Review of lock timeout analysis in DB2 9.1
The lock timeout analysis method using the
db2pd tool and
db2cos script consists of the following steps:
- A special DB2 script — named
db2cos— is adapted to execute a certain
db2pdcall each time the
db2cosscript is invoked. The
db2pdcall gathers information about locks, transactions, applications, and the statement cache and stores this information in a text file for analysis purposes.
- To execute the
db2cosscript — and therefore the contained
db2pdcommand — automatically when a lock timeout occurs, lock timeout events are registered using the
- In the case of a lock timeout event, a DBA can examine the corresponding
db2pdoutput generated by the automatic invocation of the
db2cosscript. This allows the DBA to determine the cause of the lock contention so he or she can take measures to avoid such situations in the future.
For a detailed description of those steps accompanied by a sample scenario, see the already mentioned article found in the Resources section.
Although the described method provides a lot of information that makes lock timeout analysis much simpler than in pre-Version 9 releases of DB2 for Linux, Unix, and Windows, it still has some shortcomings:
- Using this method requires manual adaptation of the
db2cosscript and registration of lock timeout events by calling
db2pdcfg. Both steps are not complex but may be obstacles for novices.
- Interpreting the
db2pdoutput to identify the applications and SQL statements involved in a lock timeout situation is not trivial and requires some exercise.
- In cases where lock timeouts are caused by transactions consisting of more than a single SQL command, the information collected by
db2pdmay not be enough to determine the SQL statement responsible for the problematic lock.
New lock timeout reporting capabilities in DB2 9.5
With its lock timeout reporting capabilities, DB2 9.5 introduces a new feature that
makes lock timeout analysis child's play. To activate lock timeout reporting, just set the DB2 registry variable
ON, and restart your DB2 instance:
Listing 1. Activate lock timeout reporting in DB2 9.5
db2set DB2_CAPTURE_LOCKTIMEOUT=ON db2stop db2start
Yes, that's all. When
DB2_CAPTURE_LOCKTIMEOUT is set to
ON, DB2 automatically creates a report file for each lock
timeout occurrence. The report file is written to the directory where the
DIAGPATH database manager configuration (DBM CFG) parameter points,
to and contains information about, the date and time of the lock timeout, the
problematic lock, the lock requester, and the lock owner.
So is the
db2cos script obsolete in DB2 9.5? No, that's not the case. Usage of the
db2cos script in combination with the
db2pd tool has a broader range of use. That means the combination of those tools can still be used to capture information about any kind of DB2 event associated with a SQL code or a DB2 internal return code, not just lock timeouts.
Now concentrate on the new DB2 9.5 registry variable
DB2_CAPTURE_LOCKTIMEOUT and walk through a sample lock timeout scenario using the DB2 SAMPLE database. If the SAMPLE database does not already exist, create it by calling the following command:
Listing 2. Creating the SAMPLE database
Lock timeouts can only occur when the database configuration (DB CFG) parameter
LOCKTIMEOUT is set to a value other than -1. A value of -1
means that an application waits indefinitely for a required lock. In most cases, that
is not the desired behavior, although -1 is the default for
LOCKTIMEOUT. For this sample scenario, assume that
LOCKTIMEOUT is changed to 10 (seconds — the value for
LOCKTIMEOUT is given in seconds):
Listing 3. Changing the value for
db2 "UPDATE DB CFG FOR SAMPLE USING LOCKTIMEOUT 10"
To force a lock timeout error, establish a first database connection and execute some SQL statements simulating a real transaction on the table EMPLOYEE:
Listing 4. First transaction giving each employee a two percent raise
db2 "CONNECT TO SAMPLE" db2 +c "UPDATE EMPLOYEE SET SALARY = SALARY * 1.02" db2 +c "SELECT LASTNAME, FIRSTNME, SALARY FROM EMPLOYEE ORDER BY LASTNAME ASC"
The transaction so far consists of an
giving each employee a two percent raise. Afterward, the new salary is queried with a
SELECT statement. Note that auto-commit is disabled by
specifying the +c option for the DB2 command line processor (CLP) calls. The
UPDATE statement results in an exclusive (X) lock on each row in the
EMPLOYEE table. Those X-locks persist until the transaction is ended by a
COMMIT or a
Without ending the transaction, establish a second database connection in a separate shell and start another transaction:
Listing 5. Second transaction giving each manager a 10 percent bonus (depending on the salary)
db2 "CONNECT TO SAMPLE" db2 +c "UPDATE EMPLOYEE SET BONUS = SALARY * 0.1 WHERE JOB = 'MANAGER'"
The intention of this transaction is to give each manager a 10 percent bonus
depending on his or her current salary. As all rows in the EMPLOYEE table are X-locked by the first transaction, the second application goes into lock wait mode. After 10 seconds (remember the
LOCKTIMEOUT setting) the expected lock timeout occurs.
