[collectd] On rrdtool CacheTimeout

[Toni Ylenius]

On Thursday 07 July 2011 07:15, Trent W. Buck wrote:
> I rolled out collectd 4.8, running in each of about 20 LXC jails. 
> The I/O load was crippling the server, so I reduced polling with
> "Interval 60" in each jail.
>
> However I would prefer to
>
>  - poll every 10s (the default);
>
>  - batch write RRDs, such that any given RRD is only written once
>    every ten minutes; and
>
>  - distribute these writes more-or-less evenly over time, i.e. avoid
> a huge I/O spike on the tenth minute.
>
> IIUC that means I should use this config:
>
>     Interval      10
>     CacheTimeout  600
>     RandomTimeout 300
>
> But this doesn't seem to be improving matters.  Polling dm-25 (the
> filesystem mounted at /var/lib/collectd) every ten seconds, before
> the change:
>
>     Device:            tps   Blk_read/s   Blk_wrtn/s   Blk_read  
> Blk_wrtn dm-25           163.70        16.80      1292.80        168 
>     12928 dm-25            88.90         4.00       707.20         40
>       7072 dm-25           279.20         8.80      2224.80        
> 88      22248 dm-25           302.40         8.80      2410.40       
>  88      24104 dm-25           122.40         7.20       972.00      
>   72       9720 dm-25           386.90         7.20      3088.00     
>    72      30880
>
> After the change:
>
>     Device:            tps   Blk_read/s   Blk_wrtn/s   Blk_read  
> Blk_wrtn dm-25            33.20        11.20       254.40        112 
>      2544 dm-25           292.10        12.00      2324.80        120
>      23248 dm-25           297.50        15.20      2364.80       
> 152      23648 dm-25            41.90         8.80       326.40      
>   88       3264 dm-25           428.30         4.00      3422.40     
>    40      34224 dm-25           283.60         4.80      2264.00    
>     48      22640
>
> Is that wrong, or am I just expecting too much of buffered rrd
> writes?
>
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Hi,

I have been using the rrdcached plugin to force writes only every few 
minutes. However I think that rrdcached should also help balancing 
write load.

Maybe someone else has better understanding about rrdcached.

Toni