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The following overview describes database parameters belonging to the Support Database Parameters group. Most support database parameters are for troubleshooting. In normal operation, it is not necessary to change the support database parameters. Changing support database parameters requires detailed knowledge of the database system.

For information about other database parameters, see

To change the values of database parameters, use one of the following database tools: Database Studio, Database Manager CLI, or CCMS (in SAP systems only). More information:

Most changes to database parameters take effect only after the database has been restarted.

However, you can change individual database parameters while the database is running, that is in the ONLINE or ADMIN operational state (see the description of individual database parameters for more information). For these changes you can define when they are to become effective:

  • Until the next restart

  • After the next restart

  • Immediately and permanently

If you change a database parameter whose value is normally calculated by the system, then the system will consider this user-defined value for all subsequent recalculations of database parameter values. The system discards and recalculates this user-defined value only if it is not within the permitted value range (for example, after the database software has been updated).

Note Note

Starting with version 7.7.03 of the database software, the names of many database parameters have changed.

Some database parameters have also been designated obsolete. These parameters have either been removed already or will be removed in one of the next database versions.

More information: Obsolete Database Parameters

End of the note.
Support Database Parameters

Name

(from version 7.7.03)

Old Name

(up to and including version 7.7.02)

Description

Change Restrictions

More Information

CommandBufferSize

_PACKET_SIZE

Size of the packages (shared memory segments) in which SQL statements and data are transferred

A package consists of a request part and a reply part.

The minimum package size is 48 KB.

Changes only apply after a restart of the database

-

DataIOClusterSize

DATA_IO_BLOCK_COUNT

Block size that the database system uses when writing data pages from the data cache to the data area.

The optimum block size depends on your hardware and software.

Changes only apply after a restart of the database

-

EnableMultipleServerTaskUKT

ALLOW_MULTIPLE_SERVERTASK_UKTS

Specifies whether the database system distributes the server tasks to the available user kernel threads. Otherwise they all run in the same user kernel thread.

Changes only apply after a restart of the database

-

EnableSymbolDemangling

SYMBOL_DEMANGLING

Specifies whether the database system performs C/C++ demangling

Can be changed in ONLINE and ADMIN operational state

-

EnableSynchronousTaskIO

_USE_IOPROCS_ONLY

YES: I/O operations are executed only by special I/O threads

NO: The kernel decides whether a task is to execute an I/O operation itself or delegate the I/O operation to a special I/O thread

Can be changed in ONLINE and ADMIN operational state

-

EnableVariableInput

EnableVariableOutput

USEVARIABLEINPUT

Specifies whether the current length of the variable contents or the maximum length of these variables is transmitted between client and kernel. In the latter case, the maximum length may be used depending on the data type of the variables.

Changes only apply after a restart of the database

-

EventFileName

_EVENTFILE

Name of the file in which the database system logs events

Changes only apply after a restart of the database

Log Files

EventFileSize

_EVENTSIZE

Size of the file in which the database system logs events (in KB)

Changes only apply after a restart of the database

-

FloatingServiceTasks

-

Maximal number of tasks in the floating service task pool

The floating service task pool contains tasks that the system can use for events and for Database Analyzer

You must configure at least 2 event tasks and 1 Database Analyzer task.

See also PreservedEventTasks and PreservedDBAnalyzerTasks

Changes only apply after a restart of the database

Concepts of the Database Systems, Tasks

InitialAllocatorSize

INIT_ALLOCATORSIZE

Initial size of the working memory that is reserved at the start of a database session

Changes only apply after a restart of the database

-

KernelDumpFileName

_KERNELDUMPFILE

Name of the dump file that is written by the kernel when the system crashes

To this file, the database system writes the contents of the data cache and the converter. Size your system so that sufficient memory space is available for this file (approximately CACHE_SIZE + 10%).

Changes only apply after a restart of the database

Parameter DIAG_HIST_NUM

KernelTraceFile

_KERNELTRACEFILE

File to which the kernel writes trace entries

The kernel only writes trace entries if you previously activated the database trace.