So far nothing new. But as the DB2 registry variable
DB2_CAPTURE_LOCKTIMEOUT is set to
timeout reporting is active and DB2 has automatically generated a lock timeout report
DIAGPATH directory. Note that the default
DIAGPATH changed in DB2 9.5 for Windows. If the
DIAGPATH parameter is not set, the
DIAGPATH points to the directory
DB2INSTPROF is the location of the instance directory and
DB2INSTANCE is the name of the DB2 instance. To determine the path stored in
DB2INSTPROF, you can display the DB2 registry contents by executing
db2set -all. The value of
DB2INSTANCE is DB2 if you created the SAMPLE database in the default instance. The name of the report file is
dbpartition is always 0 for a single-partition database.
The lock timeout report generated by DB2 looks similar to the following:
Listing 6. Lock timeout report
LOCK TIMEOUT REPORT Date: 03/01/2008 Time: 07:34:31 Instance: DB2 Database: SAMPLE Database Partition: 0 Lock Information: Lock Name: 02000600040040010000000052 Lock Type: Row Lock Specifics: Tablespace ID=2, Table ID=6, Row ID=x0400400100000000 Lock Requestor: System Auth ID: FECHNER Application Handle: [0-38] Application ID: *LOCAL.DB2.080103063343 Application Name: db2bp.exe Requesting Agent ID: 5232 Coordinator Agent ID: 5232 Coordinator Partition: 0 Lock timeout Value: 10000 milliseconds Lock mode requested: ..U Application Status: (SQLM_UOWEXEC) Current Operation: (SQLM_EXECUTE_IMMEDIATE) Lock Escalation: No Context of Lock Request: Identification: UOW ID (1); Activity ID (1) Activity Information: Package Schema: (NULLID ) Package Name: (SQLC2G13NULLID ) Package Version: () Section Entry Number: 203 SQL Type: Dynamic Statement Type: DML, Insert/Update/Delete Effective Isolation: Cursor Stability Statement Unicode Flag: No Statement: UPDATE EMPLOYEE SET BONUS = SALARY * 0.1 WHERE JOB = 'MANAGER' Lock Owner (Representative): System Auth ID: FECHNER Application Handle: [0-33] Application ID: *LOCAL.DB2.080103063332 Application Name: db2bp.exe Requesting Agent ID: 5488 Coordinator Agent ID: 5488 Coordinator Partition: 0 Lock mode held: ..X List of Active SQL Statements: Not available List of Inactive SQL Statements from current UOW: Not available
The lock timeout report consists of four sections:
- The first section provides information about the date and time of the lock timeout occurrence and about the corresponding instance and database.
- The section Lock Information shows the lock that caused the lock timeout. Besides the internal lock name, the type of the lock (row or table lock) and the table information is shown. To determine the tablename, you have to query the catalog view
SYSCAT.TABLESwith the given tablespace ID and table ID:
Listing 7. Mapping a tablespace ID/table ID to a tablename
SELECT TABSCHEMA, TABNAME FROM SYSCAT.TABLES WHERE TBSPACEID = tbspaceid AND TABLEID = tableid
- The application that experienced the lock timeout is described in the section Lock Requestor. Some of the more interesting entries in this sections are the authentication ID used to connect to the database, the application name, the requested lock mode (and whether or not the lock is the result of a lock escalation), the isolation level of the statement that required the lock, and, last but not least, the SQL statement text itself.
