原文地址：http://blog.51cto.com/tiany/1432894
Undo Segment深入解析
在undo自动管理时，设置了undo_retention以后，undo块就存在四种状态。
Active：表示正在使用该undo的事务还没有提交或回滚。
Inactive：表示该undo上没有活动的事务，该状态的undo可以被其他事务覆盖。
Expired：表示该undo持续inactive的时间超过undo_retention所指定的时间。
Freed：表示该undo块内容是空的，从来没有被使用过。
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Undo Retention
After a transaction is committed, undo data is no longer needed
for
rollback
or
transaction recovery purposes. However,
for
consistent read purposes, long-running queries may require
this
old undo information
for
producing older images of data blocks. Furthermore, the success of several Oracle Flashback features can also depend upon the availability of older undo information. For these reasons, it is desirable to retain the old undo information
for
as
long
as
possible.
When automatic undo management is enabled, there is always a current undo retention period, which is the minimum amount of time that Oracle Database attempts to retain old undo information
before
overwriting it. Old (committed) undo information that is older than the current undo retention period is said to be expired. Old undo information
with
an age that is less than the current undo retention period is said to be unexpired.
Oracle Database automatically tunes the undo retention period based
on
undo tablespace size
and
system activity. You can specify a minimum undo retention period (
in
seconds) by setting the UNDO_RETENTION initialization parameter. The database makes its best effort to honor the specified minimum undo retention period, provided that the undo tablespace has space available
for
new
transactions. When available space
for
new
transactions becomes short, the database begins to overwrite expired undo. If the undo tablespace has no space
for
new
transactions
after
all expired undo is overwritten, the database may begin overwriting unexpired undo information. If any of
this
overwritten undo information is required
for
consistent read
in
a current long-running query, the query could fail
with
the snapshot too old error message.
The following points explain the exact impact of the UNDO_RETENTION parameter
on
undo retention:
The UNDO_RETENTION parameter is ignored
for
a fixed size undo tablespace. The database may overwrite unexpired undo information when tablespace space becomes low.
For an undo tablespace
with
the AUTOEXTEND option enabled, the database attempts to honor the minimum retention period specified by UNDO_RETENTION. When space is low, instead of overwriting unexpired undo information, the tablespace auto-
extends
. If the MAXSIZE clause is specified
for
an auto-extending undo tablespace, when the maximum size is reached, the database may begin to overwrite unexpired undo information.
Automatic Tuning of Undo Retention
Oracle Database automatically tunes the undo retention period based
on
how the undo tablespace is configured.
If the undo tablespace is fixed size, the database tunes the retention period
for
the best possible undo retention
for
that tablespace size
and
the current system load. This tuned retention period can be significantly greater than the specified minimum retention period.
If the undo tablespace is configured
with
the AUTOEXTEND option, the database tunes the undo retention period to be somewhat longer than the longest-running query
on
the system
at
that time. Again,
this
tuned retention period can be greater than the specified minimum retention period.
Note:
Automatic tuning of undo retention is
not
supported
for
LOBs. This is because undo information
for
LOBs is stored
in
the segment itself
and
not
in
the undo tablespace. For LOBs, the database attempts to honor the minimum undo retention period specified by UNDO_RETENTION. However,
if
space becomes low, unexpired LOB undo information may be overwritten.
You can determine the current retention period by querying the TUNED_UNDORETENTION column of the V$UNDOSTAT view. This view contains one row
for
each
10
-minute statistics collection interval over the
last
4
days. (Beyond
4
days, the data is available
in
the DBA_HIST_UNDOSTAT view.) TUNED_UNDORETENTION is given
in
seconds.
select to_char(begin_time,
'DD-MON-RR HH24:MI'
) begin_time,
to_char(end_time,
'DD-MON-RR HH24:MI'
) end_time, tuned_undoretention
from
v$undostat order by end_time;
BEGIN_TIME END_TIME TUNED_UNDORETENTION
--------------- --------------- -------------------
04
-FEB
-05
00
:
01
04
-FEB
-05
00
:
11
12100
...
