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From Paul Davis <paul.joseph.da...@gmail.com>
Subject Re: Understanding CouchDB stats
Date Thu, 22 Oct 2009 21:46:01 GMT
>   1. What exactly is 'current' & 'count'. What are each of them recording
>   and why is 'count' less than 'current' in my system (seems counter intuitive
>   to me).

Current is a sum of the values recorded. For things like HTTP
requests, this is the total number of requsts.

Count is the number of updates for this metric in the given time span.

Stats works with two parts, a collector and an aggregator. The
collector part receives messages from through out CouchDB and holds
that data in a table. Once a second the aggregator will sweep through
the collector and update its stats.

So, if you had 20K requests between to aggregator sweeps, Current
would be incremented by 20K and Count is incremented by 1.

>   2. Can the 'mean' here be interpreted as average reads per second?

For requests, the mean is roughly the requests per second. Its not as
theoretically correct as something like RRDtool because we don't
interpolate, we just average the reads we take roughly once a second.

>   3. Is there any indication of exactly where within the 5 minute interval
>   we are?
>

No, but the current implementation (committed after 0.10.x was
branched) does not reset statistic aggregators as the old code did.
The new method is the more standard "these stats reflect all values
seen in the last 5 minutes" regardless of when you query it.

> Another metric that I'm having trouble with is the 'request_time'. Querying
> it returns data similar to:

[snip]

>   1. Again, same question about exactly what the 'count' and 'current'
>   values mean for this metric.

Oh weird. So, Count has the same meaning as before, but here Current
is the length of the last recorded request. The weirdness comes from
the fact that this is averaging a set of distinct points, where as
things like requests are averaging the relative change so current
makes a bit more sense there.

>   2. Does the 'mean' represent the average time for a request in CouchDB in
>   seconds?

milliseconds.

HTH,
Paul Davis

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