[collectd] meta and rrdcached

Florian Forster octo at verplant.org
Wed Oct 15 18:53:22 CEST 2008

Welcome back Oleg :)

On Mon, Oct 13, 2008 at 10:05:28PM +0400, Oleg King wrote:
> 1. in 4.5.0 "The concept of "notification meta data" has been
> introduced.". What is it?

There are two separate, but related, concepts in collectd: Notifications
and thresholds. The details are explained here:
The gist is: ``Notifications'' are messages together with a timestamp
and a severity. ``Thresholds'' are limits for messages. Of course, if a
value is out of bounds a ``Threshold'' can create a ``Notification''.

That notifications are simple strings is on purpose and you should *not*
parse those strings to get information. They are meant for the user and
can mean a lot of things, not just threshold stuff. For example, thanks
to a patch by Peter Holik (iIrc) the IPMI plugin can generate such a
notification if a power supply is removed or added.

However, you may still want to automatically process notifications, for
example to check for SLA violations or decide whether or not to wake the
admin via phone call or something.

This is where ``notification meta data'' comes into play: It is data
associated to the message, but it has a well defined key-value
structure. So you don't need to parse the message to get the information
you need - if provided by the source that generated the notification.

So, for example, the threshold checking code will include the following
data in machine-readable format:
With this data it should be possible to see what is going on or to
compose a (shorter) text message that is to be sent to a mobile phone..

Hope this explains it in a bit more detail..?

On Wed, Oct 15, 2008 at 09:49:06AM +0200, Sebastian Harl wrote:
> > 2. I have found rrdcached plugin source in source tree. What is it?
> > How to use it?
> RRDtool 1.4 (which is currently in development, I don't know of any
> ETA) will ship a daemon called RRDcacheD which basically is a generic
> implementation of the caching mechanism implemented in the "rrdtool"
> plugin plus some further enhancements (which might be "backported" to
> the rrdtool plugin as well some time). See [1] for further details.

One of the cool aspects is, that you can restart collectd without
writing all files to disk first, because they are cached in another

The general structure is basically the same as the one documented here:
However, Kevin Brintnall has implemented some cool additions, such as

Florian octo Forster
Hacker in training
GnuPG: 0x91523C3D
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