Wednesday, September 26, 2012


What Happens During Execution of a Data Pump Job?

Data Pump jobs use a master table, a master process, and worker processes to perform the work and keep track of progress.

Coordination of a Job

For every Data Pump Export job and Data Pump Import job, a master process is created. The master process controls the entire job, including communicating with the clients, creating and controlling a pool of worker processes, and performing logging operations.

Tracking Progress Within a Job

While the data and metadata are being transferred, a master table is used to track the progress within a job. The master table is implemented as a user table within the database. The specific function of the master table for export and import jobs is as follows:

  • For export jobs, the master table records the location of database objects within a dump file set. Export builds and maintains the master table for the duration of the job. At the end of an export job, the content of the master table is written to a file in the dump file set.
  • For import jobs, the master table is loaded from the dump file set and is used to control the sequence of operations for locating objects that need to be imported into the target database.

The master table is created in the schema of the current user performing the export or import operation. Therefore, that user must have sufficient tablespace quota for its creation. The name of the master table is the same as the name of the job that created it. Therefore, you cannot explicitly give a Data Pump job the same name as a preexisting table or view. For all operations, the information in the master table is used to restart a job.

The master table is either retained or dropped, depending on the circumstances, as follows:

  • Upon successful job completion, the master table is dropped.
  • If a job is stopped using the STOP_JOB interactive command, the master table is retained for use in restarting the job.
  • If a job is killed using the KILL_JOB interactive command, the master table is dropped and the job cannot be restarted.
  • If a job terminates unexpectedly, the master table is retained. You can delete it if you do not intend to restart the job.
  • If a job stops before it starts running (that is, it is in the Defining state), the master table is dropped.

Filtering Data and Metadata During a Job

Within the master table, specific objects are assigned attributes such as name or owning schema. Objects also belong to a class of objects (such as TABLE, INDEX, or DIRECTORY). The class of an object is called its object type. You can use the EXCLUDE and INCLUDE parameters to restrict the types of objects that are exported and imported. The objects can be based upon the name of the object or the name of the schema that owns the object. You can also specify data-specific filters to restrict the rows that are exported and imported.

Transforming Metadata During a Job

When you are moving data from one database to another, it is often useful to perform transformations on the metadata for remapping storage between tablespaces or redefining the owner of a particular set of objects. This is done using the following Data Pump Import parameters: REMAP_DATAFILE, REMAP_SCHEMA, REMAP_TABLESPACE, and TRANSFORM.

Maximizing Job Performance

To improve throughput of a job, you can use the PARALLEL parameter to set a degree of parallelism that takes maximum advantage of current conditions. For example, to limit the effect of a job on a production system, the database administrator (DBA) might wish to restrict the parallelism. The degree of parallelism can be reset at any time during a job. For example, PARALLEL could be set to 2 during production hours to restrict a particular job to only two degrees of parallelism, and during nonproduction hours it could be reset to 8. The parallelism setting is enforced by the master process, which allocates work to be executed to worker processes that perform the data and metadata processing within an operation. These worker processes operate in parallel. In general, the degree of parallelism should be set to more than twice the number of CPUs on an instance.

Loading and Unloading of Data

The worker processes are the ones that actually unload and load metadata and table data in parallel. Worker processes are created as needed until the number of worker processes is equal to the value supplied for the PARALLEL command-line parameter. The number of active worker processes can be reset throughout the life of a job.

When a worker process is assigned the task of loading or unloading a very large table or partition, it may choose to use the external tables access method to make maximum use of parallel execution. In such a case, the worker process becomes a parallel execution coordinator. The actual loading and unloading work is divided among some number of parallel I/O execution processes (sometimes called slaves) allocated from the instance wide pool of parallel I/O execution processes.


Monitoring Job Status

The Data Pump Export and Import utilities can be attached to a job in either interactive-command mode or logging mode. In logging mode, real-time detailed status about the job is automatically displayed during job execution. The information displayed can include the job and parameter descriptions, an estimate of the amount of data to be exported, a description of the current operation or item being processed, files used during the job, any errors encountered, and the final job state (Stopped or Completed).

