DB2锁问题分析与解释

来源:IT165收集  发布日期:2014-10-16 19:49:59
DB2 锁问题分析与解释


DB2 应用中经常会遇到锁超时与死锁现象,那么这种现象产生的原因是什么呢。本文以试验的形式模拟锁等待、锁超时、死锁现象,并给出这些现象的根本原因。


试验环境:


DB2 v9.7.0.6
AIX 6.1.0.0
采用默认的隔离级别CS

STUDENT表的DDL与初始内容
------------------------------------------------
-- DDL Statements for table "E97Q6C "."STUDENT"
------------------------------------------------

CREATE TABLE "E97Q6C "."STUDENT" (
"AGE" INTEGER ,
"NAME" CHAR(8) )
IN "USERSPACE1" ;


$ db2 "select * from student"


AGE NAME
----------- --------
3 xu
5 gao
2 liu
1 gu







试验1:验证insert操作与其他操作的锁等待问题


session 1中发出insert操作,在session 2中观察insert,update,delete操作是否会锁超时。


session 1
---------
$ db2 +c "insert into student values(4, 'miao')"
DB20000I The SQL command completed successfully.


session 2
---------
$ db2 "insert into student values(6, 'mu')"
DB20000I The SQL command completed successfully.
$ db2 "update student set name='gu' where age=1"
DB20000I The SQL command completed successfully.
$ db2 "delete from student where age=2"
DB20000I The SQL command completed successfully.

----------------------------------------------------------------------------

结论1:当session 1对表作insert操作时,session 2对该表的insert及其他行的update,delete操作都不会有问题


----------------------------------------------------------------------------


试验2:验证update操作与其他操作的锁等待问题
session 1中发出update操作,在session 2中观察insert,update,delete操作是否会锁超时。
--------------
session 1
---------
$ db2 commit


$ db2 "select * from student"


AGE NAME
----------- --------
3 xu
5 gao
6 mu
4 miao
1 gu


5 record(s) selected.


$ db2 +c "update student set name = 'qing' where age=4"
DB20000I The SQL command completed successfully.


session 2
---------
$ db2 "insert into student values(6, 'mu')"
DB20000I The SQL command completed successfully.
$ db2 "update student set name='gu' where age=1"
DB21034E The command was processed as an SQL statement because it was not a
valid Command Line Processor command. During SQL processing it returned:
SQL0911N The current transaction has been rolled back because of a deadlock
or timeout. Reason code "68". SQLSTATE=40001
$ db2 "delete from student where age=2"
DB21034E The command was processed as an SQL statement because it was not a
valid Command Line Processor command. During SQL processing it returned:
SQL0911N The current transaction has been rolled back because of a deadlock
or timeout. Reason code "68". SQLSTATE=40001


----------------------------------------------------------------------------
结论2:当session 1对表某一行做update操作时,session 2可以对该表作insert操作,但不允许对其他行的delete和update操作
----------------------------------------------------------------------------

试验3:验证delete操作与其他操作的锁等待问题
session 1中发出delete操作,在session 2中观察insert,update,delete操作是否会锁超时。

Session 1
---------
$ db2 commit


$ db2 +c "delete from student where age=4"
DB20000I The SQL command completed successfully.


session 2
---------
$ db2 "insert into student values(6, 'mu')"
DB20000I The SQL command completed successfully.


$ db2 "update student set name='gu' where age=1"
DB21034E The command was processed as an SQL statement because it was not a
valid Command Line Processor command. During SQL processing it returned:
SQL0911N The current transaction has been rolled back because of a deadlock
or timeout. Reason code "68". SQLSTATE=40001


$ db2 "delete from student where age=2"
DB21034E The command was processed as an SQL statement because it was not a
valid Command Line Processor command. During SQL processing it returned:
SQL0911N The current transaction has been rolled back because of a deadlock
or timeout. Reason code "68". SQLSTATE=40001
----------------------------------------------------------------------------
结论3:当应用1对表某一行做delete操作时,应用2可以对该表作insert操作,但不允许对其他行的delete和update操作
----------------------------------------------------------------------------


总的结论是:
应用对表作insert操作时,其他操作不受影响,也不受其他操作影响。
作update,delete操作时,其他的update和delete操作受影响。


为了解释以上现象的原因,我们首先看一下上面的操作需要什么样的锁。


session 1.
---------
$ db2 rollback


$ db2 +c "insert into student values(7,'han')"
DB20000I The SQL command completed successfully.


$ db2pd -db qsmiao -locks


结论:insert操作需要表级的IX锁和行级的X锁。
注:IX锁,该锁的拥有者在拥有相应行的X锁时可以更改该行的数据。


$ db2 rollback


$ db2 +c "update student set name='yan' where age=5"
DB20000I The SQL command completed successfully.


$ db2pd -db qsmiao -locks




结论:update操作需要表级的IX锁和行级的X锁。


$ db2 rollback


$ db2 +c "delete from student where age=6"

DB20000I The SQL command completed successfully.

$ db2pd -db qsmiao -locks



结论:update操作需要表级的IX锁和对应的行级的X锁(这里因为3条记录的age都为6,因此需要3个行级锁)。


现在的问题是:为什么insert和update,delete操作需要的锁一样(表级的IX锁,对应行级的X锁),但是表现的效果却不一样呢?


