一个回滚段收缩的实例
日前在整理數據庫表空間的是否，發現最大的數據文件來自回滾段。回滾段文件undotbs1的數據文件已經達到23G。
希望清理這部分數據，但一時又無從下手。于是決定深入了解一下這部分內容。
法和規劃及問題的解決。?
1，檢查回滾段信息
select?t.segment_name,t.owner,t.tablespace_name,
d.file_name,
t.initial_extent,t.min_extents,t.max_extents,t.status
from?dba_rollback_segs t, dba_data_files d
where?t.file_id = d.file_id
| seg_name | owner | tablespace | filename | init_extents | min_extents | max_extents | status |
| SYSTEM | SYS | SYSTEM | /oradata/system01.dbf | 114688 | 1 | 32765 | ONLINE |
| _SYSSMU1$ | PUBLIC | UNDOTBS1 | /oradata/undotbs01.dbf | 131072 | 2 | 32765 | ONLINE |
| _SYSSMU2$ | PUBLIC | UNDOTBS1 | /oradata/undotbs01.dbf | 131072 | 2 | 32765 | ONLINE |
| _SYSSMU3$ | PUBLIC | UNDOTBS1 | /oradata/undotbs01.dbf | 131072 | 2 | 32765 | ONLINE |
| _SYSSMU4$ | PUBLIC | UNDOTBS1 | /oradata/undotbs01.dbf | 131072 | 2 | 32765 | ONLINE |
| _SYSSMU5$ | PUBLIC | UNDOTBS1 | /oradata/undotbs01.dbf | 131072 | 2 | 32765 | ONLINE |
| _SYSSMU6$ | PUBLIC | UNDOTBS1 | /oradata/undotbs01.dbf | 131072 | 2 | 32765 | ONLINE |
| _SYSSMU12$ | PUBLIC | UNDOTBS1 | /oradata/undotbs01.dbf | 131072 | 2 | 32765 | OFFLINE |
| _SYSSMU7$ | PUBLIC | UNDOTBS1 | /oradata/undotbs01.dbf | 131072 | 2 | 32765 | ONLINE |
| _SYSSMU8$ | PUBLIC | UNDOTBS1 | /oradata/undotbs01.dbf | 131072 | 2 | 32765 | ONLINE |
| _SYSSMU9$ | PUBLIC | UNDOTBS1 | /oradata/undotbs01.dbf | 131072 | 2 | 32765 | ONLINE |
| _SYSSMU10$ | PUBLIC | UNDOTBS1 | /oradata/undotbs01.dbf | 131072 | 2 | 32765 | ONLINE |
| _SYSSMU11$ | PUBLIC | UNDOTBS1 | /oradata/undotbs02.dbf | 131072 | 2 | 32765 | OFFLINE |
| _SYSSMU13$ | PUBLIC | UNDOTBS1 | /oradata/undotbs02.dbf | 131072 | 2 | 32765 | OFFLINE |
| _SYSSMU14$ | PUBLIC | UNDOTBS1 | /oradata/undotbs02.dbf | 131072 | 2 | 32765 | OFFLINE |
這里我們看到/oradata/undotbs02.dbf中的3個公用回滾段都是offline的。而且在/oradata/undotbs01.dbf文件中也有一個回滾段屬于offline狀態。
2，檢查回滾段各段當前大小
select?ds.segment_name,ds.bytes,drs.status,ddf.file_name
from?dba_segments ds, dba_rollback_segs drs,dba_data_files ddf
where?ds.segment_name = drs.segment_name
??and?drs.file_id = ddf.file_id
| segment_name | seg_size | status | file_name |
| _SYSSMU11$ | 328.1796875 | OFFLINE | /oradata/undotbs02.dbf |
| _SYSSMU12$ | 208.0546875 | OFFLINE | /oradata/undotbs01.dbf |
| _SYSSMU13$ | 88.1796875 | OFFLINE | /oradata/undotbs02.dbf |
| _SYSSMU14$ | 95.9921875 | OFFLINE | /oradata/undotbs02.dbf |
| SYSTEM | 0.4375 | ONLINE | /oradata/system01.dbf |
| _SYSSMU1$ | 2.1171875 | ONLINE | /oradata/undotbs01.dbf |
| _SYSSMU10$ | 2.1171875 | ONLINE | /oradata/undotbs01.dbf |
