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Supporting SAP MaxDB - Frequently Asked Questions

This pages provides answers to technical and support questions about SAP MaxDB.

General Questions

Technical Aspects

Answers General Questions

Which version does this FAQ cover?

This FAQ will deal with the SAP MaxDB versions from 7.5 onwards.
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Which tools do we use?

Tool

Further Information

Comment

Database Studio

DBSTUDIO

Graphical user interface (available with version 7.7)

Database Analyzer

Bottleneck Monitoring

Embedded tool, command line

Database Manager CLI

DBMCLI

Command Line Interface

SQLCLI

SQL Query

Command Line Interface

CCMS

SAP Monitoring

Using SAP transactions

Database Manager GUI

DBMGUI

Graphic User Interface (Microsoft Windows only)
As of version 7.7 this tool was replaced by Database Studio.

SQL Studio

SQL Query

Graphic user interface (Microsoft Windows only)
As of version 7.7 this tool was replaced by Database Studio.

Further information can be found in SAP MaxDB Tools.
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Where do I find the SAP MaxDB documentation?

Note 767598 describes where the documentation can be found.
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Is there any printed/printable info on supporting SAP MaxDB beside standard documentation?

Use information in SCN and SAP MaxDB Support Guide.
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How can I check the status of a reported bug?

You can use our Problem Tracking System PTS to check the status of a reported bug: MaxDB PTS
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Where can I find information about what is new in SAP MaxDB versions?

You can find this information here: SAP MaxDB Features Described in SAP Notes.
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Answers Technical Aspect

Which operating system user owns the database software (Unix)?

As of version 7.5 the owner of the database software is a user called sdb. This user must be locked (login with this user is not necessary) and his primary group has to be sbda. Users <sid>adm have to be added to group sdba.
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How can I create a database trace?

You can activate a database trace (also called kernel trace or vtrace).

The database trace is a very detailed debugging trace generated by the database kernel on request. It is mostly used only by support staff. The information contained can be used to analyze a wide range of problems, including SQL problems and kernel communication bugs.

To activate the database trace, you can use either Database Studio, CCMS or Database Manager CLI. For more information see SAP MaxDB kernel trace.
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What can I provide in case of crashes ?

The most important log file for analyzing crashes is the database kernel log file KnlMsg/knldiag which is located in the run directory (for example <independent_data_path>\wrk\<database_name>). The KnlMsg/knldiag file contains all kernel messages (errors, warnings, and general information).

If you restart the database after a crash, the system copies the KnlMsg/knldiag file to <file_name>.old and overwrites the KnlMsg/knldiag file with new information.

If the .old file has also been already overwritten, the log files contained in the <independent_data_path>\wrk\ <database_name>\DIAGHISTORY\<database name>_<timestamp> directory can be used for analyzing the crash. The <timestamp> is in this case the time of the next database start after the crash occured - as the files are copied during this restart.
For more information see SAP MaxDB log files.
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What are system tables?

There are several system tables in a SAP MaxDB database. Some of them contain information about database objects and their connection to each other and some of them contain monitoring information (Glossary, System Table).
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When do I have to load the system tables?

The system tables need to be loaded when a database is newly installed, when a new database version is installed and when a system copy has been performed.
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How can I load the system tables?

The system tables are loaded automatically when a database is created using Database Studio. This is also the case when a new database version is installed using SDBUPD or SDBSETUP.
If you use SDBINST to install a new database version, you need to load the system tables manually. This can be done e.g. with DBM command load_systab or in Database Studio -> Configuration -> Upgrade System Tables.

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Where do I find further information about system tables?

Use the Glossary, System Table.

What is the converter?

The converter contains the information on which physical location a logical page is stored.  As of version 7.4 the converter is stored in a B* tree, striped over all data volumes. It is now possible to decrease the database size by deleting data volumes. You can find further information about the converter in SAP MaxDB Internals session.
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What is the "file directory"?

Tables are managed using a so called fileid (in older versions tabid). The relation between a table name and the fileid is stored in the database catalog.
The relation between the root page of a B* tree and the corresponding database object (using the fileid) is stored in a database structure called "file directory". The type of the database object (e.g. table or index) is also stored in this file directory.
As of version 7.6 content of the file directory can be displayed with a SELECT statement on table FILES
(SELECT * FROM files WHERE fileid = ...). See also Glossary, File Directory (Database-Internal).
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What is the X server (x_server)?

The X server is the SAP MaxDB TCP/IP listener. The X server needs to be started to allow remote communication to the database. The X server logs problems in file x_server_<host_name>.prt. See also Glossary, X Server.
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Which database caches get memory from the memory area specified with parameter CACHE_SIZE?

First the cache for the converter is created. It is important that the whole converter is available in main memory. Also file directory cache and catalog cache get memory from this memory area. The rest is used for the data cache. If one of the other caches needs to grow, pages are removed from the data cache - the data cache grows and shrinks dynamically. The database parameter CACHE_SIZE was renamed to CacheMemorySize in database version 7.7. See also Concepts of the Database System, Working Memory Areas.  
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What is a savepoint?

The data is changed in the memory (data cache). To make sure that these changes are still available after a database restart, the changed pages are written in regular intervals to the data volumes. This action is called savepoint. See also Concepts of the Database System, Savepoint.
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What is an undo log file?

Each transaction which changes data creates a so called undo log file. This undo log file contains the information which is necessary for a rollback of a transaction (the before images). As of version 7.4 this undo information is kept in the data area and it is even part of a data backup. Therefore a database can be restarted even when the log information got corrupted - all open transactions can be rolled back using the information of the undo log files in the data area. Furthermore all data backups are consistent. Log backups are not needed for a system copy.
The undo log files are deleted at the end of a transaction (OLTP). In a SAP liveCache the undo information is also used for the consistent read - the garbage collectors delete the undo information when it is not needed anymore. See also Concepts of the Database System, How Databases Log Data Changes.
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How does the mirroring of log volumes work?

We recommend to mirror the log volumes using a RAID1 system. If this is not possible, the log volume should be mirrored by the database. The database then uses two log volumes in parallel:

  • log entries are written to both volumes at the same time
  • log entries are read (during backup or restart) only from the primary log volume

The entries on both log volumes can only be overwritten when a log backup has been performed. See also Glossary, Log Settings.
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Relevant documents

SAP MaxDB Documentation Version 7.9: SAP MaxDBGlossary
Training material: No-Reorganization Principle
Internal File Size Information 

Relevant links

Wiki page SAP MaxDB Tools
Wiki page SAP MaxDB database trace
Wiki page SAP MaxDB log files
SAP MaxDB Problem Tracking System PTS

Relevant SAP Notes

767598 Available Documentation