DB2 tuning parameters

DB2 has many parameters that you can configure to optimize database performance. For complete DB2 tuning information, refer to the DB2 UDB Administration Guide... Performance.

DB2 logging

• Description: DB2 has corresponding log files for each database that provides services to administrators, including viewing database access and the number of connections. For systems with multiple hard disk drives, you can gain large performance improvements by setting the log files for each database on a different hard drive from the database files.

• How to view or set: At a DB2 command prompt, issue the command: db2 update db cfg for [database_name] using newlogpath [fully_qualified_path].

• Default value: Logs reside on the same disk as the database.

• Recommended value: Use a separate high-speed drive, preferably performance enhanced through RAID.

For more information about using AIX with DB2 see Tuning operating systems

DB2 configuration advisor

Located in the DB2 Control Center, this advisor calculates and displays recommended values for the DB2 buffer pool size, the database, and the database manager configuration parameters, with the option of applying these values. See more information about the advisor in the online help facility within the Control Center.

Use TCP/IP sockets for DB2 on Linux

• Description: On Linux platforms, whether the DB2 server resides on a local machine with WAS or on a remote machine, configure the DB2 application databases to use TCP/IP sockets for communications with the database.

• How to view or set: Locate the directions for configuring DB2 on Linux in the WAS installation documentation for the various operating systems. This document specifies setting DB2COMM for TCP/IP and corresponding changes required in the etc/service file.

• Default value: Shared memory for local databases

• Recommended value: On Linux, change the specification for the DB2 application databases and any session databases from shared memory to TCP/IP sockets.

Number of connections to DB2 - MaxAppls and MaxAgents

When configuring the data source settings for the databases, confirm the DB2 MaxAppls setting is greater than the maximum number of connections for the data source. If you are planning to establish clones, set the MaxAppls value as the maximum number of connections multiplied by the number of clones. The same relationship applies to the session manager number of connections. The MaxAppls setting must be equal to or greater than the number of connections. If you are using the same database for session and data sources, set the MaxAppls value as the sum of the number of connection settings for the session manager and the data sources.

For example, MaxAppls = (# of connections set for data source + # of connections in session manager) x # of clones.

After calculating the MaxAppls settings for the WebSphere database and each of the application databases, verify that the MaxAgents setting for DB2 is equal to or greater than the sum of all of the MaxAppls values, for example, MaxAgents = sum of MaxAppls for all databases.

DB2 buffpage

• Description: Improves database system performance. Buffpage is a database configuration parameter. A buffer pool is a memory storage area where database pages containing table rows or index entries are temporarily read and changed. Data is accessed much faster from memory than from disk.

• How to view or set: To view the current value of buffpage for database x, issue the DB2 command get db cfg for x and look for the value BUFFPAGE. To set BUFFPAGE to a value of n, issue the DB2 command update db cfg for x using BUFFPAGE n and set NPAGES to -1 as follows

  db2   <-- go to DB2 command mode, otherwise the following "select" does not work as is
      connect to x    <-- (where x is the particular DB2 database name) 
      select * from syscat.bufferpools 
         (and note the name of the default, perhaps: IBMDEFAULTBP)
         (if NPAGES is already -1, there is no need to issue following command) 
      alter bufferpool IBMDEFAULTBP size -1
      (re-issue the above "select" and NPAGES now equals -1)
  
  

  You can collect a snapshot of the database while the application is running and calculate the buffer pool hit ratio as follows...

  1. Collect the snapshot:

     1. Issue the update monitor switches using bufferpool on command.

     2. Verify that bufferpool monitoring is on by issuing the get monitor switches command.

     3. Clear the monitor counters with the reset monitor all command.

  2. Run the application.

  3. Issue the get snapshot for all databases command before all applications disconnect from the database, otherwise statistics are lost.

  4. Issue the update monitor switches using bufferpool off command.

  5. Calculate the hit ratio by looking at the following database snapshot statistics:

     • Buffer pool data logical reads

     • Buffer pool data physical reads

     • Buffer pool index logical reads

     • Buffer pool index physical reads

• Default value: 250

• Recommended value: Continue increasing the value until the snapshot shows a satisfactory hit rate.

