[collectd] collectd+rrdcached

[Bruno Prémont]

On Mon, 13 Aug 2012 12:34:56 Jeremy MAURO <jmauro at antidot.net> wrote:
> Hi everyone,
> 
> 
> I have a quick question for everybody, does anyone use colletcd+rrdcached?
> 
> 
> Because I tried to setup the rrdcached pluding to stop using rrdtools 
> plugin but when I do so the server is just under heavy IO load. Normally 
> with rrdtool plugin the load is about 2-3 (See the attachment).
> 
> 
> But when I try to activate the rrdcached the server is just unusable, 
> the load is about 25 (for a 16 CPUs) but I have something like 30% of IO 
> wait.
> 
> 
> Everything is on debian squeeze: 
> collectd(4.10.1-1+squeeze2)/rrdcached(1.4.3-1)
> 
> 
> 
> Any idea why something that is supposed to reduce load is dramatically 
> increasing it?

The caching parameters have to be properly configured on rrdcached side
(with rrdtool plugin it's the plug itself that does the caching).

When starting rrdcached, set parameters appropriate for your system,
e.g. -w, -z (the larger the values, the lower will IO load be).

Note that having a hardware RAID controller with battery-backed
write-cache and enough RAM will help keep IO load low.

rrdcached documentation:
  http://oss.oetiker.ch/rrdtool/doc/rrdcached.en.html

Naturally, it's prohibitive to pre-render all your graphs at fixed
intervals as that would kill the advantage of rrdcached!

Bruno