Buffers Monitor (SAP Library - The Alert Monitor)

Buffers Monitor

Use

You can check the utilization of the and the hit rates and swap rates for the SAP buffers using this monitor. For a general introduction to this topic, see SAP buffers.

This graphic is explained in the accompanying text

Features

This monitor contains values for the following SAP buffers, sorted by application server:

Monitoring Tree Element	Additional Names of the Buffer	Contents of the Buffer
Program	ABAP buffer PXA buffer	Compiled SAP programs
GenericKey		Wholly or partly buffered database tables
SingleRecord		Individual records from utilized database tables
Screen	Dynpro buffer Presentation buffer	Screen pages from ABAP programs
CUA	Menu buffer	Menus and pushbuttons from ABAP screen pages
TableDefinition	TTAB buffer	Table definitions from the SAP R/3 Repository
FieldDescription	FTAB buffer	Field descriptions from the SAP R/3 Repository
InitialRecords	IREC buffer	Initial record layout (initial values for the fields of a database segment) of a table
ShortNameTAB	SNTAB buffer	Combination of the TTAB and FTAB buffers

The monitor displays the following values for each buffer type:

Attribute	Description
DirectoryUsed	Percentage usage of the directory (number of entries)
SpaceUsed	Percentage usage of the buffer storage
HitRatio	Percentage of the database queries that were met from the buffer (hit rate) and did not have to be passed on to the database
Swap	Swaps due to a full buffer per minute

Activities

To start the monitor, follow the procedure below:

Start the Alert Monitor using transaction RZ20 or choose CCMS ® Control/Monitoring ® Alert Monitor.
On the CCMS Monitor Sets screen, expand the SAP CCMS Monitor Templates set.
Start the Buffers monitor from the list by double clicking it.

Procedure if Buffer Is Full or Swap Rate Is too High

SAP buffers use intelligent procedures to replace old buffered data with new requested data. Allocating too much space to a buffer can also lead to other problems (operating system paging) without significantly increasing the hit rate (buffer quality).

The goal of the buffer setting is therefore to have a sufficiently large buffer to maintain a high rate, and to do so with a low swap rate and a minimal effect on operating system paging.

Here are a few guidelines about when certain SAP buffers must be increased in size:

Increase the size of the buffer if the swap rate (the replacing of buffer objects) is very high, or if the hit rate is below the target threshold value.
Increase the size of the buffer if alerts are frequently triggered in the associated node. Frequent alerts indicate that the buffer exchange mechanism cannot create enough free space for effective buffer operation.
As far as possible, you should always increase the amount of space available for the GenericKey buffer if it becomes full.
Increase the size of the program buffer if the hit rate becomes too low, or the swaps become too frequent. Development systems require a larger program buffer than normal systems. It is, however, almost impossible to avoid swapping in a development system.

If undesired side effects occur after increasing the size of the SAP buffer, monitor the operating system paging for significant increase and the SAP R/3 Memory Management subtree (in the Entire System monitor) for new alerts.

Procedure if Hit Rate Is Too Low

Hit rate alerts indicate that bad buffer performance is damaging the performance of the application server. As the buffer is of great importance for good system performance, you should investigate the buffer quality problem in great detail and take corrective measures.

In general, bad buffer quality means that the buffer is too small. If a buffer is too small, it is more likely that requested objects (such as table entries or programs) are not found in the buffer. This leads to a lower hit rate, and, if the buffer is already full, to increased swapping. The more often requested objects replace older objects in the buffer, the more swapping takes place.

To improve the hit rate, you should increase the size of the buffer. You should ensure, however, that the buffer quality has not been reduced by a temporary problem: after a system or application server is started, all buffers perform badly, as the buffers must first be filled. The default setting for the monitoring architecture is therefore not to trigger any buffer alerts during the first 20 minutes of operation.

The performance of a program buffer can also be reduced if developers are working in the system or if objects are imported into the system using the SAP R/3 transport system.

Note

With the exception of the SingleRecord buffer, the hit rate for buffers should always be above 95 percent if the buffers are large enough. The performance of the SingleRecord buffer can be regarded as good if the hit rate is between 80 and 90 percent.

Increasing the Size of SAP Buffers

To increase the size of a buffer, change the system profile parameters that specify the size of the buffer. For some buffers, you can set the size of the buffer and the maximum number of entries in separate parameters. For other buffers, there is only a size parameter, and the system automatically calculates the required system parameters.

Name of the Buffer	Responsible System Parameters	Typical Size/Number of Entries
Program	Size: abap/buffersize [KB] Number of entries is determined by size	240,000–400,000 KB
GenericKey	Size: zcsa/table_buffer_area [Bytes] Entries: zcsa/db_max_buftab	50 MB 10,000 entries
SingleRecord	Size: rtbb/buffer_length [KB] Entries: rtbb/max_tables	30,000 KB
Screen	Size: zcsa/presentation_buffer_area [Bytes] Number of entries is determined by size	150–200 MB
CUA	Size: rsdb/cua/buffersize [KB] Number of entries is determined by size	3,000-6,000 KB
TableDefinition	Size is determined by the number of entries Entries: rsdb/ntab/entrycount	30,000 entries
FieldDescription	Size: rsdb/ntab/ftabsize [KB] Entries: 2 * rsdb/ntab/entrycount + 1	20,000-30,000 KB
InitialRecords	Size: rsdb/ntab/irbdsize [KB] Entries: 2 * rsdb/ntab/entrycount + 1	4,000 KB
ShortNameTAB	Size: rsdb/ntab/sntabsize [KB] Entries: 2 * rsdb/ntab/entrycount + 1	2,500 KB

Note

For more information about setting buffer sizes, see Typical Parameter Settings for SAP Buffers. See also SAP Note 0103747.

It is often not possible to set the size of a buffer so that it never becomes full. Above a certain size, which is greatly dependent on the hardware and the operating system, the performance advantage of the buffer is canceled out by increased operating system paging. A buffer that occupies too much main memory forces the operating system to perform expensive paging.

The goal of buffer management is to assign the buffers enough memory so that they achieve a high hit rate with low swap rates, without causing much operating system paging. After each change of the buffer size, you should monitor paging and memory usage to ensure that the new buffer size does not cause any other performance problems.

As a rule of thumb, we recommend that you assign space to the SAP buffers rather than the database buffers, if main memory becomes limited. In SAP R/3 Systems, every application server has its own buffer. These buffers should not be confused with the buffers of the SAP R/3 database system.