Job status can be displayed on request in interactive-command mode. The information displayed can include the job description and state, a description of the current operation or item being processed, files being written, and a cumulative status.

A log file can also be optionally written during the execution of a job. The log file summarizes the progress of the job, lists any errors that were encountered along the way, and records the completion status of the job. An alternative way to determine job status or to get other information about Data Pump jobs, would be to query the DBA_DATAPUMP_JOBS, USER_DATAPUMP_JOBS, or DBA_DATAPUMP_SESSIONS views.


Monitoring the Progress of Executing Jobs

Data Pump operations that transfer table data (export and import) maintain an entry in the V$SESSION_LONGOPS dynamic performance view indicating the job progress (in megabytes of table data transferred). The entry contains the estimated transfer size and is periodically updated to reflect the actual amount of data transferred.

File Allocation

There are three types of files managed by Data Pump jobs:

  • Dump files to contain the data and metadata that is being moved
  • Log files to record the messages associated with an operation
  • SQL files to record the output of a SQLFILE operation. A SQLFILE operation is invoked using the Data Pump Import SQLFILE parameter and results in all of the SQL DDL that Import will be executing based on other parameters, being written to a SQL file

Specifying Files and Adding Additional Dump Files

For export operations, you can specify dump files at the time the job is defined, as well as at a later time during the operation. For example, if you discover that space is running low during an export operation, you can add additional dump files by using the Data Pump Export ADD_FILE command in interactive mode. For import operations, all dump files must be specified at the time the job is defined. Log files and SQL files will overwrite previously existing files. Dump files will never overwrite previously existing files. Instead, an error will be generated.

Default Locations for Dump, Log, and SQL Files

Because Data Pump is server-based, rather than client-based, dump files, log files, and SQL files are accessed relative to server-based directory paths. Data Pump requires you to specify directory paths as directory objects. A directory object maps a name to a directory path on the file system. For example, the following SQL statement creates a directory object named dpump_dir1 that is mapped to a directory located at /usr/apps/datafiles.

SQL> CREATE DIRECTORY dpump_dir1 AS '/usr/apps/datafiles';

The reason that a directory object is required is to ensure data security and integrity. For example:

  • If you were allowed to specify a directory path location for an input file, you might be able to read data that the server has access to, but to which you should not.
  • If you were allowed to specify a directory path location for an output file, the server might overwrite a file that you might not normally have privileges to delete.

On Unix and Windows NT systems, a default directory object, DATA_PUMP_DIR, is created at database creation or whenever the database dictionary is upgraded. By default, it is available only to privileged users. If you are not a privileged user, before you can run Data Pump Export or Data Pump Import, a directory object must be created by a database administrator (DBA) or by any user with the CREATE ANY DIRECTORY privilege.

After a directory is created, the user creating the directory object needs to grant READ or WRITE permission on the directory to other users. For example, to allow the Oracle database to read and write files on behalf of user hr in the directory named by dpump_dir1, the DBA must execute the following command:


Note that READ or WRITE permission to a directory object only means that the Oracle database will read or write that file on your behalf. You are not given direct access to those files outside of the Oracle database unless you have the appropriate operating system privileges. Similarly, the Oracle database requires permission from the operating system to read and write files in the directories.

Data Pump Export and Import use the following order of precedence to determine a file's location:

  • If a directory object is specified as part of the file specification, then the location specified by that directory object is used. (The directory object must be separated from the filename by a colon.). If a directory object is not specified for a file, then the directory object named by the DIRECTORY parameter is used. If a directory object is not specified, and if no directory object was named by the DIRECTORY parameter, then the value of the environment variable, DATA_PUMP_DIR, is used. This environment variable is defined using operating system commands on the client system where the Data Pump Export and Import utilities are run. The value assigned to this client-based environment variable must be the name of a server-based directory object, which must first be created on the server system by a DBA. For example, the following SQL statement creates a directory object on the server system. The name of the directory object is DUMP_FILES1, and it is located at '/usr/apps/dumpfiles1'.

