[jira] [Commented] (CASSANDRA-4894) log number of combined/merged rows during a compaction

["Jonathan Ellis (JIRA)" <ji...@apache.org>]

    [ https://issues.apache.org/jira/browse/CASSANDRA-4894?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13534986#comment-13534986 ] 

Jonathan Ellis commented on CASSANDRA-4894:
-------------------------------------------

+1
                
> 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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