- The last section Lock Owner (Representative) lists the application holding the problematic lock. As with the other application, you can see information about the authentication ID, the application name, and the lock mode. In some cases more than one application may hold a (share) lock that blocks the lock requestor. In those cases only one of the lock owners is shown in the lock timeout report. That's why the section has the addition Representative.
At the beginning of this article, three shortcomings were mentioned of the lock
timeout analysis approach using
db2pd. The first one was usability. The
db2pd approach requires
several steps to set up lock timeout monitoring. The new approach is much simpler,
you just set
DB2_CAPTURE_LOCKTIMEOUT=ON and that's it. The
second shortcoming was complexity, because it requires some exercise to read a
db2pd output and to correlate the different
db2pd sections. With the new approach, DB2 generates a report file
for you with all the necessary information in one place. But what about the last
shortcoming mentioned: Not enough information about the SQL statements that were
involved in the lock timeout situation? Until now you only know the SQL statement
that was blocked by the existing lock, but you have no information about the
statement that caused the lock. Concerning this point, the new lock timeout reporting
functionality provides improvements too. Now see how it works.
Collect SQL statement history information
To get information about the SQL statements that were executed by the lock owner's application, create a deadlock event monitor with the
DETAILS HISTORY option and activate the event monitor. For example, a suitable deadlock event monitor could be created and activated as follows:
Listing 8. Creating a deadlock event monitor with the
DETAILS HISTORY option
db2 "CREATE EVENT MONITOR evmondeadlock FOR DEADLOCKS WITH DETAILS HISTORY WRITE TO FILE 'path'" db2 "SET EVENT MONITOR evmondeadlock STATE 1"
You may ask, "Why do I need a deadlock event monitor for monitoring lock timeouts?" The answer is that lock timeout reporting builds partly on functionality delivered by deadlock event monitor code. When a deadlock event monitor is created with the
DETAILS HISTORY option, DB2 keeps track of the SQL statements already executed in a transaction. In case of a deadlock or lock timeout, this information can be used to present a history of SQL statements that may be involved in the deadlock or lock timeout occurrence.
With the active deadlock event monitor, walk through the lock timeout scenario described above again. This time DB2 writes a lock timeout report, as shown in Listing 9:
Listing 9. Lock timeout report containing SQL statement history information
LOCK TIMEOUT REPORT Date: 03/01/2008 Time: 15:10:13 Instance: DB2 Database: SAMPLE Database Partition: 0 Lock Information: Lock Name: 02000600040040010000000052 Lock Type: Row Lock Specifics: Tablespace ID=2, Table ID=6, Row ID=x0400400100000000 Lock Requestor: System Auth ID: FECHNER Application Handle: [0-202] Application ID: *LOCAL.DB2.080103140934 Application Name: db2bp.exe Requesting Agent ID: 2356 Coordinator Agent ID: 2356 Coordinator Partition: 0 Lock timeout Value: 10000 milliseconds Lock mode requested: ..U Application Status: (SQLM_UOWEXEC) Current Operation: (SQLM_EXECUTE_IMMEDIATE) Lock Escalation: No Context of Lock Request: Identification: UOW ID (1); Activity ID (1) Activity Information: Package Schema: (NULLID ) Package Name: (SQLC2G13NULLID ) Package Version: () Section Entry Number: 203 SQL Type: Dynamic Statement Type: DML, Insert/Update/Delete Effective Isolation: Cursor Stability Statement Unicode Flag: No Statement: UPDATE EMPLOYEE SET BONUS = SALARY * 0.1 WHERE JOB = 'MANAGER' Lock Owner (Representative): System Auth ID: FECHNER Application Handle: [0-188] Application ID: *LOCAL.DB2.080103140511 Application Name: db2bp.exe Requesting Agent ID: 5488 Coordinator Agent ID: 5488 Coordinator Partition: 0 Lock mode held: ..X List of Active SQL Statements: Not available List of Inactive SQL Statements from current UOW: Entry: #1 Identification: UOW ID (6); Activity ID (2) Package Schema: (NULLID ) Package Name: (SQLC2G13) Package Version: () Section Entry Number: 201 SQL Type: Dynamic Statement Type: DML, Select (blockable) Effective Isolation: Cursor Stability Statement Unicode Flag: No Statement: SELECT LASTNAME, FIRSTNME, SALARY FROM EMPLOYEE ORDER BY LASTNAME ASC Entry: #2 Identification: UOW ID (6); Activity ID (1) Package Schema: (NULLID ) Package Name: (SQLC2G13) Package Version: () Section Entry Number: 203 SQL Type: Dynamic Statement Type: DML, Insert/Update/Delete Effective Isolation: Cursor Stability Statement Unicode Flag: No Statement: UPDATE EMPLOYEE SET SALARY = SALARY * 1.02
The beginning of this lock timeout report is identical to what you have already
seen. But this time the Lock Owner section contains additional, valuable
information. Under the heading List of Inactive SQL Statements from current
UOW, you see all the SQL statements that were executed in the lock owner's
transaction prior to the occurrence of the lock timeout. From this list of SQL
statements, you can identify the statement(s) that are responsible for the
problematic lock. In this scenario, it is the
UPDATE statement to increase the salary of each employee.
