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From "Cyril Scetbon (JIRA)" <j...@apache.org>
Subject [jira] [Commented] (CASSANDRA-7731) Average live/tombstone cells per slice
Date Wed, 13 Aug 2014 05:38:12 GMT

    [ https://issues.apache.org/jira/browse/CASSANDRA-7731?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14095171#comment-14095171
] 

Cyril Scetbon commented on CASSANDRA-7731:
------------------------------------------

[~snazy] in the first part, you're totally right about the use of exponentially decaying reservoirs.
I didn't see that at first. Cool. Yeah, you could use a better name for variables but as long
as it does what it should that's fine for me :) The message is clear in CFSTAT about this.
Renaming them to be clearer could help developers too.
For the second part, that's a yes. I think we really need to know the last max for live and
tombstone cells number of reads. We hit 2 development bugs related to this and monitoring
that could really help ! So using 2 more histograms (with biased=true) for those counters
 should help and is a must have.

Can you just confirm that calling CFSTAT does not reset internal counters at the end of the
call ? I understand that for the histograms above it doesn't, but what about the others ?

> Average live/tombstone cells per slice
> --------------------------------------
>
>                 Key: CASSANDRA-7731
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-7731
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>            Reporter: Cyril Scetbon
>            Assignee: Robert Stupp
>            Priority: Minor
>
> I think you should not say that slice statistics are valid for the [last five minutes
|https://github.com/apache/cassandra/blob/cassandra-2.0/src/java/org/apache/cassandra/tools/NodeCmd.java#L955-L956]
in CFSTATS command of nodetool. I've read the documentation from yammer for Histograms and
there is no way to force values to expire after x minutes except by [clearing|http://grepcode.com/file/repo1.maven.org/maven2/com.yammer.metrics/metrics-core/2.1.2/com/yammer/metrics/core/Histogram.java#96]
it . The only thing I can see is that the last snapshot used to provide the median (or whatever
you'd used instead) value is based on 1028 values.
> I think we should also be able to detect that some requests are accessing a lot of live/tombstone
cells per query and that's not possible for now without activating DEBUG for SliceQueryFilter
for example and by tweaking the threshold. Currently as nodetool cfstats returns the median
if a low part of the queries are scanning a lot of live/tombstone cells we miss it !



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