Changes only apply after a restart of the database

Concepts of the Database System, Load Balancing Between User Kernel Threads

LoadBalancingWorkloadDeviation

LOAD_BALANCING_EQ

For load balancing

Specifies which time delay should be regarded as equal when comparing waiting tasks (as a percentage)

Changes only apply after a restart of the database

-

LoadBalancingWorkloadThreshold

LOAD_BALANCING_DIF

For load balancing

Specifies by how much longer the task to be moved has to wait in its user kernel thread than the task that has been waiting the longest in the target user kernel thread (as a percentage)

Changes only apply after a restart of the database

-

LogIOClusterSize

LOG_IO_BLOCK_COUNT

Block size that the database system uses when writing log pages from the log queues to the log area

The optimum block size depends on your hardware and software.

Changes only apply after a restart of the database

-

PreservedDBAnalyzerTasks

-

Minimal number of Database Analyzer tasks in the floating service task pool

See also FloatingServiceTasks

Changes only apply after a restart of the database

Database Analyzer documentation

PreservedEventTasks

_MAXEVENTTASKS

Minimal number of event tasks in the floating service task pool

See also FloatingServiceTasks

The database system uses event tasks both for Event Dispatcher and for the DBM commands event_wait,event_receive, event_available, auto_extend and auto_update_statistics.

Changes only apply after a restart of the database

Database Manager CLI,

MaxPagerTasks

MAXPAGER

Maximum number of pagers; calculated by the database system from MAXDATAVOLUMES and other parameters

Not changeable

You can override this parameter with XP_MAXPAGER.

-

MaxSavepointTimeInterval

_RESTART_TIME

Minimum time between two savepoints (in s); corresponds to the time that is required for a restart after a system crash

Furthermore, the database system always writes a savepoint in the following cases:

  • When you have created a new index

  • When the number of pages released by a savepoint is larger than the number of free blocks in the data volumes

Changes only apply after a restart of the database

-

MaxServerTaskStackSize

MAX_SERVERTASK_STACK

Maximum size of the stack that is used by the server tasks

Changes only apply after a restart of the database

-

MaxSpecialTaskStackSize

MAX_SPECIALTASK_STACK

Maximum size of the stack that is used by special tasks (all except user tasks and server tasks)

Changes only apply after a restart of the database

-

MaxTaskStackSize

_MAXTASK_STACK

Size of the stack that is used by the user tasks

Changes only apply after a restart of the database

-

MaxTempFilesPerIndexCreation

_IDXFILE_LIST_SIZE

Number of temporary result files in the case of parallel indexing

The database system indexes large tables using multiple server tasks. These server tasks write their results to temporary files. When the number of these files reaches the value of this parameter, the database system has to merge the files before it can generate the actual index. This results in a decline in performance.

Can be changed in ONLINE and ADMIN operational state

-

MaxVolumes

MAXVOLUMES

Maximum number of data and log volumes including mirrored volumes

Changes only apply after a restart of the database

-

MiniDumpType

MINI_DUMP

Defines whether the system writes the memory dumps knlmini.dmp and srvmini.dmp (for postmortem debugging) and how much information these dumps contain

Changes only apply after a restart of the database

-

OMSStreamTimeout

OMS_STREAM_TIMEOUT

Maximum wait time for all database sessions that may elapse until the reply to an OMS stream request to a client (in s)

Can be changed in ONLINE and ADMIN operational state

-

ReadAheadLobThreshold

_READAHEAD_BLOBS

Number of pages from which large LOB values are imported in advance by additional server tasks

If a LOB value is too large to be transferred to the client in a single request package, the database system splits it into several request packages. To increase performance, server tasks can import further parts of the LOB value while the first request package is being sent.

Can be changed in ONLINE and ADMIN operational state

-

ReservedCommandBufferReplySize

_MINREPLY_SIZE

Minimum size of the memory that is available in a package (shared memory segment) for the reply

0: The whole package is available for the request/reply.