| _SYSSMU2$ | 2.1171875 | ONLINE | /oradata/undotbs01.dbf |
| _SYSSMU3$ | 2.1171875 | ONLINE | /oradata/undotbs01.dbf |
| _SYSSMU4$ | 2.1171875 | ONLINE | /oradata/undotbs01.dbf |
| _SYSSMU5$ | 2.1171875 | ONLINE | /oradata/undotbs01.dbf |
| _SYSSMU6$ | 2.1171875 | ONLINE | /oradata/undotbs01.dbf |
| _SYSSMU7$ | 2.1171875 | ONLINE | /oradata/undotbs01.dbf |
| _SYSSMU8$ | 2.1171875 | ONLINE | /oradata/undotbs01.dbf |
| _SYSSMU9$ | 2.1171875 | ONLINE | /oradata/undotbs01.dbf |
結果是令人沮喪的，23G的空間只有500M左右是used，并且used空間的99％的數據還是在offline的回滾段中的。這里，我們需要做的處理應該不是對回滾段的收縮。
事實上目前online的回滾段空間是非常小的，接近2M。而大部分的空間在置入free后沒有回收。我們來檢查一下回滾段的空閑塊信息。從而推測一下freelist的組成。
select?a.segment_name,dfs.tablespace_name,dfs.block_id,dfs.bytes,dfs.blocks
from?dba_free_space dfs,
(
select?ds.segment_name,ds.header_block,drs.file_id,
lead(ds.header_block) over(partition?by?drs.file_id?order?by?ds.header_block) next_block
from?dba_segments ds, dba_rollback_segs drs
where?ds.segment_name = drs.segment_name
) a
where?dfs.block_id?between?a.header_block?and?nvl(a.next_block,100000000000)
??and?dfs.file_id = a.file_id
??and?dfs.tablespace_name =?'UNDOTBS1'
| segment_name | tablespace | block_id | bytes | blocks |
| _SYSSMU10$ | UNDOTBS1 | 185 | 65536 | 8 |
| _SYSSMU10$ | UNDOTBS1 | 201 | 65536 | 8 |
| _SYSSMU10$ | UNDOTBS1 | 265 | 65536 | 8 |
| _SYSSMU10$ | UNDOTBS1 | 281 | 196608 | 24 |
| _SYSSMU10$ | UNDOTBS1 | 313 | 327680 | 40 |
| _SYSSMU10$ | UNDOTBS1 | 361 | 262144 | 32 |
| _SYSSMU10$ | UNDOTBS1 | 905 | 3145728 | 384 |
| _SYSSMU10$ | UNDOTBS1 | 1673 | 1048576 | 128 |
| _SYSSMU10$ | UNDOTBS1 | 3721 | 33554432 | 4096 |
| _SYSSMU10$ | UNDOTBS1 | 7945 | 143654912 | 17536 |
| _SYSSMU10$ | UNDOTBS1 | 25865 | 1048576 | 128 |
| _SYSSMU10$ | UNDOTBS1 | 26377 | 1048576 | 128 |
| _SYSSMU10$ | UNDOTBS1 | 26889 | 8388608 | 1024 |
| _SYSSMU10$ | UNDOTBS1 | 28297 | 1048576 | 128 |
| _SYSSMU10$ | UNDOTBS1 | 28681 | 1048576 | 128 |
| _SYSSMU10$ | UNDOTBS1 | 28937 | 502267904 | 61312 |
| _SYSSMU10$ | UNDOTBS1 | 90633 | 1048576 | 128 |
| _SYSSMU10$ | UNDOTBS1 | 92041 | 13631488 | 1664 |
| _SYSSMU10$ | UNDOTBS1 | 93833 | 279969792 | 34176 |
| _SYSSMU10$ | UNDOTBS1 | 131081 | 57671680 | 7040 |
| _SYSSMU10$ | UNDOTBS1 | 139145 | 76546048 | 9344 |
| _SYSSMU10$ | UNDOTBS1 | 149513 | 84934656 | 10368 |
| _SYSSMU10$ | UNDOTBS1 | 160521 | 134217728 | 16384 |
| _SYSSMU10$ | UNDOTBS1 | 177161 | 11534336 | 1408 |
| _SYSSMU10$ | UNDOTBS1 | 179849 | 29360128 | 3584 |