The buffer pool hit ratio indicates the percentage of time that the database manager did not need to load a page from disk to service a page request. That is, the page was already in the buffer pool. The greater the buffer pool hit ratio, the lower the frequency of disk input and output. Calculate the buffer pool hit ratio as follows:

• P = buffer pool data physical reads + buffer pool index physical reads

• L = buffer pool data logical reads + buffer pool index logical reads

• Hit ratio = (1-(P/L)) * 100%

DB2 query optimization level

• Description: Sets the amount of work and resources that DB2 puts into optimizing the access plan. When a database query is executed in DB2, various methods are used to calculate the most efficient access plan. The range is from zero to 9. An optimization level of 9 causes DB2 to devote a lot of time and all of its available statistics to optimizing the access plan.

• How to view or set: The optimization level is set on individual databases and can be set with either the command line or with the DB2 Control Center. Static SQL statements use the optimization level specified on the prep and bind commands. If the optimization level is not specified, DB2 uses the default optimization as specified by the dft_queryopt setting. Dynamic SQL statements use the optimization class specified by the current query optimization special register, which is set using the SQL Set statement. For example, the following statement sets the optimization class to 1

  Set current query optimization = 1
  

  If the current query optimization register is not set, dynamic statements are bound using the default query optimization class.

• Default value: 5

• Recommended value: Set the optimization level for the needs of the application. Use high levels only when there are very complicated queries.

DB2 reorgchk

• Description: Obtains the current statistics for data and rebinding. Use this parameter because SQL statement performance can deteriorate after many updates, deletes or inserts.

• How to view or set: Use the DB2 reorgchk update statistics on table all command to issue runstats on all user and system tables for the database to which you are currently connected. Rebind packages using the bind command. If runstats exists, issue the db2 -v "select tbname, nleaf, nlevels, stats_time from sysibm.sysindexes" command on DB2 CLP. If no runstats exist, nleaf and nlevels are -1, and stats_time has an empty entry , for example "-". If runstats was previously done, the real-time stamp from completion of the runstats also displays under stats_time. If you think the time shown for the previous runstats is too old, execute runstats again.

• Default value: None

• Recommended value: None

DB2 MinCommit

• Description: Delays the writing of log records to a disk until a minimum number of commits is performed, reducing the database manager overhead associated with writing log records. For example, if MinCommit is set to 2, a second commit causes output to the transaction log for the first and second commits. The exception occurs when a one-second timeout forces the first commit to be output if a second commit does not ocurr within one second.

  In test applications, up to 90% of the disk input and output was related to the DB2 transaction log. Changing MinCommit from 1 to 2 reduced the results to 45%.

  Adjust this parameter if the disk input and output wait is more than 5% and there is DB2 transaction log activity from multiple sources. When a lot of activity occurs from multiple sources, it is less likely that a single commit has to wait for another commit, or the one second timeout. Do not adjust this parameter if you have an application with a single thread performing a series of commits because each commit can hit the one second delay.

• How to view or set:

  1. Issue the DB2 get db cfg for xxxxxxcommand, where xxxxxx is the name of the application database, to list database configuration parameters.

  2. Look for "Group commit count (MINCOMMIT)".

  3. Set a new value by issuing the DB2 update db cfg for xxxxxx using mincommit n command, where xxxxxx is the name of the application database and n is a value between 1 and 25 inclusive.

  The new setting takes effect immediately.

  The following are several metrics that are related to DB2 MinCommit:

  • The disk input and output wait can be observed on AIX with the command vmstat 5. This shows statistics every 5 seconds. Look for the wa column under the CPU area.

  • The percentage of time a disk is active can be observed on AIX with the command iostat 5. This shows statistics every 5 seconds. Look for the %tm_act column.

  • The DB2 get snapshot for db on xxxxxx command , where xxxxxx is the name of the application database, shows counters for log pages read and log pages written.

• Default value: 1.

• Recommended value: 1 or 2, if the circumstance permits.

DB2 Deadlock Event Monitor

• Description:For DB2 V8 or later, deadlock event monitor is turned on by default when a database is created. This event monitor captures critical information about all connections involved in deadlock when the particular event occurs. Although there is not much overhead on faster SMP systems, it can be turned off.

• How to view or set: In DB2 command window, issue following commands

  db2 connect to [db name]
  db2 set event monitor db2detaildeadlock state 0 
  db2 drop event monitor db2detaildeadlock
  db2 connect reset
  db2 terminate
  

  .

• Default value: Event monitor is ON by default.

• Recommended value: Turn OFF event monitor, if it is not required.

For more information about using AIX with DB2 see Tuning operating systems

 

See Also

Tuning performance parameter index

 

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