[rrd-users] buffers flush every midnight

Tobias Oetiker tobi at oetiker.ch
Thu Oct 30 09:39:21 CET 2008


Today Maxim Kuleshov wrote:

> Hi!
> In self-developed monitoring system I use rrd to manage counters data.
> There are tens of thousands of device ports and thus - the same number
> of .rrd files. Most of them are updated every 5 minutes.
> For the performance optimization I had to tune some of vm.dirty_*
> parameters, so with reasonable amount of RAM (used for cache) all runs
> smoothly.
> But every GMT midnight (local time - 04:00 MSD or 03:00 MSK) system
> runs very slowly for about 15 minutes. Analysis of collected performance
> data shows that 'Buffers' value greatly decreases during that period
> (and then again - increase). So, I guess for unknown reason kernel
> flushes all buffered data, therefore large amount of disk I/O occurs.
> I have a suspicion that the problem relates to rrd update mechanism.
> May be once a day during rrdupdate additional work is performed,
> extra file system read/write occurs or something like this?

if  you have an RRA storing daily values, then this is bound to get
updated at midnight GMT ...

this will use more memory as more diskblocks go to cache
and it will cause a lot of disk activity as the data blocks get
read in for the updates.


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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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