07
-FEB
-05
23
:
21
07
-FEB
-05
23
:
31
86700
07
-FEB
-05
23
:
31
07
-FEB
-05
23
:
41
86700
07
-FEB
-05
23
:
41
07
-FEB
-05
23
:
51
86700
07
-FEB
-05
23
:
51
07
-FEB
-05
23
:
52
86700
576
rows selected.
|
回滚段自动管理模式下UNDO block分配算法：
1、如果当前extent有空闲的数据块，则使用当前extent
2、如果当前extent的下一个extent已经处于过期（expired）状态，那么环绕（wrap）到一个extent，然后使用该extent的第一个数据块。
3、如果下一个extent没有expired，则从undo tablespace中分配空间。如果有剩余空间则使用新分配extent的第一个数据块。此时undo tablespace的使用率开始增加。
4、如果没有剩余空闲的extent，则从offline状态的回滚段中偷取（steal）过期的extent，加入当前回滚段，并使用第一个数据块。
5、如果offline状态的回滚段中没有expired extent，则从online状态的回滚段中偷取（steal）过期区加入到当前的回滚段中，并使用extent中的第一个数据块。
6、如果undo tablespace可以扩展，则扩展undo tablespace，并将新extent加入到当前的回滚段中，同时使用第一个数据块，此时undo所占的操作系统空间开始增加。
7、如果undo tablespace 不能扩展，则自动调整（下降幅度为10%）回滚段的保留时间，然后偷取在更短保留时间下的未过期的extent，如果还未找到过期的extent，则继续以10%的速度减少回滚段的保留时间，重复几次。
8、随机从其他offline状态的回滚段中偷取未过期（unexpired）的extent。
9、尝试使用当前回滚段中未过期的extent，如果所有的区都为ACTIVE状态，则进入下一步。
10、如果以上所有的尝试都失败，则报ORA-30036的错误。
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[oracle@rh6 ~]$ oerr ora
30036
30036
,
00000
,
"unable to extend segment by %s in undo tablespace '%s'"
// *Cause: the specified undo tablespace has no more space available.
// *Action: Add more space to the undo tablespace before retrying
// the operation. An alternative is to wait until active
// transactions to commit.
案例：
10
:
34
:
45
SYS@ prod>select tablespace_name,contents
from
dba_tablespaces;
TABLESPACE_NAME CONTENTS
------------------------------ ---------
SYSTEM PERMANENT
SYSAUX PERMANENT
TEMP TEMPORARY
USERS PERMANENT
UNDOTBS2 UNDO
EXAMPLE PERMANENT
TBS1 PERMANENT
7
rows selected.
Elapsed:
00
:
00
:
00.03
10
:
34
:
56
SYS@ prod>create undo tablespace undotbs1
10
:
35
:
15
2
datafile
'/u01/app/oracle/oradata/prod/undotbs1.dbf'
size 1m;
Tablespace created.
Elapsed:
00
:
00
:
01.38
10
:
35
:
40
SYS@ prod>show parameter undo
NAME TYPE VALUE
------------------------------------ ----------- ------------------------------
undo_management string AUTO
undo_retention integer
900
undo_tablespace string UNDOTBS2
10
:
35
:
49
SYS@ prod>alter system set undo_tablespace=undotbs1;
System altered.
Elapsed:
00
:
00
:
00.13
10
:
36
:
03
SYS@ prod>
10
:
36
:
03
SYS@ prod>select s.sid,s.serial#,s.username,u.segment_name,count(u.extent_id)
"Extent count"
,
10
:
37
:
37
2
t.used_ublk,t.used_urec,s.program
10
:
38
:
00
3
from
v$session s,v$transaction t,dba_undo_extents u
10
:
38
:
24
4
where
s.taddr=t.addr
and
u.segment_name like
'_SYSSMU'
||t.xidusn||
'_%$'
and
u.status=
'ACTIVE'
10
:
39
:
50
5
GROUP BY s.sid,s.serial#,s.username,u.segment_name,t.used_ublk,t.used_urec,s.program
10
:
40
:
37
6
order by t.used_ublk desc,t.used_urec desc,s.sid,s.serial#,s.username,s.program;
开启新的session，执行DML:
10
:
41
:
45
SYS@ prod>conn scott/tiger
Connected.
10
:
42
:
45
SCOTT@ prod>
insert
into
t1 select *
from
t1
where
rownum <
1000
;
999
rows created.
查看undo tablespace 使用情况：
10
:
43
:
29
SYS@ prod> select s.sid,s.serial#,s.username,u.segment_name,count(u.extent_id)
"Extent count"
,
2
t.used_ublk,t.used_urec,s.program
3
from
v$session s,v$transaction t,dba_undo_extents u
4
where
s.taddr=t.addr
and
u.segment_name like
'_SYSSMU'
||t.xidusn||
'_%$'
and
u.status=
'ACTIVE'
5
GROUP BY s.sid,s.serial#,s.username,u.segment_name,t.used_ublk,t.used_urec,s.program
6
* order by t.used_ublk desc,t.used_urec desc,s.sid,s.serial#,s.username,s.program
SID SERIAL# USERNAME SEGMENT_NAME Extent count USED_UBLK USED_UREC PROGRAM
---------- ---------- ---------- -------------------- ------------ ---------- ---------- ----------
37
36
SCOTT _SYSSMU12_2144756092
1
1
11
sqlplus@rh
$
6
(TNS V1-
V3)
Elapsed:
00
:
00
:
00.20
10
:
45
:
01
SCOTT@ prod>
insert
into
t1 select *
from
t1
where
rownum <
20000
;
19999
rows created.
