[jira] [Commented] (CASSANDRA-7331) Improve Droppable Tombstone compaction

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

    [ https://issues.apache.org/jira/browse/CASSANDRA-7331?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14017196#comment-14017196 ] 

Jonathan Ellis commented on CASSANDRA-7331:
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/cc [~krummas] [~yukim]

> Improve Droppable Tombstone compaction
> --------------------------------------
>
>                 Key: CASSANDRA-7331
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-7331
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>            Reporter: sankalp kohli
>            Priority: Minor
>
> I was thinking about this idea so creating a JIRA to discuss it. 
> Currently we do compaction for stables which have more than configurable number of droppable tombstones. 
> Also there is another JIRA CASSANDRA-7019 to do compactions involving multiple stables from different levels which will be triggered based of same threshold. 
> One of the areas of improvement here to pick better candidates will be to find out if a tombstone can actually get rid of data in other stables. 
> We can add a byte to tombstone to keep track of whether it has knocked off the actual data(for which it is there) or not. 
> All tombstones will start out with 0 as its value. When it compacts with other stables and causes data to be deleted, it will be incremented. 
> For cases where there are multiple updates and then a delete, this value can be more than 1 depending on how many updates came in before delete. 
> If we have this, by looking at these numbers in tombstones, we can find a stable which by compacting, we will get rid of maximum data. We can also add a global number per stable which sums up these numbers. 
> I am not sure how this will work with range tombstones and whether this will be useful.   



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