[mrtg] MRTG with rrdcached

Steve Shipway s.shipway at auckland.ac.nz
Tue Sep 14 03:09:23 CEST 2010

So far, from my investigations, it seems that you can make MRTG/rrdcached work, but there are a few important points -

1.       It will not work at all via TCP, only via unix sockets since create and info are not supported by rrdcached, and MRTG tests for existence of the rrd file by looking for it in the filesystem.

2.       You need to modify the mrtg code about line 894 to so that updatev is not used (as rrdcached doesn't support updatev)
    if ( $RRDs::VERSION >= 1.2 and ($RRDs::VERSION <1.4 or !$ENV{RRDCACHED_ADDRESS})){

3.       If you make the updatev check above, then you also kill off any thereshold checking unless you make a similar change lower down, which would force cache flushing and remove much of the cache benefit.  Fortunately, we don't use thresholding.

4.       Since the web frontend doesn't query via the cache (since rrdcached doesn't support graph, fetch or info) you can end up with displayed data lagging behind slightly.

Seems that in order for this to work smoothly, we need the rrdcached code to be improved to support create, info,fetch and preferably tune; and also the MRTG code to be enhanced to be rrdcached-aware.  Possibly rrdcached could be modified to fork off an rrdtool process to handle the difficult things like 'create', though 'info' and 'fetch' would probably need to be done internally.

I've noticed there was some discussion on this in the rrdtool mailing lists back in Jan/Feb this year, but nothing more since.


Steve Shipway
ITS Unix Services Design Lead
University of Auckland, New Zealand
Floor 1, 58 Symonds Street, Auckland
Phone: +64 (0)9 3737599 ext 86487
DDI: +64 (0)9 924 6487
Mobile: +64 (0)21 753 189
Email: s.shipway at auckland.ac.nz<mailto:s.shipway at auckland.ac.nz>
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