[jira] [Commented] (CASSANDRA-5850) change gc_grace_seconds default to 28 days

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

    [ https://issues.apache.org/jira/browse/CASSANDRA-5850?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=14036057#comment-14036057 ] 

Robert Coli commented on CASSANDRA-5850:
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For the record, my current view on this is :

- 34 days default for gc_grace_seconds
- start repair on the first of the month
- have a minimum of 3 and maximum of 7 (leap year) days in which to complete repair
- only "burn" 3 to 7 days of runtime every 34, a better ratio than 3 days out of every 7

> change gc_grace_seconds default to 28 days
> ------------------------------------------
>
>                 Key: CASSANDRA-5850
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-5850
>             Project: Cassandra
>          Issue Type: Improvement
>          Components: Core
>    Affects Versions: 2.0 beta 2
>            Reporter: Robert Coli
>            Priority: Trivial
>         Attachments: gc_grace_seconds_to_2419200_seconds_aka_28_days.patch
>
>
> Current default for gc_grace_seconds is 10 days. Attached patch changes all instances of this 10 day default to 28 days. 
> Rationale :
> - 10 days is arbitrary, there is nothing special about the current value
> - human societies do not operate on cycles which are a multiple of 10 days, they operate on a cycle of 7 day weeks
> - operators must run repair once every gc_grace_seconds, and with typical data sizes (and compaction/streaming throttling) this might run for a significant fraction of 10 days
> - repair often fails, and detecting and working around that failure might also take a significant fraction of 10 days
> - repair is the heaviest operation one can run on a cassandra cluster and operators are therefore motivated to run it ~3x less frequently by default
> - the worst case impact is keeping data around for 18 days longer than the previous default, and this only occurs in CFs which actually take DELETE operation
> - 28 days is an even multiple of 7 days and easily comprehensible as a default time in which to schedule repair



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