[syslog-ng] disk based buffering

Szalai, Attila Attila.Szalai at morganstanley.com
Wed Sep 28 13:51:39 CEST 2016

The trick is (if I remember correctly) that even in reliable disk buffer, the content of the buffer is not _flushed_ to the disk with every write. It is (and this is maybe only in my fading memories) a memory mapped file and that is the kernel responsibility to synchronize it. This is why it not safe to kernel crashes. But if you stop the syslog-ng, the content should appear in the queue.

From: syslog-ng-bounces at lists.balabit.hu [mailto:syslog-ng-bounces at lists.balabit.hu] On Behalf Of Scheidler, Balázs
Sent: Wednesday, September 28, 2016 12:48 PM
To: Fabien Wernli; Syslog-ng users' and developers' mailing list
Subject: Re: [syslog-ng] disk based buffering

log-qout-size() controls the size of the memory queue that sits next to the disk buffer, and it defaults to 64 elements. However it shouldn't apply to reliable disk buffers (e.g. reliable() setting), because in that case everything touches the disk before syslog-ng would be sending them on.
Obviously reliable(yes) is a lot slower.


On Wed, Sep 28, 2016 at 9:02 AM, Fabien Wernli <wernli at in2p3.fr<mailto:wernli at in2p3.fr>> wrote:
On Wed, Sep 28, 2016 at 03:02:19AM +0200, thejaguar at tutanota.de<mailto:thejaguar at tutanota.de> wrote:
> So with dqtool ( cool) I noticed that actual problem is that syslog-ng is not writing to disk queue immediately when a message is generated.  The message goes to usual place like var/log/message etc but not in the queue where it should because it has not been transmitted to remote host.  Now I am thinking there has to be a memory limit before it starts flushing/dumping unsent message to disk queue ? I tried mem-buf-size(1) and log-fifo-size(1) but none of them works. Is it a static value or is it configurable ?

If I'm not mistaken the message will go to the destination's memory queue
first; if the latter is full, only then will it make it to the diskq.

You can check the status of the queue using the control socket
(syslog-ng-ctl stats)

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