| _SYSSMU10$ | UNDOTBS1 | 183561 | 33554432 | 4096 |
| _SYSSMU10$ | UNDOTBS1 | 191753 | 8388608 | 1024 |
| _SYSSMU10$ | UNDOTBS1 | 195977 | 396361728 | 48384 |
| _SYSSMU10$ | UNDOTBS1 | 245385 | 696254464 | 84992 |
| _SYSSMU10$ | UNDOTBS1 | 331401 | 1445986304 | 176512 |
| _SYSSMU10$ | UNDOTBS1 | 507913 | 4160749568 | 507904 |
| _SYSSMU10$ | UNDOTBS1 | 1015817 | 671088640 | 81920 |
| _SYSSMU10$ | UNDOTBS1 | 1105929 | 536870912 | 65536 |
| _SYSSMU10$ | UNDOTBS1 | 1204361 | 67108864 | 8192 |
| _SYSSMU10$ | UNDOTBS1 | 1217033 | 22020096 | 2688 |
| _SYSSMU10$ | UNDOTBS1 | 1221513 | 2475687936 | 302208 |
| _SYSSMU10$ | UNDOTBS1 | 1523721 | 4160749568 | 507904 |
| _SYSSMU10$ | UNDOTBS1 | 2031625 | 4160749568 | 507904 |
| _SYSSMU10$ | UNDOTBS1 | 2539529 | 2196701184 | 268152 |
| _SYSSMU14$ | UNDOTBS1 | 3337 | 33554432 | 4096 |
| _SYSSMU14$ | UNDOTBS1 | 7561 | 131072000 | 16000 |
| _SYSSMU14$ | UNDOTBS1 | 23817 | 1048576 | 128 |
| _SYSSMU14$ | UNDOTBS1 | 24969 | 1048576 | 128 |
| _SYSSMU14$ | UNDOTBS1 | 25353 | 10485760 | 1280 |
| _SYSSMU14$ | UNDOTBS1 | 27145 | 67108864 | 8192 |
| _SYSSMU14$ | UNDOTBS1 | 35593 | 729808896 | 89088 |
| _SYSSMU14$ | UNDOTBS1 | 129417 | 72351744 | 8832 |
| _SYSSMU14$ | UNDOTBS1 | 139273 | 83886080 | 10240 |
| _SYSSMU14$ | UNDOTBS1 | 152201 | 142606336 | 17408 |
| _SYSSMU14$ | UNDOTBS1 | 169993 | 179306496 | 21888 |
| _SYSSMU14$ | UNDOTBS1 | 192009 | 2097152 | 256 |
| _SYSSMU14$ | UNDOTBS1 | 192521 | 25165824 | 3072 |
| _SYSSMU14$ | UNDOTBS1 | 196105 | 18874368 | 2304 |
| _SYSSMU14$ | UNDOTBS1 | 199177 | 41943040 | 5120 |
| _SYSSMU14$ | UNDOTBS1 | 206089 | 38797312 | 4736 |
| _SYSSMU14$ | UNDOTBS1 | 211849 | 192937984 | 23552 |
| _SYSSMU14$ | UNDOTBS1 | 236425 | 201326592 | 24576 |
| _SYSSMU14$ | UNDOTBS1 | 262025 | 645922816 | 78848 |
| _SYSSMU14$ | UNDOTBS1 | 342921 | 1351614464 | 164992 |
| _SYSSMU14$ | UNDOTBS1 | 507913 | 805306368 | 98304 |
| _SYSSMU14$ | UNDOTBS1 | 606233 | 65536 | 8 |
| _SYSSMU14$ | UNDOTBS1 | 606249 | 65536 | 8 |
| _SYSSMU14$ | UNDOTBS1 | 606321 | 217251840 | 26520 |
| _SYSSMU14$ | UNDOTBS1 | 633865 | 176095232 | 21496 |
????結果顯示，大部分的free space來自10和14兩個回滾段，我們先嘗試對這兩個回滾段進行收縮。
4，收縮回滾段
SQL> alter rollback segment 10 shrink to 100M;
alter rollback segment 10 shrink to 100M
ORA-02245: invalid ROLLBACK SEGMENT name
5，修改數據文件大小
SQL> alter database datafile '/oradata/undotbs02.dbf' resize 1000M;
alter database datafile '/oradata/undotbs02.dbf' resize 1000M
*
ERROR at line 1:
ORA-03297: file contains used data beyond requested RESIZE value
?
