[rrd-developers] rrdcached + collectd issues

Thorsten von Eicken tve at voneicken.com
Wed Oct 14 05:07:45 CEST 2009

kevin brintnall wrote:
> On Mon, Oct 12, 2009 at 11:33:40AM -0700, Thorsten von Eicken wrote:
>> Thorsten von Eicken wrote:
>> - I'm wondering how we could overcome the RRD working set issue. Even
>> with rrdcached and long cache periods (e.g. I use 1 hour) it seems that
>> the system comes to a crawl if the RRD working set exceeds memory. One
>> idea that came to mind is to use the caching in rrdcached to convert
>> the random small writes that are typical for RRDs to more of a
>> sequential access pattern. If we could tweak the RRD creation and the
>> cache write-back algorithm such that RRDs are always accessed in the
>> same order, and we manage to get the RRDs allocated on disk in that
>> order, then we could use the cache to essentially do one sweep through
>> the disk per cache flush period (e.g. per hour in my case). Of course
>> on-demand flushes and other things would interrupt this sweep, but the
>> bulk of accesses could end up being more or less sequential. I believe
>> that doing the cache write-back in a specific order is not too
>> difficult, what I'm not sure of is how to make it such that the RRD
>> files get allocated on disk in the that order too. Any thoughts?
> Thorsten,
> Read back about a year in the archives.  Daniel Pocock 
> <Daniel.Pocock at barclayscapital.com> started some work on "striping" RRD
> files together..  i.e. you would have one large disk file that contained
> multiple RRD databases, where the rows were clustered on disk by time..
> I'm not sure how far he got on it.
Yeah, that sounds like a really exciting approach if you have a lot of 
time! I believe it's not too difficult if you don't implement RRDtools' 
notion of aggregation. But if do... good luck... I was looking for 
creative ways to get some disk sweeping without turning RRDtool upside 
down. Ha, maybe I should just use a FAT filesystem, that would make it 
easy to get a file -> block mapping :-).

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