Author: Mounir Babari. Senior Technical Support Engineer UK. IBM B2B & Commerce - Industry Solutions. 1 Reclaiming wasted space in Oracle database

Transcription

1 Reclaiming wasted space in Oracle database Author: Mounir Babari Senior Technical Support Engineer UK IBM B2B & Commerce - Industry Solutions 1 Reclaiming wasted space in Oracle database

2 Reclaiming space in Oracle can be sometimes challenging. It is a common question from users. Some customers manage to reclaim space from tables and indexes but find it hard for the lob segments. Starting with Oracle 10g R2, Oracle provides a solution that allows users to reclaim LOB space (LOB data and LOB index). In this document, I will demonstrate how to reclaim the unused space using the segment advisor and manual commands with Oracle 11g. I will use IBM Sterling Integrator 5 to fill a table called TRANS_DATA that contain a blob column called DATA_OBJECT. SQL> Desc TRANS_DATA Name Null Type DATA_ID DATA_OBJECT PAGE_INDEX DATA_TYPE WF_ID REFERENCE_TABLE NOT NULL VARCHAR2(255) NOT NULL BLOB NOT NULL NUMBER(38) NOT NULL NUMBER(38) NUMBER VARCHAR2(255) First, we need to find the size of the tablespaces: Using SQL: select b.tablespace_name, tbs_size SizeMb, a.free_space FreeMb from (select tablespace_name, round(sum(bytes)/1024/1024,2) as free_space from dba_free_space group by tablespace_name) a, (select tablespace_name, sum(bytes)/1024/1024 as tbs_size from dba_data_files group by tablespace_name) b where a.tablespace_name(+)=b.tablespace_name 2 Reclaiming wasted space in Oracle database

3 This information can also be found with the Oracle Enterprise manager under Servers/Tablespaces. The tablespace called SI51GM has 875MB allocated with 201MB free. Now let us load this tablespace SI51GM using IBM Sterling Integrator: Some of the free space was reused and the tablespace has increased to 950MB 3 Reclaiming wasted space in Oracle database

4 Let us upload a second large document The size of the tablespace is 1025 MB now!! We need to delete this data and observe the space size: Procedure for Sterling Integrator to delete the data from the table update archive_info set archive_flag=1,archive_date=sysdate where wf_id= update archive_info set archive_flag=1,archive_date=sysdate where wf_id= run the purge service to delete the rows from the table TRANS_DATA The data was removed from the tables. However, the space is not reclaimed and the tablespace size has actually increased to 1100MB!! 4 Reclaiming wasted space in Oracle database

5 The Database usage indicator is in the red zone!! We are only using about 50MB for this blob column, we use the dbms_lob to find the size of the binary objects: select sum(dbms_lob.getlength(data_object))/1024/1024, count(*) from trans_data We have about 500 MB used between TRANS_DATA table and its LOB segment: select segment_name,segment_type,tablespace_name,round(bytes/1024/1024) MB from user_segments where tablespace_name ='SI51GM' order by bytes desc 5 Reclaiming wasted space in Oracle database

6 Reclaiming the space with the Segment Advisor The Segment Advisor identifies segments that have space available for reclamation. It performs its analysis by examining usage and growth statistics in the Automatic Workload Repository (AWR), and by sampling the data in the segment. It is configured to run during maintenance windows as an automated maintenance task, and you can also run it on demand (manually). The Segment Advisor automated maintenance task is known as the Automatic Segment Advisor. The Segment Advisor creates several types of results: recommendations, findings, actions, and objects. You can view results in the following ways: With Enterprise Manager By querying the DBA_ADVISOR_* views By calling the DBMS_SPACE.ASA_RECOMMENDATIONS procedure For further reading see also: Oracle Database Reference for details on the DBA_ADVISOR_* views. Oracle Database PL/SQL Packages and Types Reference for details on the DBMS_SPACE.ASA_RECOMMENDATIONS function. Segment advisor on the SI51GM tablespace 6 Reclaiming wasted space in Oracle database

