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	See
Read Consistency	Multiversion Concurrency Control
Transaction Rollback	Rolling Back Transactions
Database Recovery	Rollback Segments and Rolling Back

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