[collectd] [PATCH] Random write timeout for rrdtool plugin

Sebastian Harl sh at tokkee.org
Mon Aug 17 11:09:42 CEST 2009

Hi Mariusz,

On Mon, Aug 17, 2009 at 02:20:29AM +0200, Mariusz Gronczewski wrote:
> i was thinking how to "spread out" writes to rrd files a bit, because
> now its big spike every CacheTimeout or little smaller "square" on
> graph if u use WritesPerSecond. So ive written little patch which
> "spreads out" writing by changing Cache timeout every time rrdtool
> plugin finds data to save. Basically instead of moving data older than
> CacheTimeout to write queue it moves it if its older than CacheTimeout
> +- RandomTimeout.

Hrm … I'm not sure about the benefits from that. By using 'WritesPer-
Second' you should be able to limit I/O to some amount that's feasibly
for you hard-drive and thus you should be able to limit the impact on
your system to a reasonable value. I don't see any disadvantages from
those "square spikes" as long as the hight of those spikes is within
reasonable limits.

Anyway, OTOH, I think think it hurts to introduce some option like that.
However, I think we can get rid of that randomness. What you're basical-
ly trying to do is to adapt 'WritesPerSecond' to the number of files to
be updated and the value of 'CacheTimeout'. We don't need to approximate
that but we can even calculate exact numbers.

So, I'd suggest to do something along the following lines:

 * Introduce a boolean config option called, e.g. 'SpreadWritesPer-
   Second' (maybe someone can come up with a better name).
 * If that option is set, the current number of writes per second is
   determined from the cache size divided by the cache timeout but
   limited to 'WritesPerSecond'.

What do you think? Any comments?


Sebastian "tokkee" Harl +++ GnuPG-ID: 0x8501C7FC +++ http://tokkee.org/

Those who would give up Essential Liberty to purchase a little Temporary
Safety, deserve neither Liberty nor Safety.         -- Benjamin Franklin

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