Elapsed:
00
:
00
:
00.07
10
:
44
:
52
SYS@ prod>select s.sid,s.serial#,s.username,u.segment_name,count(u.extent_id)
"Extent count"
,
2
t.used_ublk,t.used_urec,s.program
3
from
v$session s,v$transaction t,dba_undo_extents u
4
where
s.taddr=t.addr
and
u.segment_name like
'_SYSSMU'
||t.xidusn||
'_%$'
and
u.status=
'ACTIVE'
5
GROUP BY s.sid,s.serial#,s.username,u.segment_name,t.used_ublk,t.used_urec,s.program
6
* order by t.used_ublk desc,t.used_urec desc,s.sid,s.serial#,s.username,s.program
SID SERIAL# USERNAME SEGMENT_NAME Extent count USED_UBLK USED_UREC PROGRAM
---------- ---------- ---------- -------------------- ------------ ---------- ---------- ----------
37
36
SCOTT _SYSSMU16_2726800344
2
7
109
sqlplus@rh
$
6
(TNS V1-
V3)
Elapsed:
00
:
00
:
00.01
10
:
45
:
13
SYS@ prod>
10
:
48
:
16
SYS@ prod>select a.usn,a.name,b.xacts,b.extents,b.status
from
v$rollname a,v$rollstat b
10
:
49
:
01
2
where
a.usn=b.usn;
USN NAME XACTS EXTENTS STATUS
---------- ------------------------------ ---------- ---------- ---------------
0
SYSTEM
0
6
ONLINE
11
_SYSSMU11_2517864848$
0
2
ONLINE
12
_SYSSMU12_2144756092$
0
2
ONLINE
13
_SYSSMU13_527038519$
0
3
ONLINE
14
_SYSSMU14_2951869305$
0
2
ONLINE
15
_SYSSMU15_2206823906$
0
2
ONLINE
16
_SYSSMU16_2726800344$
1
2
ONLINE
17
_SYSSMU17_2098084560$
0
2
ONLINE
10
:
50
:
45
SYS@ prod>select SEGMENT_NAME,TABLESPACE_NAME,EXTENT_ID,STATUS
from
dba_undo_extents
SEGMENT_NAME TABLESPACE_NAME EXTENT_ID STATUS
------------------------- ------------------------------ ---------- ---------
_SYSSMU17_2098084560$ UNDOTBS1
0
UNEXPIRED
_SYSSMU17_2098084560$ UNDOTBS1
1
UNEXPIRED
_SYSSMU16_2726800344$ UNDOTBS1
0
ACTIVE
_SYSSMU16_2726800344$ UNDOTBS1
1
ACTIVE
_SYSSMU15_2206823906$ UNDOTBS1
0
UNEXPIRED
_SYSSMU15_2206823906$ UNDOTBS1
1
UNEXPIRED
_SYSSMU14_2951869305$ UNDOTBS1
0
UNEXPIRED
_SYSSMU14_2951869305$ UNDOTBS1
1
UNEXPIRED
_SYSSMU13_527038519$ UNDOTBS1
0
UNEXPIRED
_SYSSMU13_527038519$ UNDOTBS1
1
UNEXPIRED
_SYSSMU13_527038519$ UNDOTBS1
2
UNEXPIRED
_SYSSMU12_2144756092$ UNDOTBS1
0
UNEXPIRED
_SYSSMU12_2144756092$ UNDOTBS1
1
UNEXPIRED
_SYSSMU11_2517864848$ UNDOTBS1
0
UNEXPIRED
_SYSSMU11_2517864848$ UNDOTBS1
1
UNEXPIRED
_SYSSMU30_1737877121$ UNDOTBS2
0
EXPIRED
_SYSSMU30_1737877121$ UNDOTBS2
1
UNEXPIRED
SEGMENT_NAME TABLESPACE_NAME EXTENT_ID STATUS
------------------------- ------------------------------ ---------- ---------
_SYSSMU30_1737877121$ UNDOTBS2
2
EXPIRED
_SYSSMU29_2754652023$ UNDOTBS2
0
EXPIRED
_SYSSMU29_2754652023$ UNDOTBS2
1
EXPIRED
_SYSSMU29_2754652023$ UNDOTBS2
2
EXPIRED
_SYSSMU29_2754652023$ UNDOTBS2
