks are subsequently reused for other extents.

Non-Clustered Tables, Snapshots, and Snapshot Logs

When you drop a non-clustered table, Oracle reclaims all the extents of its data and index segments for the tablespaces that they were in and makes the extents available for other objects in the tablespace. Subsequently, when other segments require large extents, Oracle identifies and combines contiguous reclaimed extents to form the requested larger extents.

Clustered Tables and Snapshots

Indexes

Rollback Segments

Temporary Segments

Determining Sizes and Limits of Segment Extents

Segments

There are four types of segments used in Oracle databases:

• data segments

• index segments

• rollback segments

• temporary segments

Data Segments

Every cluster in an Oracle database uses a single data segment to hold the data for all of its tables. Oracle creates the data segment for the cluster when you issue the CREATE CLUSTER command.

The storage parameters for a table, snapshot, snapshot log, or cluster control the way that its data segment's extents are allocated. You can set these storage parameters directly with the CREATE TABLE/ SNAPSHOT/ SNAPSHOT LOG/CLUSTER or ALTER TABLE/ SNAPSHOT/ SNAPSHOT LOG/CLUSTER commands; these affect the efficiency of data retrieval and storage for the data segment associated with the object. For more information on the various CREATE and ALTER commands, see the Oracle7 Server SQL Reference.

Index Segments

Rollback Segments

For specific information about how rollback segments function in these situations, see the appropriate sections of this book:

Topic	Section Name	Page
Read Consistency	Multiversion Concurrency Control	10-5
Transaction Rollback	Rolling Back Transactions	12-6
Database Recovery	Rollback Segments and Rolling Back	24-4

Contents of a Rollback Segment

Database users or administrators cannot access or read rollback segments; only Oracle can write to or read them. (They are owned by the user SYS, no matter which user creates them.)

Because rollback entries change data blocks, Oracle also records changes to them in the redo log. This second recording of the rollback information is very important for active transactions not yet committed at the time of the system crash. If a system crash occurs, Oracle automatically restores the rollback segment information, including the rollback entries for active transactions, as part of instance or media recovery. Oracle performs rollbacks of transactions that had not been committed or rolled back at the time of the failure after recovery is complete.

When Rollback Information Is Required

Rollback segments record the data prior to change on a per transaction basis. For every transaction, Oracle links each new change to the previous change. If you must roll back the transaction, Oracle applies the changes in the chain to the data blocks in an order that restores the data to its previous state.

Similarly, when Oracle needs to provide a read-consistent set of results for a query, it can use information in rollback segments to create a set of data consistent with respect to a single point in time.

All types of rollbacks use the same procedures:

• statement level rollback (due to statement or deadlock execution error)

• rollback to a savepoint

• rollback of a transaction due to user request

• rollback of a transaction due to abnormal process termination

• rollback of all outstanding transactions when an instance terminates abnormally

• rollback of incomplete transactions during recovery

Transactions and Rollback Segments

• Oracle can assign a transaction automatically to the next available rollback segment. The transaction assignment occurs when you issue the first DML or DDL statement in the transaction. Oracle never assigns read-only transactions (transactions that contain only queries) to a rollback segment, regardless of whether the transaction begins with a SET TRANSACTION READ ONLY statement.

• An application can assign a transaction explicitly to a specific rollback segment. At the start of a transaction, a developer or user can specify a particular rollback segment that Oracle should use when executing the transaction. This allows the developer or user to select a large or small rollback segment, as appropriate for the transaction.

When you commit a transaction, Oracle releases the rollback information, but does not immediately destroy it. The information remains in the rollback segment to create read-consistent views of pertinent data for queries that started before the transaction committed. To guarantee that rollback data is available for as long as possible for such views, Oracle writes the extents of rollback segments sequentially. When the last extent of the rollback segment becomes full, Oracle continues writing rollback data by wrapping around to the first extent in the segment. A long-running transaction (idle or active) may require a new extent to be allocated for the rollback segment. See Figure 3 - 7, Figure 3 - 8, and Figure 3 - 9 for more information about how transactions use the extents of a rollback segment.

Each rollback segment can handle a certain number of transactions from one instance. Unless you explicitly assign transactions to particular rollback segments, Oracle distributes active transactions across available rollback segments so that all rollback segments are assigned approximately the same number of active transactions. Distribution does not depend on the size of the available rollback segments. Therefore, in environments where all transactions generate the same amount of rollback information, all rollback segments can be the same size.