Note that this functionality is a major improvement to the
db2pd approach. With the
db2pd approach, you would only
see the last statement executed by the lock owner's application — in this
scenario the query on the EMPLOYEE table. But as the query did not cause the
problematic X-lock, you would still not know which statement was responsible for the
lock. With the new approach —
DB2_CAPTURE_LOCKTIMEOUT with a deadlock event monitor — you
have the history of all SQL statements executed in the lock owner's transaction,
which makes it possible to identify the
UPDATE as the relevant statement.
Hints for using lock timeout reporting
A deadlock event monitor with statement history capabilities affects all
applications and increases the monitor heap usage by the DB2 database manager. So
this kind of deadlock event monitor should be used with caution. You should always
start by setting only
activate a deadlock event monitor with the
DETAILS HISTORY option only if required.
When working with lock timeout reporting, you may notice that the number of lock timeout report files in the
DIAGPATH continuously increases. DB2 does not delete those report files, so you as a DBA are responsible for deleting them or moving them to a different place so that there is always enough space on the
Even with the functionality of lock timeout reporting, it may not always be possible to determine the cause of lock timeouts easily. For example this may be the case when static SQL or DB2 internal locks are involved in lock timeout occurrences. The Lock timeout reporting chapter in the DB2 9.5 documentation provides a short list of those limitations (see the Resources section below). However, lock timeout reporting in DB2 9.5 is surely a functionality many DBAs waited for and will make analyzing lock timeouts much easier than ever before.
This article introduces the new lock timeout reporting capabilities of DB2 9.5. The new capabilities are compared to the approach described in "Analyzing lockwait situations in DB2 for Linux, UNIX, and Windows" to demonstrate the strengths of this new DB2 9.5 feature.
- "Analyzing lockwait situations in DB2 for Linux, UNIX, and Windows" (developerWorks, Jul 2007): Read about lockwait and lock timeout monitoring.
- See all the articles in the series.
- The Lock timeout reporting chapter of the "IBM DB2 Database for Linux, UNIX, and Windows Information Center": Find limitations that should be considered when using the lock timeout reporting feature.
- "DB2 9 Fundamentals exam 730 prep, Part 6: Data concurrency" (developerWorks, Jul 2006): Get an introduction to DB2 data concurrency concepts as well as a nice overview of the different kinds of locks used by DB2.
- "Understanding locking in DB2 Universal Database" (developerWorks, Nov 2005): Discover the principles behind DB2's locking strategy.
- "Improve concurrency with DB2 9.5 optimistic locking" (developerWorks, Jan 2008): Find the new DB2 9.5 optimistic locking support that can help avoiding database concurrency problems.
- "DB2 9.5 Information Center for Linux, UNIX, and Windows": Access the complete DB2 9.5 documentation online in HTML format.
- DB2 9 for Linux UNIX and Windows Support: Search for APARs, download fixpacks, get DB2 documentation in PDF format, and more
- Visit the developerWorks resource page for DB2 for Linux, UNIX, and Windows to read articles and tutorials and connect to other resources to expand your DB2 skills.
- developerWorks Information Management zone: Learn more about DB2. Find technical documentation, how-to articles, education, downloads, product information, and more.
- Stay current with developerWorks technical events and webcasts.
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