?
6，重建回滾表空間，將當前回滾段執行該表空間，并刪除舊的回滾段表空間
創建新的回滾空間
SQL> create undo tablespace undotbs2 datafile '/oradata/undotbs03.dbf' size 1000M;
Tablespace created.
切換回滾空間為新的空間
SQL> alter system set undo_tablespace = undotbs2 scope = both;
System altered.
等待源undo表空間的所有undo segment offline
select?segment_name,tablespace_name,status?from?dba_rollback_segs
where?tablespace_name =?'UNDOTBS1'
| segment_name | tablespace | status |
| _SYSSMU1$ | UNDOTBS1 | OFFLINE |
| _SYSSMU2$ | UNDOTBS1 | OFFLINE |
| _SYSSMU3$ | UNDOTBS1 | OFFLINE |
| _SYSSMU4$ | UNDOTBS1 | OFFLINE |
| _SYSSMU5$ | UNDOTBS1 | OFFLINE |
| _SYSSMU6$ | UNDOTBS1 | OFFLINE |
| _SYSSMU7$ | UNDOTBS1 | OFFLINE |
| _SYSSMU8$ | UNDOTBS1 | OFFLINE |
| _SYSSMU9$ | UNDOTBS1 | OFFLINE |
| _SYSSMU10$ | UNDOTBS1 | OFFLINE |
| _SYSSMU11$ | UNDOTBS1 | OFFLINE |
| _SYSSMU12$ | UNDOTBS1 | OFFLINE |
| _SYSSMU13$ | UNDOTBS1 | OFFLINE |
| _SYSSMU14$ | UNDOTBS1 | OFFLINE |
刪除原表空間untotbs1
SQL> drop tablespace undotbs1 including contents;
Tablespace dropped.
刪除原表空間包含文件
$ rm –I undotbs01.dbf
$ rm –I undotbs02.dbf
?
?
?
附錄1
DBA_ROLLBACK_SEGS
DBA_ROLLBACK_SEGS describes rollback segments.
| Column | Datatype | NULL | Description |
| SEGMENT_NAME | VARCHAR2(30) | NOT NULL | Name of the rollback segment |
| OWNER | VARCHAR2(6) | ? | Owner of the rollback segment |
| TABLESPACE_NAME | VARCHAR2(30) | NOT NULL | Name of the tablespace containing the rollback segment |
| SEGMENT_ID | NUMBER | NOT NULL | ID number of the rollback segment |
| FILE_ID | NUMBER | NOT NULL | File identifier number of the file containing the segment head |
| BLOCK_ID | NUMBER | NOT NULL | ID number of the block containing the segment header |
| INITIAL_EXTENT | NUMBER | ? | Initial extent size in bytes |
| NEXT_EXTENT | NUMBER | ? | Secondary extent size in bytes |
| MIN_EXTENTS | NUMBER | NOT NULL | Minimum number of extents |
| MAX_EXTENTS | NUMBER | NOT NULL | Maximum number of extent |
| PCT_INCREASE | NUMBER | NOT NULL | Percent increase for extent size |
| STATUS | VARCHAR2(16) | ? | Rollback segment status |
| INSTANCE_NUM | VARCHAR2(40) | ? | Rollback segment owning Oracle Real Application Cluster instance number |
| RELATIVE_FNO | NUMBER | NOT NULL | Relative file number of the segment header |
?