Additional Information: The number of transactions that a rollback segment can handle is an operating system-specific function of the data block size. See your Oracle operating system-specific documentation for more information.

How Extents Are Used and Allocated for Rollback Segments

A transaction writes sequentially to a single rollback segment. Each transaction writes to only one extent of the rollback segment at any given time. Furthermore, many active transactions (transactions in progress, not committed or rolled back) can write concurrently to a single rollback segment, even the same extent of a rollback segment; however, each block in a rollback segment's extent can contain information for a single transaction only.

When a transaction runs out of space in the current extent and needs to continue writing, Oracle must find an available extent of the same rollback segment in which to write. Oracle has two options:

• It can reuse an extent already allocated to the rollback segment.

• It can acquire (and allocate) a new extent for the rollback segment.

Figure 3 - 7. Use of Allocated Extents in a Rollback Segment

As the transactions continue writing and fill the current extent, Oracle checks the next extent already allocated for the rollback segment to determine if it is available. In Figure 3 - 8, when E4 is completely full, T1 and T2 continue any further writing to the next extent allocated for the rollback segment that is available; in this figure, E1 is this extent. This figure shows the cyclical nature of extent use in rollback segments.

Figure 3 - 8. Cyclical Use of the Allocated Extents in a Rollback Segment

To continue writing rollback information for a transaction, Oracle always tries to reuse the next extent in the ring first. However, if the next extent contains active data, then Oracle must allocate a new extent. Oracle can allocate new extents for a rollback segment until the number of extents reaches the value set for the rollback segment's storage parameter MAXEXTENTS.

Figure 3 - 9 shows when a new extent must be allocated for a rollback segment. The uncommitted transactions are long running (either idle, active, or persistent in-doubt distributed transactions). At this time, they are writing to the fourth extent, E4, in the rollback segment. However, when E4 is completely full, the transactions cannot continue further writing to the next extent in sequence, E1, because it contains active rollback entries. Therefore, Oracle allocates a new extent, E5, for this rollback segment, and the transactions continue writing to this new extent.

Figure 3 - 9. Allocation of a New Extent for a Rollback Segment

How Extents Are Deallocated from a Rollback Segment

(INITIAL + NEXT + NEXT + ... up to MINEXTENTS) bytes 

When you drop a rollback segment, Oracle returns all extents of the rollback segment to its tablespace. The returned extents are then available to other segments in the tablespace.

Instances and Types of Rollback Segments

Any number of private and public rollback segments can exist in a database. As an instance opens a database, the instance attempts to acquire one or more rollback segments according to the following rules:

• The instance must acquire at least one rollback segment. If the instance is the only instance accessing the database, it acquires the SYSTEM segment; if the instance is one of several instances accessing the database, it acquires the SYSTEM rollback segment and at least one other rollback segment. If it cannot, Oracle returns an error, and the instance cannot open the database.

• The instance first tries to acquire all private rollback segments specified by the instance's ROLLBACK_SEGMENTS parameter. If one instance opens a database and attempts to acquire a private rollback segment already claimed by another instance, the second instance trying to acquire the rollback segment receives an error during startup. An error is also returned if an instance attempts to acquire a private rollback segment that does not exist.

• The instance always attempts to acquire at least the number of rollback segments equal to the quotient of the values for the following initialization parameters:

		CEIL(TRANSACTIONS/TRANSACTIONS_PER_ROLLBACK_SEGMENT) 

CEIL is a SQL function that returns the smallest integer greater than or equal to the numeric input. In the example above, if TRANSACTIONS equal 155 and TRANSACTIONS_PER_ROLLBACK_SEGMENT equal 10, then the instance will try to acquire at least 16 rollback segments.

• If the instance already has acquired enough private rollback segments in Step 2, no further action is required. However, if an instance requires more rollback segments, the instance attempts to acquire public rollback segments. (An instance can open the database even if the instance cannot acquire the number of rollback segments given by the division above.)

Once an instance claims a public rollback segment, no other instance can use that segment until either the rollback segment is taken offline or the instance that claimed the rollback segment is shut down.

