Undo Management
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The document discusses various ways to manage undo space in an Oracle database including manual and automatic undo management. It provides SQL commands for configuring undo management and calculating the optimal undo retention parameter and undo tablespace size.
![Anar Godjaev
select begin_time, end_time, tuned_undoretention from
v$undostat;
-- calculating undo space
select
ceil(((UR * (UPS * DBS)) + (DBS * 24))/1024/1024) as MB
from
(select value as UR from v$parameter where
name='undo_retention'),
(select (sum(undoblks)/sum(((end_time - begin_time) * 84600)))
as UPS
from v$undostat),
(select value as DBS from v$parameter where name =
'db_block_size');
set transaction read only;
set transaction isolation level serializable; -- possible
negative impact on performance
-- Actual Undo Size
SELECT SUM(a.bytes) "UNDO_SIZE"
FROM v$datafile a,
v$tablespace b,
dba_tablespaces c
WHERE c.contents = 'UNDO'
AND c.status = 'ONLINE'
AND b.name = c.tablespace_name
AND a.ts# = b.ts#;
-- Undo Blocks per Second
SELECT MAX(undoblks/((end_time-begin_time)*3600*24))
"UNDO_BLOCK_PER_SEC"
FROM v$undostat;
-- DB Block Size
SELECT TO_NUMBER(value) "DB_BLOCK_SIZE [KByte]"
FROM v$parameter
WHERE name = 'db_block_size';](https://image.slidesharecdn.com/undo-20management-131025060914-phpapp01/85/Undo-Management-2-320.jpg)
![Anar Godjaev
-- optimal undo retention
SELECT d.undo_size/(1024*1024) "ACTUAL UNDO SIZE [MByte]",
SUBSTR(e.value,1,25) "UNDO RETENTION [Sec]",
ROUND((d.undo_size / (to_number(f.value) *
g.undo_block_per_sec)))
"OPTIMAL UNDO RETENTION [Sec]"
FROM (
SELECT SUM(a.bytes) undo_size
FROM v$datafile a,
v$tablespace b,
dba_tablespaces c
WHERE c.contents = 'UNDO'
AND c.status = 'ONLINE'
AND b.name = c.tablespace_name
AND a.ts# = b.ts#
) d,
v$parameter e,
v$parameter f,
( SELECT MAX(undoblks/((end_time-begin_time)*3600*24))
undo_block_per_sec FROM v$undostat ) g
WHERE e.name = 'undo_retention'
AND f.name = 'db_block_size';
-- optimal undo size
SELECT d.undo_size/(1024*1024) "ACTUAL UNDO SIZE [MByte]",
SUBSTR(e.value,1,25) "UNDO RETENTION [Sec]",
(TO_NUMBER(e.value) * TO_NUMBER(f.value) *
g.undo_block_per_sec) / (1024*1024)
"NEEDED UNDO SIZE [MByte]"
FROM (
SELECT SUM(a.bytes) undo_size
FROM v$datafile a,
v$tablespace b,
dba_tablespaces c
WHERE c.contents = 'UNDO'
AND c.status = 'ONLINE'
AND b.name = c.tablespace_name
AND a.ts# = b.ts#](https://image.slidesharecdn.com/undo-20management-131025060914-phpapp01/85/Undo-Management-3-320.jpg)
Undo Management
- 1. Anar Godjaev Undo Management optimal undo_retention parameter = actual undo size / (db_block_size/undo_block_per_sec) -- ********************** -- manual undo management -- ********************** create rollback segment r01 tablespacerbs; create public rollback_segment r01 tablespacerbs; alter rollback segment r01 offline; drop rollback segment r01; -- ************************* -- automatic undo management -- ************************* alter system set undo_management = auto; alter system set undo_tablespace = <ts_name>; alter system set undo_suppress_errors = true|false ; alter system set undo_retention = 900; --seconds select name, value