Thanks Ian, the bug does seem to be very relevant to this issue. I'm not
sure what patches Jason got applied, but it seems like the perl script or
something like it could be adopted as part of gmetad to automatically adjust
the RRAs to the specified intervals if custom RRAs have not also been
specified. Specifying a custom interval is easy, specifying and
understanding RRAs is hard. In the meantime, I just went back to 15 second
intervals. :)
-Matt
_____
From: Ian Cunningham [mailto:[EMAIL PROTECTED]
Sent: Wednesday, May 16, 2007 2:57 PM
To: Matthew Chambers
Cc: ganglia-developers@lists.sourceforge.net
Subject: Re: [Ganglia-developers] About gmetad data source intervals
Matthew Chambers wrote:
I recently discovered the solution to a problem I was having and want some
more detail. I initially set up gmetad.conf with my data sources using
default intervals (15sec). I then decided I wanted 10sec intervals so I
changed the conf and restarted the service. I soon discovered that my hour
graphs looked like bar graphs. After much gnashing of teeth (and learning
about rrdtool, and help from #rrdtool), I discovered that the default RRAs
that were initially created by gmetad were dependent on the 15sec interval
(actually, >= 15 sec). In other words, the RRA for the hourly data was not
big enough to store 10 second intervals because it was created for 15 second
intervals; as a consequence, the next largest RRA was used, the daily RRA
which had ~24 times lower resolution! It seems to me that when gmetad
starts up, it should check the RRAs and should either warn that the existing
RRAs are too small or it should resize the RRAs automatically. Is this a
known issue?
Thanks,
Matt Chambers
Matt Chambers,
This specific issue is not captured to my knowledge, but that it would
happen is expected and can be explained by Bug 33
<http://bugzilla.ganglia.info/cgi-bin/bugzilla/show_bug.cgi?id=33> . I do
know of two things that will help you.
1. Jason wrote a perl
<http://bugzilla.ganglia.info/cgi-bin/bugzilla/attachment.cgi?id=19&action=v
iew> script that will tune and resize all of your RRD's.
2. You can specify your own RRA spec in gmetad.conf to store more data. Look
to rrdtool for better documentation on what the RRA spec means. The default
based on 15 seconds looks like this:
RRAs "RRA:AVERAGE:0.5:1:244" \ #one sample every 15 sec for 61 min
"RRA:AVERAGE:0.5:24:244" \ #one sample every 6 min for 24.4 hours
"RRA:AVERAGE:0.5:168:244" \ #one sample every 42 min for 7.11 days
"RRA:AVERAGE:0.5:672:244" \ #one sample every 168 min for 28.4 days
"RRA:AVERAGE:0.5:5760:374" #one sample every day for 374 days
(remove the comments and put on one line)
Cheers,
Ian C.
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