SQL> CREATE DIRECTORY DUMP_FILES1 AS '/usr/apps/dumpfiles1';

  • Then, a user on a UNIX-based client system using csh can assign the value DUMP_FILES1 to the environment variable DATA_PUMP_DIR. The DIRECTORY parameter can then be omitted from the command line. The dump file employees.dmp, as well as the log file export.log, will be written to '/usr/apps/dumpfiles1'.

%expdp hr/hr TABLES=employees DUMPFILE=employees.dmp

  • If none of the previous three conditions yields a directory object and you are a privileged user, then Data Pump attempts to use the value of the default server-based directory object, DATA_PUMP_DIR. This directory object is automatically created at database creation or when the database dictionary is upgraded. You can use the following SQL query to see the path definition for DATA_PUMP_DIR:

SQL> SELECT directory_name, directory_path FROM dba_directories WHERE directory_name='DATA_PUMP_DIR';

If you are not a privileged user, access to the DATA_PUMP_DIR directory object must have previously been granted to you by a DBA.  Do not confuse the default DATA_PUMP_DIR directory object with the client-based environment variable of the same name.

Using Directory Objects When Automatic Storage Management Is Enabled

If you use Data Pump Export or Import with Automatic Storage Management (ASM) enabled, you must define the directory object used for the dump file so that the ASM disk-group name is used (instead of an operating system directory path). A separate directory object, which points to an operating system directory path, should be used for the log file. For example, you would create a directory object for the ASM dump file as follows:


Then you would create a separate directory object for the log file:

SQL> CREATE or REPLACE DIRECTORY dpump_log as '/homedir/user1/';

To enable user hr to have access to these directory objects, you would assign the necessary privileges, for example:


You would then use the following Data Pump Export command:

> expdp hr/hr DIRECTORY=dpump_dir DUMPFILE=hr.dmp LOGFILE=dpump_log:hr.log


Setting Parallelism

For export and import operations, the parallelism setting (specified with the PARALLEL parameter) should be less than or equal to the number of dump files in the dump file set. If there are not enough dump files, the performance will not be optimal because multiple threads of execution will be trying to access the same dump file.

Using Substitution Variables

Instead of, or in addition to, listing specific filenames, you can use the DUMPFILE parameter during export operations to specify multiple dump files, by using a substitution variable (%U) in the filename. This is called a dump file template. The new dump files are created as they are needed, beginning with 01 for %U, then using 02, 03, and so on. Enough dump files are created to allow all processes specified by the current setting of the PARALLEL parameter to be active. If one of the dump files becomes full because its size has reached the maximum size specified by the FILESIZE parameter, it is closed, and a new dump file (with a new generated name) is created to take its place.

If multiple dump file templates are provided, they are used to generate dump files in a round-robin fashion. For example, if expa%U, expb%U, and expc%U were all specified for a job having a parallelism of 6, the initial dump files created would be expa01.dmp, expb01.dmp, expc01.dmp, expa02.dmp, expb02.dmp, and expc02.dmp.

For import and SQLFILE operations, if dump file specifications expa%U, expb%U, and expc%U are specified, then the operation will begin by attempting to open the dump files expa01.dmp, expb01.dmp, and expc01.dmp. If the dump file containing the master table is not found in this set, the operation expands its search for dump files by incrementing the substitution variable and looking up the new filenames (for example, expa02.dmp, expb02.dmp, and expc02.dmp). The search continues until the dump file containing the master table is located. If a dump file does not exist, the operation stops incrementing the substitution variable for the dump file specification that was in error. For example, if expb01.dmp and expb02.dmp are found but expb03.dmp is not found, then no more files are searched for using the expb%U specification. Once the master table is found, it is used to determine whether all dump files in the dump file set have been located.


Moving Data Between Different Database Versions

Because most Data Pump operations are performed on the server side, if you are using any version of the database other than COMPATIBLE, you must provide the server with specific version information. Otherwise, errors may occur. To specify version information, use the VERSION parameter.