3
UNEXPIRED
_SYSSMU28_707429450$ UNDOTBS2
0
UNEXPIRED
_SYSSMU28_707429450$ UNDOTBS2
1
EXPIRED
_SYSSMU28_707429450$ UNDOTBS2
2
EXPIRED
_SYSSMU27_3269963619$ UNDOTBS2
0
EXPIRED
_SYSSMU27_3269963619$ UNDOTBS2
1
EXPIRED
_SYSSMU27_3269963619$ UNDOTBS2
2
EXPIRED
_SYSSMU27_3269963619$ UNDOTBS2
3
UNEXPIRED
_SYSSMU27_3269963619$ UNDOTBS2
4
EXPIRED
_SYSSMU27_3269963619$ UNDOTBS2
5
EXPIRED
_SYSSMU26_2968904537$ UNDOTBS2
0
EXPIRED
_SYSSMU26_2968904537$ UNDOTBS2
1
EXPIRED
_SYSSMU26_2968904537$ UNDOTBS2
2
EXPIRED
SEGMENT_NAME TABLESPACE_NAME EXTENT_ID STATUS
------------------------- ------------------------------ ---------- ---------
_SYSSMU26_2968904537$ UNDOTBS2
3
EXPIRED
_SYSSMU26_2968904537$ UNDOTBS2
4
UNEXPIRED
_SYSSMU26_2968904537$ UNDOTBS2
5
EXPIRED
_SYSSMU25_2810228709$ UNDOTBS2
0
EXPIRED
_SYSSMU25_2810228709$ UNDOTBS2
1
EXPIRED
_SYSSMU25_2810228709$ UNDOTBS2
2
EXPIRED
_SYSSMU25_2810228709$ UNDOTBS2
3
EXPIRED
_SYSSMU25_2810228709$ UNDOTBS2
4
EXPIRED
_SYSSMU25_2810228709$ UNDOTBS2
5
EXPIRED
_SYSSMU25_2810228709$ UNDOTBS2
6
EXPIRED
_SYSSMU25_2810228709$ UNDOTBS2
7
EXPIRED
_SYSSMU25_2810228709$ UNDOTBS2
8
EXPIRED
_SYSSMU25_2810228709$ UNDOTBS2
9
EXPIRED
_SYSSMU25_2810228709$ UNDOTBS2
10
EXPIRED
_SYSSMU25_2810228709$ UNDOTBS2
11
EXPIRED
_SYSSMU25_2810228709$ UNDOTBS2
12
EXPIRED
_SYSSMU25_2810228709$ UNDOTBS2
13
EXPIRED
SEGMENT_NAME TABLESPACE_NAME EXTENT_ID STATUS
------------------------- ------------------------------ ---------- ---------
_SYSSMU25_2810228709$ UNDOTBS2
14
EXPIRED
_SYSSMU25_2810228709$ UNDOTBS2
15
EXPIRED
_SYSSMU25_2810228709$ UNDOTBS2
16
EXPIRED
_SYSSMU25_2810228709$ UNDOTBS2
17
EXPIRED
_SYSSMU25_2810228709$ UNDOTBS2
18
EXPIRED
_SYSSMU25_2810228709$ UNDOTBS2
19
EXPIRED
_SYSSMU25_2810228709$ UNDOTBS2
20
EXPIRED
_SYSSMU25_2810228709$ UNDOTBS2
21
EXPIRED
_SYSSMU25_2810228709$ UNDOTBS2
22
EXPIRED
_SYSSMU25_2810228709$ UNDOTBS2
23
EXPIRED
_SYSSMU25_2810228709$ UNDOTBS2
24
EXPIRED
_SYSSMU25_2810228709$ UNDOTBS2
25
EXPIRED
_SYSSMU25_2810228709$ UNDOTBS2
26
EXPIRED
_SYSSMU25_2810228709$ UNDOTBS2
27
EXPIRED
_SYSSMU25_2810228709$ UNDOTBS2
28
EXPIRED
_SYSSMU25_2810228709$ UNDOTBS2
29
EXPIRED
_SYSSMU25_2810228709$ UNDOTBS2
30
EXPIRED
SEGMENT_NAME TABLESPACE_NAME EXTENT_ID STATUS
------------------------- ------------------------------ ---------- ---------
_SYSSMU25_2810228709$ UNDOTBS2
31
EXPIRED
_SYSSMU25_2810228709$ UNDOTBS2
32
EXPIRED
_SYSSMU25_2810228709$ UNDOTBS2
33
EXPIRED
_SYSSMU25_2810228709$ UNDOTBS2
34
EXPIRED
_SYSSMU25_2810228709$ UNDOTBS2
35
UNEXPIRED
_SYSSMU24_386518199$ UNDOTBS2
0
EXPIRED
