[collectd] On rrdtool CacheTimeout

[Mariusz Gronczewski]

2011/7/7 Trent W. Buck <twb-mailman-collectd at cyber.com.au>:
> 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?
>
How many RRD files do you have ?
You can try using WritesPerSecond option to limit amount of writes and
CacheFlush to ensure no data older than X will be in cache, like that:


        CacheFlush 1500
        WritesPerSecond 20



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