Keep the following information in mind when you are using Data Pump Export and Import to move data between different database versions:

  • If you specify a database version that is older than the current database version, certain features may be unavailable. For example, specifying VERSION=10.1 will cause an error if data compression is also specified for the job because compression was not supported in 10.1.
  • On a Data Pump export, if you specify a database version that is older than the current database version, then a dump file set is created that you can import into that older version of the database. However, the dump file set will not contain any objects that the older database version does not support. For example, if you export from a version 10.2 database to a version 10.1 database, comments on indextypes will not be exported into the dump file set.
  • Data Pump Import can always read dump file sets created by older versions of the database.
  • Data Pump Import cannot read dump file sets created by a database version that is newer than the current database version, unless those dump file sets were created with the version parameter set to the version of the target database. Therefore, the best way to perform a downgrade is to perform your Data Pump export with the VERSION parameter set to the version of the target database.
  • When operating across a network link, Data Pump requires that the remote database version be either the same as the local database or one version older, at the most. For example, if the local database is version 10.2, the remote database must be either version 10.1 or 10.2. If the local database is version 10.1, then 10.1 is the only version supported for the remote database.


Original Export and Import Versus Data Pump Export and Import

If you are familiar with the original Export (exp) and Import (imp) utilities, it is important to understand that many of the concepts behind them do not apply to Data Pump Export (expdp) and Data Pump Import (impdp). In particular:

  • Data Pump Export and Import operate on a group of files called a dump file set rather than on a single sequential dump file.
  • Data Pump Export and Import access files on the server rather than on the client. This results in improved performance. It also means that directory objects are required when you specify file locations.
  • The Data Pump Export and Import modes operate symmetrically, whereas original export and import did not always exhibit this behavior. For example, suppose you perform an export with FULL=Y, followed by an import using SCHEMAS=HR. This will produce the same results as if you performed an export with SCHEMAS=HR, followed by an import with FULL=Y.
  • Data Pump Export and Import use parallel execution rather than a single stream of execution, for improved performance. This means that the order of data within dump file sets and the information in the log files is more variable.
  • Data Pump Export and Import represent metadata in the dump file set as XML documents rather than as DDL commands. This provides improved flexibility for transforming the metadata at import time.
  • Data Pump Export and Import are self-tuning utilities. Tuning parameters that were used in original Export and Import, such as BUFFER and RECORDLENGTH, are neither required nor supported by Data Pump Export and Import.
  • At import time there is no option to perform interim commits during the restoration of a partition. This was provided by the COMMIT parameter in original Import.
  • There is no option to merge extents when you re-create tables. In original Import, this was provided by the COMPRESS parameter. Instead, extents are reallocated according to storage parameters for the target table.
  • Sequential media, such as tapes and pipes, are not supported.
  • The Data Pump method for moving data between different database versions is different than the method used by original Export/Import. With original Export, you had to run an older version of Export (exp) to produce a dump file that was compatible with an older database version. With Data Pump, you can use the current Export (expdp) version and simply use the VERSION parameter to specify the target database version.
  • When you are importing data into an existing table using either APPEND or TRUNCATE, if any row violates an active constraint, the load is discontinued and no data is loaded. This is different from original Import, which logs any rows that are in violation and continues with the load.
  • Data Pump Export and Import consume more undo tablespace than original Export and Import. This is due to additional metadata queries during export and some relatively long-running master table queries during import. As a result, for databases with large amounts of metadata, you may receive an ORA-01555: snapshot too old error. To avoid this, consider adding additional undo tablespace or increasing the value of the UNDO_RETENTION initialization parameter for the database.
  • If a table has compression enabled, Data Pump Import attempts to compress the data being loaded. Whereas, the original Import utility loaded data in such a way that if a even table had compression enabled, the data was not compressed upon import.
  • Data Pump supports character set conversion for both direct path and external tables. Most of the restrictions that exist for character set conversions in the original Import utility do not apply to Data Pump. The one case in which character set conversions are not supported under the Data Pump is when using transportable tablespaces.

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