附錄2?V$ROLLSTAT
This view contains rollback segment statistics.
| Column | Datatype | Description |
| USN | NUMBER | Rollback segment number |
| LATCH | NUMBER | Latch for the rollback segment |
| EXTENTS | NUMBER | Number of extents in the rollback segment |
| RSSIZE | NUMBER | Size (in bytes) of the rollback segment. This value differs by the number of bytes in one database block from the value of the BYTES column of the ALL/DBA/USER_SEGMENTS views. See Also:?Oracle9i Database Administrator's Guide. |
| WRITES | NUMBER | Number of bytes written to the rollback segment |
| XACTS | NUMBER | Number of active transactions |
| GETS | NUMBER | Number of header gets |
| WAITS | NUMBER | Number of header waits |
| OPTSIZE | NUMBER | Optimal size of the rollback segment |
| HWMSIZE | NUMBER | High water mark of rollback segment size |
| SHRINKS | NUMBER | Number of times the size of a rollback segment decreases |
| WRAPS | NUMBER | Number of times rollback segment is wrapped |
| EXTENDS | NUMBER | Number of times rollback segment size is extended |
| AVESHRINK | NUMBER | Average shrink size |
| AVEACTIVE | NUMBER | Current size of active extents, averaged over time. |
| STATUS | VARCHAR2(15) | Rollback segment status:
|
| CUREXT | NUMBER | Current extent |
| CURBLK | NUMBER | Current block |
?
?
附錄3?V$UNDOSTAT
V$UNDOSTAT displays a histogram of statistical data to show how well the system is working. The available statistics include undo space consumption, transaction concurrency, and length of queries executed in the instance. You can use this view to estimate the amount of undo space required for the current workload. Oracle uses this view to tune undo usage in the system. The view returns null values if the system is in manual undo management mode.
Each row in the view keeps statistics collected in the instance for a 10-minute interval. The rows are in descending order by the BEGIN_TIME column value. Each row belongs to the time interval marked by (BEGIN_TIME, END_TIME). Each column represents the data collected for the particular statistic in that time interval. The first row of the view contains statistics for the (partial) current time period. The view contains a total of 1008 rows, spanning a 7 day cycle.
| Column | Datatype | Description |
| BEGIN_TIME | DATE | Identifies the beginning of the time interval |
| END_TIME | DATE | Identifies the end of the time interval |
| UNDOTSN | NUMBER | Represents the last active undo tablespace in the duration of time. The tablespace ID of the active undo tablespace is returned in this column. If more than one undo tablespace was active in that period, the active undo tablespace that was active at the end of the period is reported. |
| UNDOBLKS | NUMBER | Represents the total number of undo blocks consumed. You can use this column to obtain the consumption rate of undo blocks, and thereby estimate the size of the undo tablespace needed to handle the workload on your system. |
| TXNCOUNT | NUMBER | Identifies the total number of transactions executed within the period |
| MAXQUERYLEN | NUMBER | Identifies the length of the longest query (in number of seconds) executed in the instance during the period. You can use this statistic to estimate the proper setting of the UNDO_RETENTION initialization parameter. |
| MAXCONCURRENCY | NUMBER | Identifies the highest number of transactions executed concurrently within the period |
| UNXPSTEALCNT | NUMBER | Number of attempts to obtain undo space by stealing unexpired extents from other transactions |
| UNXPBLKRELCNT | NUMBER | Number of unexpired blocks removed from certain undo segments so they can be used by other transactions |
| UNXPBLKREUCNT | NUMBER | Number of unexpired undo blocks reused by transactions |
| EXPSTEALCNT | NUMBER | Number of attempts to steal expired undo blocks from other undo segments |
| EXPBLKRELCNT | NUMBER | Number of expired undo blocks stolen from other undo segments |
| EXPBLKREUCNT | NUMBER | Number of expired undo blocks reused within the same undo segments |
| SSOLDERRCNT | NUMBER | Identifies the number of times the error ORA-01555 occurred. You can use this statistic to decide whether or not the UNDO_RETENTION initialization parameter is set properly given the size of the undo tablespace. Increasing the value of UNDO_RETENTION can reduce the occurrence of this error. |
| NOSPACEERRCNT | NUMBER | Identifies the number of times space was requested in the undo tablespace and there was no free space available. That is, all of the space in the undo tablespace was in use by active transactions. The corrective action is to add more space to the undo tablespace. |
?
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