from v$parameter where name like 'undo_management' select name, value from v$parameter where name like 'undo_tablespace' select name, value from v$parameter where name like 'undo_retention' -- creating undo tablespace create undo tablespacets_undodatafile 'C:ts_undo_01.dbf' size 20m; alter tablespacets_undo add datafile 'C:ts_undo_02.dbf' size 40m; alter system set undo_tablespace = ts_undo; drop tablespacets_undo; alter tablespacets_undo retention guarantee; select retention, tablespace_name from dba_tablespaces;
- 2. Anar Godjaev select begin_time, end_time, tuned_undoretention from v$undostat; -- calculating undo space select ceil(((UR * (UPS * DBS)) + (DBS * 24))/1024/1024) as MB from (select value as UR from v$parameter where name='undo_retention'), (select (sum(undoblks)/sum(((end_time - begin_time) * 84600))) as UPS from v$undostat), (select value as DBS from v$parameter where name = 'db_block_size'); set transaction read only; set transaction isolation level serializable; -- possible negative impact on performance -- Actual Undo Size SELECT SUM(a.bytes) "UNDO_SIZE" FROM v$datafile a, v$tablespace b, dba_tablespaces c WHERE c.contents = 'UNDO' AND c.status = 'ONLINE' AND b.name = c.tablespace_name AND a.ts# = b.ts#; -- Undo Blocks per Second SELECT MAX(undoblks/((end_time-begin_time)*3600*24)) "UNDO_BLOCK_PER_SEC" FROM v$undostat; -- DB Block Size SELECT TO_NUMBER(value) "DB_BLOCK_SIZE [KByte]" FROM v$parameter WHERE name = 'db_block_size';
- 3. Anar Godjaev -- optimal undo retention SELECT d.undo_size/(1024*1024) "ACTUAL UNDO SIZE [MByte]", SUBSTR(e.value,1,25) "UNDO RETENTION [Sec]", ROUND((d.undo_size / (to_number(f.value) * g.undo_block_per_sec))) "OPTIMAL UNDO RETENTION [Sec]" FROM ( SELECT SUM(a.bytes) undo_size FROM v$datafile a, v$tablespace b, dba_tablespaces c WHERE c.contents = 'UNDO' AND c.status = 'ONLINE' AND b.name = c.tablespace_name AND a.ts# = b.ts# ) d, v$parameter e, v$parameter f, ( SELECT MAX(undoblks/((end_time-begin_time)*3600*24)) undo_block_per_sec FROM v$undostat ) g WHERE e.name = 'undo_retention' AND f.name = 'db_block_size'; -- optimal undo size SELECT d.undo_size/(1024*1024) "ACTUAL UNDO SIZE [MByte]", SUBSTR(e.value,1,25) "UNDO RETENTION [Sec]", (TO_NUMBER(e.value) * TO_NUMBER(f.value) * g.undo_block_per_sec) / (1024*1024) "NEEDED UNDO SIZE [MByte]" FROM ( SELECT SUM(a.bytes) undo_size FROM v$datafile a, v$tablespace b, dba_tablespaces c WHERE c.contents = 'UNDO' AND c.status = 'ONLINE' AND b.name = c.tablespace_name AND a.ts# = b.ts#
- 4. Anar Godjaev ) d, v$parameter e, v$parameter f, ( SELECT MAX(undoblks/((end_time-begin_time)*3600*24)) undo_block_per_sec FROM v$undostat ) g WHERE e.name = 'undo_retention' AND f.name = 'db_block_size'; -- undo consumer sessions SELECT TO_CHAR (s.SID) || ',' || TO_CHAR (s.serial#) sid_serial, NVL (s.username, 'None') orauser, s.program, r.NAME undoseg, t.used_ublk * TO_NUMBER (x.VALUE) / 1024 || 'K' "Undo", t1.tablespace_name FROM SYS.v_$rollname r, SYS.v_$session s, SYS.v_$transaction t, SYS.v_$parameter x, dba_rollback_segs t1 WHERE s.taddr = t.addr AND r.usn = t.xidusn(+) AND x.NAME = 'db_block_size' AND t1.segment_id = r.usn AND t1.tablespace_name = 'UNDOTBS1';