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 document.
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 a redundant array of independent disk (RAID)
configuration.
For more information about using AIX with
DB2 see Tuning AIX 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.
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 = (number of connections set for the data source
+ number of connections in the session manager) multiplied by the number of
clones.
After calculating the MaxAppls settings for the WebSphere Application Server
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:
- Collect the snapshot:
- Issue the update monitor switches using bufferpool on command.
- Make sure that bufferpool monitoring is on by issuing the get monitor
switches command.
- Clear the monitor counters with the reset monitor all command.
- Run the application.
- Issue the get snapshot for all databases command before all applications
disconnect from the database, otherwise statistics are lost.
- Issue the update monitor switches using bufferpool off command.
- 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 is 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
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 perform the runstats operation on all user
and system tables for the database to which you are currently connected.
Rebind packages using the bind command. If statistics are available,
issue the db2 -v "select tbname, nleaf, nlevels, stats_time from sysibm.sysindexes" command
on DB2 CLP. If no statistic updates exist, nleaf and nlevels are -1, and
stats_time has an empty entry (for example: "-"). If the runstats command
was previously run, the real-time stamp from completion of the runstats operation
also displays under stats_time. If you think the time shown for the previous
runstats operation is too old, run the runstats command again.
- Default value: None
- Recommended value: None
DB2 locktimeout
- Description: Specifies the number of seconds that an application
waits to obtain a lock. Setting this property helps avoid global deadlocks
for applications.
- How to view or set: To view the current value of the lock timeout
property for database xxxxxx, issue the DB2 get db cfg for xxxxxx command
and look for the value, LOCKTIMEOUT. To set LOCKTIMEOUT to a value of n,
issue the DB2 update db cfg for xxxxxx command using LOCKTIMEOUT n,
where xxxxxx is the name of the application database and n is
a value between 0 and 30 000 inclusive.
- Default value: -1, meaning lock timeout detection is turned off.
In this situation, an application waits for a lock if one is not available
at the time of the request, until either of the following events occurs:
- The lock is granted
- A deadlock occurs
- Recommended value: If your database access pattern tends toward
a majority of writes, set this value so that it gives you early warning when
a timeout occurs. A setting of 30 seconds suits this purpose. If your pattern
tends toward a majority of reads, either accept the default lock timeout value,
or set the property to a value greater than 30 seconds.
DB2 maxlocks
- Description: Specifies the percentage of the lock list that is
reached when the database manager performs escalation, from row to table,
for the locks held by the application. Although the escalation process does
not take much time, locking entire tables versus individual rows decreases
concurrency, and potentially decreases overall database performance for subsequent
attempts to access the affected tables.
- How to view or set: To view the current value of the maxlocks property
for database xxxxxx, issue the DB2 get db cfg for xxxxxx command
and look for the MAXLOCKS value. To set MAXLOCKS to a value of n, issue
the DB2 update db cfg for xxxxxx command using MAXLOCKS n,
where xxxxxx is the name of the application database and n is
a value between 1 and 100 inclusive.
- Default value: Refer to the current database information for property
default values per operating system.
- Recommended value: If lock escalations are causing performance
concerns, you might need to increase the value of this parameter or the locklist
parameter, which is described in the following paragraph. You can use the
database system monitor to determine if lock escalations are occurring.
DB2 locklist
- Description: Specifies the amount of storage that is allocated
to the lock list.
- How to view or set: To view the current value of the locklist property
for database xxxxxx, issue the DB2 get db cfg for xxxxxx command
and look for the LOCKLIST value . To set LOCKLIST to a value of n,
issue the DB2 update db cfg for xxxxxx command using LOCKLIST n,
where xxxxxx is the name of the application database and n is
a value between 4 and 60 000 inclusive.
- Default value: Refer to the current database information for property
default values per operating system.
- Recommended value: If lock escalations are causing performance
concerns, you might need to increase the value of this parameter or the maxlocks
parameter, which is described in the previous paragraph. You can use the database
system monitor to determine if lock escalations are occurring. Refer to the DB2
Administration Guide: Performance document for more details.