[rrd-developers] rfc: later caching

Tobias Oetiker tobi at oetiker.ch
Fri Apr 17 08:38:10 CEST 2009

disclaimer: this is about 1.5 not 1.4 !

Yesterday I was giving an rrdtool talk at goOpen 2009 a question in
the talk lead to a discussion about the design of the caching
model. The question was, why we were caching at the 'input' stage
and not at the output stage, since the writing to disk is what is
hurting us ...

What came out of the discussion is this:

* the current rrdcached is sitting quite
  close to the beginning of the rrd data path.

* the advantage of this is, that it is quite simple to hook it
  up there.

* the (big) disadvantage is that updatev does not work anymore, and
  for larger deployments updatev is a cornerstone function in
  driving holt winters based alerting.

* moving the cached to a place right before the updates get written
  into the rrd file would solve this problem.

* it would require the cached to read the header information of the
  rrdfiles once and cache them internally so that it can calculate
  the updates without accessing the disk, but since header
  information is quite small, a decent sized machine could
  easily keep hundreds of thousands of headers in the cache daemon.

comments ?


Tobi Oetiker, OETIKER+PARTNER AG, Aarweg 15 CH-4600 Olten, Switzerland
http://it.oetiker.ch tobi at oetiker.ch ++41 62 775 9902 / sb: -9900

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