7 The dbms_advisor is used in the background: DECLARE taskname varchar2(100); taskdesc varchar2(128); task_id number; object_id number; timelimit varchar2(25); numdaystoretain varchar2(25); objectname varchar2(100); objecttype varchar2(100); BEGIN taskname := 'SEGMENTADV_ '; taskdesc :='Get shrink advice based on object growth trend'; numdaystoretain :='30'; dbms_advisor.create_task('segment Advisor',?,taskname,taskdesc,NULL); dbms_advisor.create_object(taskname, 'TABLESPACE', 'SI51GM', ' ', ' ', NULL, object_id); dbms_advisor.set_task_parameter(taskname, 'RECOMMEND_ALL', 'TRUE'); dbms_advisor.set_task_parameter(taskname, 'DAYS_TO_EXPIRE', numdaystoretain); END; 7 Reclaiming wasted space in Oracle database

8 8 Reclaiming wasted space in Oracle database

9 The Segment Advisor identified the following segments that have space available for reclamation. The segment advisor found 247MB to reclaim from the TRANS_DATA table. Below is the SQL query to shrink the table: PL/SQL begin EXECUTE IMMEDIATE 'alter table "SI51GM"."TRANS_DATA" shrink space'; end; 9 Reclaiming wasted space in Oracle database

10 We can see that some space was freed from the table but not from the LOB segment: 10 Reclaiming wasted space in Oracle database

11 Running the Segment Advisor manually: We need to run the segment advisor manually to analyze the LOB segment for the TRANS_DATA table. The following procedures enables you to include the schema object's dependent objects in the Segment Advisor run. For example, if you select a table and select the Run Segment Advisor command, Enterprise Manager displays the table's dependent objects, such as partitions, index segments, LOB segments, and so on. You can then select dependent objects to include in the run. To run the Segment Advisor manually with the Segment Advisor Wizard: 1. From the database Home page, under Related Links, click Advisor Central. The Advisor Central page appears Under Advisors, click Segment Advisor. The first page of the Segment Advisor wizard appears. 2. Follow the wizard steps to schedule the Segment Advisor job, and then click Submit on the final wizard page. The Advisor Central page reappears, with the new Segment Advisor job at the top of the list under the Results heading. The job status is SCHEDULED. (If you don't see your job, use the search fields above the list to display it.) 3. Check the status of the job. If it is not COMPLETED, click the Refresh button at the top of the page repeatedly. (Do not use your browser's Refresh icon.) When the job status changes to COMPLETED, select the job by clicking in the Select column, and then click View Result. 11 Reclaiming wasted space in Oracle database

12 12 Reclaiming wasted space in Oracle database

13 Some space has been reclaimed from different objects but not from the lob segment: 13 Reclaiming wasted space in Oracle database

14 After running the shrink space command on the blob column we get more space reclaimed: The syntax used to reclaim LOB space is: SQL> ALTER TABLE <table_name> MODIFY LOB (<lob_column>) (SHRINK SPACE); The alter table <table_name> shrink space statement was introduced in Oracle 10g R1. The ability to extend the SHRINK SPACE command to LOBs was introduced in Oracle 10g R2 SQL> alter table trans_data modify lob (data_object) (shrink space); After running the previous command, we notice that the SYS_LOB C00002$$ is not even in the top list: SQL Query to list all the lob segments for a given tablespace: select segment_name,segment_type,tablespace_name, round(bytes/1024/1024) MB from user_segments where tablespace_name='si51gm' and segment_type='lobsegment' order by bytes desc 14 Reclaiming wasted space in Oracle database

15 The space now is reclaimed and the Sterling Integrator database screen usage indicator is back to the green zone: 15 Reclaiming wasted space in Oracle database

16 DBA tips: The LOB segment must reside in an Automatic Segment Space Management ASSM tablespace. Row movement on a table is only required if you are moving rows from the table itself. It is not required if all you are doing is shrinking its LOB segment(s). Shrinking a LOB segment does generate redo. For example, shrinking a 15GB LOB will generate approximately 15GB of redo! Make sure you have an adequate amount of disk space for any archived redo log files before manually shrinking a LOB segment. The alter table <table_name> shrink space statement has an optional CASCADE clause that shrinks all dependent objects (including LOBS) along with the table data itself. With Oracle 10g R1, when shrinking a table, the CASCADE option DOES NOT shrink LOB segments for that table. With Oracle 10g R2 and higher, when shrinking a table, the CASCADE option DOES shrink all LOB segments (and indexes) for that table 16 Reclaiming wasted space in Oracle database