[rrd-users] Re: how to merge 2 rrd-files

McGlinchy, Alistair Alistair.McGlinchy at marks-and-spencer.com
Wed Jun 9 20:43:15 MEST 2004

Alex et al,

> > Example 1:
> > You are running MRTG using 32bit SNMP counters and you want 
> to switch 
> > to 64 SNMP counters.  If you just change your configs you will get 
> > enormous spikes in each graph as the 64 bit counters will be much 
> > bigger than the 32 bit, but may still be valid data for the RRD. My 
> > solution is to run MRTG with 64 bit counters and have it create a 
> > brand new RRD. Then you can merge the old RRD data from 32 bit 
> > collections into the new RRD.
> Step 1:  stop updating using the 32-bit counter
> Step 2:  insert NaN in the stream (rrdtool update ... N:U)
> Step 3:  start updating using the 64-bit counter

Good idea! I hadn't realised that explicitly setting U would reset the
last value. I had assumed it was a null-op. That'll save some grunt work
... cheers!

> > Example 2:
> > Your collection server crashes and you are in a mad panic. 
> You start 
> > up collections on a separate server while you fix your 
> server.  What 
> > do you do with the data collected on the backup server when your 
> > normal collection box is fixed...?
> Step 1: update both databases simultaneously (so that they're 
> in sync) Step 2: a:  rrdtool dump database1.rrd > database1.xml
>         b:  rrdtool dump database2.rrd > database2.xml
> Step 3: copy interesting parts from both xml files into a 
> third Step 4: rrdtool restore newdatabase.xml newdatabase.rrd

Um... this is exactly what the perl script I posted does. Note that for
the standard MRTG RRD "interesting" means 8 different places with some
non-trivial alignment issues. That's not something I want to do manually
with even one RRD file let alone the other 17000 RRDs that are updated
every 5min on our production server. I need a tool that can re-merge in
the old data without disrupting live collections.

> This is the most complex solution.  Then again, how often are 
> you going to make this mistake?

I still have to plan for what I will do if/when my collection server
dies.  What did happen that was almost as bad was when a sysadmin
accidentally increased the system clock by a month on a Saturday I came
in on a Monday and had flat-lines for a month on all my graphs. I would
be interested to hear suggestions on the most effective way to tidy up
from that.




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