为了解决这个问题,看一下他们的执行计划吧:


$ db2expln -d qsmiao -g -statement "insert into student values(5, 'gao')" -terminal




$ db2expln -d qsmiao -g -statement "update student set name='qing' where age=4" -terminal





$ db2expln -d qsmiao -g -statement "delete from student where age=6" -terminal




从上面的执行计划中可以看到原因:insert操作不需要表扫描,而update和delete操作都需要全表扫描,而且会在扫描的时候试图对每一行加U锁。
导致锁超时的原因就是表扫描
例如session 1要更新表的某一行,会在该行加上X锁。之后, session 2试图更新该表的另一行,进行全表扫描时,就会试图对A占用的那一行加上U锁,但无能为力,最终导

致锁超时。

为了验证该说法,可以抓取锁等待的消息,


session 1
---------
$ db2 +c "update student set name='hehe' where age = 4"
DB20000I The SQL command completed successfully.


session 2
---------
$ db2 +c "delete from student where age=6"
<-------这时会hang住,因为它在等session 1的锁


session 3
---------
$ db2pd -db qsmiao -wlocks <---在锁超时发生之前,抓取锁等待的消息

Locks being waited on :
AppHandl [nod-index] TranHdl Lockname Type Mode Conv Sts CoorEDU AppName AuthID AppID
15393 [000-15393] 2 00020004000000000000000952 Row ..X G 7818 db2bp E97Q6C *LOCAL.e97q6c.141016035113
15408 [000-15408] 16 00020004000000000000000952 Row ..U W 10153 db2bp E97Q6C *LOCAL.e97q6c.141016035219


可以看到,是因为U锁和X锁的不兼容导致锁等待,最后导致锁超时。



为了解决该锁等待问题,可以在查询谓词所涉及的列age上建立索引,避免全表扫描


试验4:通过建立索引,消除锁等待现象


session 1
---------
$ db2 rollback


$ db2 +c "lock table student in share mode"


$ db2 +c "create index stu_idx on student(age)"


$ db2 commit


$ db2 +c "update student set name='hehe' where age = 4"
DB20000I The SQL command completed successfully.


session 2
---------
$ db2 +c "delete from student where age=6" <--没有发生锁等待现象,直接成功
DB20000I The SQL command completed successfully.


可以看到,已经通过索引解决了该锁超时问题,如果读者有兴趣的话,可以看下建立索引之后的访问计划。






下面模拟一个死锁现象
试验5:模拟死锁,过程如下
第一步:session 1 获得 锁 LOCK1


第二步:session 2 获得 锁 LOCK2


第三步:session 2 申请 锁 LOCK1


第四步:session 1 申请 锁 LOCK2


为了避免死锁之前产生锁超时,先将锁超时控制参数设为-1(表示永远等待)
update db cfg using locktimeout -1
之后重启数据库


session 1
---------
$ db2 +c "update student set name = 'an' where age = 1" <--获得锁LOCK1,成功
DB20000I The SQL command completed successfully.


session 2
---------
$ db2 +c "update student set name = 'two' where age = 4" <--获得锁LOCK2,成功
DB20000I The SQL command completed successfully.


$ db2 +c "update student set name = 'four' where age = 1" <--申请锁LOCK1,hang住,因为LOCK1被session 1持有


session 1
---------
$ db2 +c "update student set name = 'three' where age = 4" <--申请锁LOCK2,hang住,因为LOCK2被session 2持有




这时已经发生了死锁,10s之后,这两个session有一个会报出如下死锁(reason code 2)错误,另一个session成功执行
SQL0911N The current transaction has been rolled back because of a deadlock
or timeout. Reason code "2". SQLSTATE=40001


参考资料:
标准表的锁定方式和存取方案,这里您可以看到详细的加锁方式
http://www-01.ibm.com/support/knowledgecenter/SSEPGG_9.7.0/com.ibm.db2.luw.admin.perf.doc/doc/r0005275.html?lang=zh


附,只能在发生死锁或者锁等待的时候才能用db2pd查看锁的信息。下面附上如何采用事件监控器监控死锁/锁超时。事件监控器可以抓取一段时间内的锁事件
db2 update db cfg for sample using MON_LOCKWAIT hist_and_values MON_DEADLOCK hist_and_values MON_LOCKTIMEOUT hist_and_values MON_LW_THRESH 10000
db2 "CREATE EVENT MONITOR LOCKEVMON FOR LOCKING WRITE TO UNFORMATTED EVENT TABLE (TABLE LOCKEVMON)"
db2 set event monitor LOCKEVMON state=1


重现问题


db2 flush event monitor LOCKEVMON
db2 set event monitor LOCKEVMON state=0


cp /home/db2users/e97q6c/sqllib/samples/java/jdbc/db2evmonfmt.java ./
cp /home/db2users/e97q6c/sqllib/samples/java/jdbc/DB2EvmonLocking.xsl ./
export PATH=/home/db2users/e97q6c/sqllib/java/jdk64/bin:$PATH


javac db2evmonfmt.java


java db2evmonfmt -d qsmiao -ue LOCKEVMON -ftext -u e97q6c -p e97q6c > deadlock.txt
more deadlock.txt 可以看到有关的SQL语句。


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