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https://issues.apache.org/jira/browse/CASSANDRA-4894?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13535852#comment-13535852
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Sylvain Lebresne commented on CASSANDRA-4894:
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I tend to think that the log is more suited to human readable information than
machine parseable ones (don't get me wrong, I'm sure lots of people do have
script to manipulate the log, I'm just not sure it's the idea tool for that,
especially considering we haven't really concerned ourselves with machine
parseability so far).
I do agree though that compaction stats is probably important enough that you
may want to monitor that closely and I would in fact be in favor of saving
compaction stats in a System table (much like we plan to save repair stats at
some point). That would be imo the good place to be machine friendly.
That being said, if you guys prefer the shorter format, and provided that at
the time of this ticket the log will be the only place where that information
will be available, I'm fine with that. But I do anticipate people asking what
those numbers means on irc quite a few time, and I do think having things
self-descriptive is a better user experience than having to go find
documentation somewhere on the net, even if said doc exists. I could be wrong
though and again, not a big deal in any case.
> log number of combined/merged rows during a compaction
> ------------------------------------------------------
>
> Key: CASSANDRA-4894
> URL: https://issues.apache.org/jira/browse/CASSANDRA-4894
> Project: Cassandra
> Issue Type: Improvement
> Components: Core
> Reporter: Matthew F. Dennis
> Assignee: Yuki Morishita
> Priority: Minor
> Fix For: 1.2.1
>
> Attachments: 4894-1.2.txt
>
>
> we already log some details about compactions but it would be useful to know
> how many rows were merged (resulting in "useful" work) and how many were
> unique (representing "wasted work").
> the simple approach requires two additional counters (one for unique rows,
> one for merged rows). As the merge join is progressing if two or more rows
> are combined, tick the joined counter. If a row is simply copied tick the
> unique counter.
> a more complete solution would be to keep a separate count for each number of
> merges. This would require number_of_files_being_merged counters. If no
> rows were merged, tick counters[0], if two rows were merged tick counters[1]
> and so on
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