SAP Library - Find your answers

The primary function of the redo log is to record all the changes made to data. Changes made to entries are written to the on-line redo log files. So that they can be used in roll forward operations during database recovery.
When a change has happened, the entry will be first stored in the SGA. SGA has the following components:
1. Shared pool
2. Database buffer cache (DB cache)
3. Redolog Buffer
4. Large pool
5. Java Pool

Shared pool, consists of shared SQL area and shared PL/SQL area. DB cache will have the most recently used blocks & the blocks that needs to be written to the datafile. Redolog Buffer contains the changed/redo data and Large pool is used only in parallel sever environment.
The SGA may also use a Java pool, if java stored procedures are used.
When a SQL statement is executed, the output (data) will be stored in these DB cache as dirty blocks, and then moved into redo log buffers, which will be updated at a later stage.

NTAB buffer:

The name table (nametab) contains the table and field definitions that are activated in the SAP System. An entry is made in the Repository buffer when a mass activator or a user (using the ABAP Dictionary, Transaction SE11) requests to activate a table. The corresponding name table is then generated from the information that is managed in the Repository.

The Repository buffer is mainly known as the nametab buffer (NTAB), but it is also known as the ABAP Dictionary buffer.

Table definitions: TTAB

Field descriptions: FTAB

Initial record layouts: IREC Buffer (Contains the record layout initialized depending on the field type)

Short Nametab: SNTAB Buffer (A short summary of TTAB and FTAB buffers)

The description of a table in the Repository is distributed among several tables (for field definition, data element definition and domain definition). This information is summarized in the name table. The name table is saved in the following database tables:

DDNTT (table definitions)
DDNTF (field descriptions)

The Repository buffer consists of four buffers in shared memory, one for each of the following
The Short nametab and Initial record layouts are not saved in the database. Instead, they are derived from the contents of tables DDNTT and DDNTF.

When access to a table is requested, the database access agent embedded in each work process first reads the Short nametab buffer for information about the table. If the information is insufficient (for example, the SELECT statement uses a non-primary key) it accesses the Table definitions buffer and then the Field descriptions buffer.

By reading the Repository buffers, the database access agent knows whether the table is buffered or not. Using this information, it accesses the table buffers (partial buffer or generic buffer) or the database

Program buffer: Also called as
SAP executable buffer
ABAP buffer
PXA (Program Execution Area)

Stores the compiled executable versions of ABAP programs (loads).

The contents of this buffer are stored in tables D010L (ABAP loads), D010T (texts) and D010Y (symbol table)

Calender Buffer:
The SAP calendar buffer stores all defined factory and public holiday calendars.
Calendars are stored in the database tables TFACS and THOCS.

There are two kinds of SAPgui buffers:
Presentation buffers
Menu buffers
The following table shows the SAPgui buffers and their functions:

The buffer has a directory structure. This means that if the shared memory is configured too small, only the required data is loaded; there is no LRU displacement of the contents of the buffer.
Presentation buffer (Screen Buffer): Stores the generated screens (DYNPRO loads)
Menu buffer (CUA Buffer): Stores objects from the SAPgui.

For example, menus, pushbutton definitions.
The buffer has directory structure and supports LRU displacement.
The menu buffer is adjusted by amending its instance profile parameters

The CUA buffer holds objects from the GUI interface, such as menus, pushbuttons, and so on. In the database, these objects are held in tables D342L and D345T. Its size is expressed in KB. The standard SAP recommendation is: Up to 2500 KB for small systems, up to 3000 KB for medium systems and up to 6000 KB for large systems.

Use

Unit: 8 KB

Default value: 0

Recommended value: UNIX: 1000, but the value depends on the server size and the host system. You can find the values in the configuration examples (Contents button). For more information on Windows NT, see Parameter Overview for Windows NT.

Absolute upper limit: 250000 blocks (2 GB) for special applications (file system must be large enough)

Changes: The customer can make changes.

Use

Integration

rdisp/ROLL_SHM : Buffered data is in the shared memory; accessing this data here is much faster than accessing a file.

Unit: 8 KB

Default value: 0

Recommended value: UNIX: 8000, IBM AS/400: 0, under Microsoft Windows NT 128 blocks (1 MB)

Unless the Tune summary in the CCMS (Transaction ST02) (under Monitoring the Memory Management System) specifies that a larger roll buffer is required, do not set a value larger than 12500 (100 MB).

Absolute upper limit: 250000 blocks (2 GB) for special applications (file system must be large enough)

You can find the values in the configuration examples (Contents button). For more information on Windows NT, see Parameter Overview for Windows NT.

Changes: The customer can make changes.