[rrd-users] buffers flush every midnight

[Tobias Oetiker]

Maxim,

> On Thu, 30 Oct 2008 09:39:21 +0100 (CET)
> Tobias Oetiker <tobi at oetiker.ch> wrote:
>
> >
> > if  you have an RRA storing daily values, then this is bound to get
> > updated at midnight GMT ...
> >
>
> Yes, I have. Typical rrd created like this:
>
> 	DS:ds0:DERIVE:600:0:U
> 	DS:ds1:DERIVE:600:0:U
> 	RRA:AVERAGE:0.5:1:600
> 	RRA:AVERAGE:0.5:6:700
> 	RRA:AVERAGE:0.5:24:775
> 	RRA:AVERAGE:0.5:288:797
> 	RRA:MAX:0.5:1:600
> 	RRA:MAX:0.5:6:700
> 	RRA:MAX:0.5:24:775
> 	RRA:MAX:0.5:288:797
>
>
> > 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.
> >
>
> You seem to be right... Two-hour values computation is slightly slowed
> down too. Not that critical, but noticeably.
>
> One unexplained thing is that 'Buffers' values decreases, but not
> 'Cached'. As far as I know, in modern kernels 'Buffers' shows amount of
> RAM allocated for file-system metadata cache and actual content stored
> in 'Cached' memory.

if you are using a 1.3 version of rrdtool it will try to tell the
kernel to NOT cache the daily values since it will not be using
them for another 24h ...

cheers
tobi

>
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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