syslog-ng Premium Edition 7.0.30 - Administration Guide

How it works

1. syslog-ng PE puts messages into the disk-buffer file (set via disk-buf-size()) when the destination becomes unavailable or when it is not able to process logs as fast as they arrive through the sources.
2. When the disk-buffer file is full, syslog-ng PE puts messages into the memory buffer (set via mem-buf-length()). When the memory buffer gets full too, then syslog-ng PE stops the source (flow-control mechanism).

Figure 38: Flow control, normal disk buffering, no ALTP

How to set key parameters

Set flags(flow-control) in the log path.

The memory buffer must be large enough to store the incoming messages of every source:

mem-buf-length() > sum of log-iw-size() of sources connected to this destination

Configure the disk-buffer option. For details, see Example: Example configuration of the normal disk-buffer option.

disk-buffer(
	mem-buf-length(20000) # storing 20000 messages in memory, sum log-iw-size of sources should be less than 20000 to use flow-control
	disk-buf-size(2147483648) # storing 2 GB of messages on disk
	reliable(no)
)

Benefits

This configuration minimizes the loss of log messages in the following situations:

• Unreachable destination server(s): Only as many incoming log messages are read as can be "delivered". When flow control is used in combination with disk buffering, messages that have been written to the disk-buffer file and/or the memory buffer are considered delivered. When the memory buffer becomes full, syslog-ng PE stops reading messages from the configured sources. This means that no log messages get lost.
• Message loss outside of syslog-ng PE: The greatest advantage of this configuration over when the disk-buffer option is not used at all is that when the log-iw-size() control window is full, the flow-control mechanism stops reading logs from the sources much later. This is because when it is not possible to send logs directly to the destinations, they are first written to the disk and then the memory buffer. It is only after both the disk-buffer file and the memory buffer have been filled to their full capacity that the sources are stopped. This enables you to minimize the loss of log messages during peak hours or when the network is temporarily down.

• Message loss when syslog-ng PE is stopped or restarted: When syslog-ng is stopped or restarted, the contents of the memory buffer and the disk-buffer file are flushed to disk, meaning that no log loss occurs.

  NOTE: In rare cases, the buffers stored on the disk can become corrupted, in which case syslog-ng PE may not able to process all the logs stored in the disk-buffer file.

Drawbacks

One drawback of using the disk-buffer option is that the processing of log messages by syslog-ng PE is slower.

This configuration does not provide protection against the loss of log messages in the following situations:

• TCP error: With a TCP connection, when messages are sent from the destination drivers to the destination servers, messages are written to the TCP socket. The TCP socket sends an acknowledgment to the destination drivers once it has successfully processed messages. A message is considered "delivered" when no error occurs during the process of writing the data to the socket, and the acknowledgment is received. Note, however, that if something goes wrong after messages have been successfully written to the TCP socket, log messages can still get lost. Also note that TCP errors can occur on both the source and the destination side, and both can cause the loss of log messages.
• When syslog-ng PE is not able to operate normally (for example, when syslog-ng PE crashes due to some unforeseen event): Log messages that were in the output buffer when the issue occurred get lost because those messages are stored in the memory.