Changes only apply after a restart of the database

-

RTEDumpFileName

_RTEDUMPFILE

File name of the file to which the kernel writes information about the runtime environment if the system crashes

Changes only apply after a restart of the database

-

ShowMaxStackUsage

SHOW_MAX_STACK_USE

Logs the maximum stack usage of each task in the kernel log

Only set this parameter to YES for diagnosis purposes as it compromises performance.

Changes only apply after a restart of the database

-

TaskCluster01 to TaskCluster03

_TASKCLUSTER_01 to _03

These parameters describe how the database system distributes user tasks to the threads. Only change these parameters after consulting support.

Changes only apply after a restart of the database

-

TaskSpecificCatalogCacheMinSize

CAT_CACHE_MINSIZE

Minimum size of the task-specific catalog cache (in bytes)

Changes only apply after a restart of the database

Concepts of the Database System, Working Memory Areas

UpdateStatParallelServerTask

UPDATESTAT_PARALLEL_SERVERS

Specifies how many parallel server tasks the database system uses for updating the SQL optimizer statistics

Can be changed in ONLINE and ADMIN operational state

-

UseAsynchronousIO

_USE_ASYNC_IO

Specifies whether operating system functions or special I/O threads are used for asynchronous I/O operations

Changes only apply after a restart of the database

-

UseColumnCompression

COLUMNCOMPRESSION

Specifies whether the column values can be stored in variable length or not.

Default value: YES (columns have variable length, so the values can be compressed if necessary)

Only for columns that are not primary key columns

Can be changed in ONLINE and ADMIN operational state

-

UseExtendedOracleSQLMode

_SERVERDB_FOR_SAP

Specifies whether the database is being used in an SAP system

Can be changed in ONLINE and ADMIN operational state

-

UseFilesystemCacheForBackup

-

Specifies whether the database system uses the operating system's file system cache for I/O operations (O_DIRECT flag) when opening volumes or data carriers for backup or recovery

YES: The database system does not use the O_DIRECT flag

NO: The database system uses the O_DIRECT flag (if this flag is supported by the file system; note that this flag is ignored for Linux kernels < 2.4.18).

Changes only apply after a restart of the database

-

UseFilesystemCacheForVolume

(USE_OPEN_DIRECT)

Caution Caution

This parameter replaces the USE_OPEN_DIRECT parameter, but with a different logic. For backward compatibility, DeprecatedParameter29 has been introduced as a direct replacement for USE_OPEN_DIRECT, see Obsolete Database Parameters.

After migrating from an earlier database version and if USE_OPEN_DIRECT was set explicitly for example using a script, the system changes UseFilesystemCacheForVolume to the opposite value. Note though that after you have changed UseFilesystemCacheForVolume for the first time, it does not depend on USE_OPEN_DIRECT any longer.

End of the caution.

Specifies whether the database system uses the operating system's file system cache for I/O operations O_DIRECT flag) when it is opening volumes for normal database operation

YES: The database system does not use the O_DIRECT flag

NO: The database system uses the O_DIRECT flag (if this flag is supported by the file system; note that this flag is ignored for Linux kernels < 2.4.18).

Changes only apply after a restart of the database

-

UseStrategyCache

OPTIM_CACHE

Specifies whether the database system determines the search strategy only once or every time it executes a parsed SQL statement

For prepared statements with parameters, it may be sufficient to determine the search strategy only once.

Can be changed in ONLINE and ADMIN operational state

-

UseSystemPageCache

USE_SYSTEM_PAGE_CACHE

Specifies whether the database uses the system page cache for buffering memory pages that are no longer required

Changes only apply after a restart of the database

-

UseSystemTriggers

ENABLE_SYSTEM_TRIGGERS

Specifies whether system triggers are called when the database is restarted

Changes only apply after a restart of the database

-

UseVolumeLock

SET_VOLUME_LOCK

Specifies whether the database system sets a lock when a volume is added, which prevents the same volume from being added again later

In the following cases, it may make sense to set this parameter to NO:

  • NFS-mounted volume

  • Hot standby system

Changes only apply after a restart of the database

-

More Information

Glossary