Concepts of System Logging 
SAP servers records events and problems in system logs. There are two types of logging if your SAP system is executed on a UNIX host: Local and central loggoing. Every SAP application server has a local log that contains the messages sent from this server. You can also work with central logging. Every application server copies its local logs to a central log for this.
Only local logs (one for each application server) are created on Microsoft Windows NT hosts. They are not summarized in a central log.
How are Log Files Written?
Every SAP application server keeps a local log. System messages are logged in a ring buffer on the server. If this log file reaches the maximum permitted size then the system starts to overwrite the oldest data.
If necessary you can also configure a server so that messages are also displayed alongside the local file in a central log file (central logging is not available on Microsoft Windows NT host systems). A central log file is kept in a selected application server. The individual SAP application servers (or instances) send their local messages to this system which collects the messages from the different instances and writes them to the central log.
A central log consists of two files: an active and an old file. The active file contains the current log. If it reaches the maximum size defined in the system profile then the log file is switched. The system deletes the old log file, turns the previously active file into the old file, and creates a new active log file.
Note
The system does not display a message when the old log file is replaced.
Accuracy of Log Files
A local log is always up-to-date. This does not always apply for a central log. There can be short delays between recording a message in a local log and writing this message to the central log since send processes are not always active in local systems. The time period that a send process is inactive for varies depending on the intensity of logging in that particular system.
It can take longer to build a central log file if a message is sent from a system where there is normally very little logging activity. Longer delays and loss of messages in local systems can be caused by network problems or by a failure of the collect process on the computer where the contral log is stored.