[jira] [Commented] (CASSANDRA-6758) Measure data consistency in the cluster

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

    [ https://issues.apache.org/jira/browse/CASSANDRA-6758?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13910212#comment-13910212 ] 

Benedict commented on CASSANDRA-6758:
-------------------------------------

This doesn't seem like a bad idea at all. The only problem that I can see is that the "first half" of the repair process is actually one of the more expensive actions a cluster can perform, as the entire cluster needs to walk all of its data to compute its merkle tree. I wonder if it would be possible to calculate and save an abbreviated merkle tree when writing each sstable, that could be combined cheaply to give this answer.

> Measure data consistency in the cluster
> ---------------------------------------
>
>                 Key: CASSANDRA-6758
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-6758
>             Project: Cassandra
>          Issue Type: New Feature
>            Reporter: Jimmy Mårdell
>            Priority: Minor
>
> Running multi-DC Cassandra can be a challenge as the cluster easily tends to get out-of-sync. We have been thinking it would be nice to measure how out of sync a cluster is and expose those metrics somehow.
> One idea would be to just run the first half of the repair process and output the result of the differencer. If you use Random or the Murmur3 partitioner, it should be enough to calculate the merkle tree over a small subset of the ring as the result can be extrapolated.
> This could be exposed in nodetool. Either a separate command or perhaps a dry-run flag to repair?
> Not sure about the output format. I think it would be nice to have one value ("% consistent"?) within a DC, and also one value for every pair of DC's perhaps?



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