_SYSSMU24_386518199$ UNDOTBS2
1
UNEXPIRED
_SYSSMU24_386518199$ UNDOTBS2
2
EXPIRED
_SYSSMU23_4084707454$ UNDOTBS2
0
EXPIRED
_SYSSMU23_4084707454$ UNDOTBS2
1
UNEXPIRED
_SYSSMU23_4084707454$ UNDOTBS2
2
EXPIRED
_SYSSMU22_3375463809$ UNDOTBS2
0
UNEXPIRED
_SYSSMU22_3375463809$ UNDOTBS2
1
UNEXPIRED
_SYSSMU22_3375463809$ UNDOTBS2
2
EXPIRED
_SYSSMU22_3375463809$ UNDOTBS2
3
EXPIRED
_SYSSMU21_2312338076$ UNDOTBS2
0
EXPIRED
_SYSSMU21_2312338076$ UNDOTBS2
1
EXPIRED
SEGMENT_NAME TABLESPACE_NAME EXTENT_ID STATUS
------------------------- ------------------------------ ---------- ---------
_SYSSMU21_2312338076$ UNDOTBS2
2
UNEXPIRED
86
rows selected.
Elapsed:
00
:
00
:
00.06
10
:
52
:
33
SYS@ prod>alter session set nls_date_format=
'yyyy-mm-dd hh24:mi:ss'
;
Session altered.
Elapsed:
00
:
00
:
00.02
10
:
53
:
03
SYS@ prod>select BEGIN_TIME,END_TIME,UNDOTSN,UNDOBLKS,TUNED_UNDORETENTION
from
v$undostat;
BEGIN_TIME END_TIME UNDOTSN UNDOBLKS TUNED_UNDORETENTION
------------------- ------------------- ---------- ---------- -------------------
2014
-07
-01
10
:
52
:
51
2014
-07
-01
10
:
53
:
08
2
0
921
2014
-07
-01
10
:
42
:
51
2014
-07
-01
10
:
52
:
51
2
74
921
2014
-07
-01
10
:
32
:
51
2014
-07
-01
10
:
42
:
51
2
43
1260
估算undo tablespace 的大小：
UndoTablesapce = UR * (UPS * DBS)
UR: undo_retention
UPS:在业务高峰期每秒产生的undo blocks的数量
DBS:undo tablespace的数据库的大小
10
:
53
:
08
SYS@ prod>select (UR * (UPS * DBS)) AS
"BYTES"
10
:
56
:
12
2
from
(select value AS UR
from
v$parameter
where
name=
'undo_retention'
),
10
:
57
:
04
3
(select undoblks/((end_time-begin_time)*
900
)
as
UPS
10
:
58
:
39
4
FROM v$undostat
10
:
58
:
50
5
where
undoblks=(select max(undoblks)
from
v$undostat)),
10
:
59
:
29
6
(select block_size
as
DBS
10
:
59
:
51
7
FROM dba_tablespaces
11
:
00
:
01
8
where
tablespace_name=(select upper(value)
from
v$parameter
where
name=
'undo_tablespace'
));
BYTES
----------
87293952
Elapsed:
00
:
00
:
00.07
11
:
00
:
45
SYS@ prod>
11
:
00
:
45
SYS@ prod>select (UR * (UPS * DBS))/
1024
/
1024
AS
"BYTES"
11
:
01
:
42
2
from
(select value AS UR
from
v$parameter
where
name=
'undo_retention'
),
11
:
01
:
42
3
(select undoblks/((end_time-begin_time)*
900
)
as
UPS
11
:
01
:
42
4
FROM v$undostat
11
:
01
:
42
5
where
undoblks=(select max(undoblks)
from
v$undostat)),
11
:
01
:
42
6
(select block_size
as
DBS
11
:
01
:
42
7
FROM dba_tablespaces
11
:
01
:
42
8
where
tablespace_name=(select upper(value)
from
v$parameter
where
name=
'undo_tablespace'
));
BYTES
----------
83.25
Elapsed:
00
:
00
:
00.10
11
:
01
:
44
SYS@ prod>
|
以上部分内容参考了《Oracle DBA实战攻略》书中的内容，这里表示感谢！