DB2 锁问题分析与解释

Posted 2020-09-30 mthoutai

DB2 锁问题分析与解释


DB2 应用中常常会遇到锁超时与死锁现象，那么这样的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
採用默认的隔离级别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操作与其它操作的锁等待问题


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及其它行的update,delete操作都不会有问题


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


试验2：验证update操作与其它操作的锁等待问题
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能够对该表作insert操作，但不同意对其它行的delete和update操作
----------------------------------------------------------------------------

试验3：验证delete操作与其它操作的锁等待问题
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能够对该表作insert操作。但不同意对其它行的delete和update操作
----------------------------------------------------------------------------


总的结论是：
应用对表作insert操作时。其它操作不受影响,也不受其它操作影响。

作update，delete操作时，其它的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须要什么样的锁。


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操作须要表级的IX锁和行级的X锁。

注：IX锁。该锁的拥有者在拥有对应行的X锁时能够更改该行的数据。


$ 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操作须要表级的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操作须要表级的IX锁和相应的行级的X锁（这里由于3条记录的age都为6,因此须要3个行级锁）。


如今的问题是：为什么insert和update,delete操作须要的锁一样（表级的IX锁。相应行级的X锁），可是表现的效果却不一样呢？


为了解决问题。看一下他们的运行计划吧：


$ 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




从上面的运行计划中能够看到原因：insert操作不须要表扫描，而update和delete操作都须要全表扫描，并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
比如session 1要更新表的某一行，会在该行加上X锁。之后, session 2试图更新该表的还有一行，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，终于导

致锁超时。

为了验证该说法，能够抓取锁等待的消息。


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。由于它在等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     


能够看到，是由于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，能够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.


能够看到，已经通过索引攻克了该锁超时问题。假设读者有兴趣的话，能够看下建立索引之后的訪问计划。






以下模拟一个死锁现象
试验5:模拟死锁，步骤例如以下
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，先将锁超时控制參数设为-1（表示永远等待）
update db cfg using locktimeout -1
之后重新启动数据库


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。由于LOCK1被session 1持有


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




这时已经发生了死锁，10s之后，这两个session有一个会报出例如以下死锁（reason code 2）错误。还有一个session成功运行
SQL0911N  The current transaction has been rolled back because of a deadlock 
or timeout.  Reason code "2".  SQLSTATE=40001


參考资料：
标准表的锁定方式和存取方案，这里您能够看到具体的加锁方式
http://www-01.ibm.com/support/knowledgecenter/SSEPGG_9.7.0/com.ibm.db2.luw.admin.perf.doc/doc/r0005275.html?lang=zh


附。仅仅能在发生死锁或者锁等待的时候才干用db2pd查看锁的信息。以下附上怎样採用事件监控器监控死锁/锁超时。事件监控器能够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 sample -ue LOCKEVMON -ftext -u e97q6c -p e97q6c > deadlock.txt
more deadlock.txt 能够看到有关的SQL语句。

请參考

http://www.ibm.com/developerworks/cn/data/library/techarticles/dm-1004lockeventmonitor/

http://blog.csdn.net/qingsong3333/article/details/51206329