Note: A database used by the Oracle Parallel Server optionally can have only public and no private segments, as long as the number of segments in the database is high enough to ensure that each instance that opens the database can acquire at least two rollback segments, one of which is the SYSTEM rollback segment (see the following section). However, when using the Oracle Parallel Server, you may want to use private rollback segments. See Oracle7 Parallel Server Concepts & Administration for more information about rollback segment use in an Oracle Parallel Server.

The Rollback Segment SYSTEM

If there are multiple rollback segments, Oracle tries to use the SYSTEM rollback segment only for special system transactions and distributes user transactions among other rollback segments; if there are too many transactions for the non-SYSTEM rollback segments, Oracle uses the SYSTEM segment as necessary. In general, after database creation, you should create at least one additional rollback segment in the SYSTEM tablespace.

Rollback Segment States

The rollback segment states are the following:

OFFLINE

Has not been acquired (brought online) by any instance.

ONLINE

Has been acquired (brought online) by an instance; may contain data from active transactions.

NEEDS RECOVERY

Contains data from uncommitted transactions that cannot be rolled back (because the data files involved are inaccessible), or is corrupted.

PARTLY AVAILABLE

Contains data from an in-doubt transaction (that is, an unresolved distributed transaction).

INVALID

Has been dropped (The space once allocated to this rollback segment will later be used when a new rollback segment is created).

Figure 3 - 10 shows how a rollback segment moves from one state to another.

Figure 3 - 10. Rollback Segment States and State Transitions

PARTLY AVAILABLE and NEEDS RECOVERY Rollback Segments The PARTLY AVAILABLE and NEEDS RECOVERY states are very similar: a rollback segment in either state usually contains data from an unresolved transaction. The differences between the two states are the following:

• A PARTLY AVAILABLE rollback segment is being used by an in-doubt distributed transaction, that cannot be resolved because of a network failure. A NEEDS RECOVERY rollback segment is being used by a transaction (local or distributed) that cannot be resolved because of a local media failure, such as a missing or corrupted datafile, or is itself corrupted.

• Oracle or a DBA can bring a PARTLY AVAILABLE rollback segment online. In contrast, you must take a NEEDS RECOVERY rollback segment OFFLINE before it can be brought online. (If you recover the database and thereby resolve the transaction, Oracle automatically changes the state of the NEEDS RECOVERY rollback segment to OFFLINE.)

• A DBA can drop a NEEDS RECOVERY rollback segment. (This allows the DBA to drop corrupted segments.) A PARTLY AVAILABLE segment cannot be dropped; you must first resolve the in-doubt transaction, either automatically by the RECO process or manually. (See Oracle7 Server Distributed Systems, Volume I for information about failures in distributed transactions.)

Also, until you resolve the in-doubt transaction, the transaction continues to hold the extents it acquired in the rollback segment, preventing other transactions from using them. Thus, the rollback segment might need to acquire new extents for the active transactions, and therefore grow. To prevent the rollback segment from growing, a database administrator might prefer to create a new rollback segment for transactions to use until the in-doubt transaction is resolved, rather than bring the PARTLY AVAILABLE segment online.

Viewing the State of a Rollback Segment The data dictionary table DBA_ROLLBACK_SEGS lists the status (state) of each rollback segment, along with other rollback segment information.

Deferred Rollback Segments

Temporary Segments

Operations Requiring Temporary Segments

• CREATE INDEX

• SELECT ... ORDER BY

• SELECT DISTINCT ...

• SELECT ... GROUP BY

• SELECT ... UNION

• SELECT ... INTERSECT

• SELECT ... MINUS

• unindexed joins

• certain correlated subqueries

How Temporary Segments Are Allocated

Oracle creates temporary segments in the temporary tablespace of the user issuing the statement. You specify this tablespace with a CREATE USER or an ALTER USER command using the TEMPORARY TABLESPACE option. Otherwise, the default temporary tablespace is the SYSTEM tablespace. For more information about assigning a user's temporary segment tablespace, see Chapter 17, "Database Access".

Because allocation and deallocation of temporary segments occur frequently, it is reasonable to create a special tablespace for temporary segments. By doing so, you can distribute I/O across disk devices, and you may avoid fragmentation of the SYSTEM and other tablespaces that otherwise would hold temporary segments.

The redo log does not contain entries for changes to temporary segments used for sort operations.

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