[jira] [Commented] (CASSANDRA-4658) Explore improved vnode-aware replication strategy

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

    [ https://issues.apache.org/jira/browse/CASSANDRA-4658?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13504670#comment-13504670 ] 

Jonathan Ellis commented on CASSANDRA-4658:
-------------------------------------------

changed title to reflect most recent understanding that NTS is probably adequate for now.
                
> Explore improved vnode-aware replication strategy
> -------------------------------------------------
>
>                 Key: CASSANDRA-4658
>                 URL: https://issues.apache.org/jira/browse/CASSANDRA-4658
>             Project: Cassandra
>          Issue Type: Bug
>    Affects Versions: 1.2.0 beta 1
>            Reporter: Nick Bailey
>             Fix For: 1.2.1
>
>
> It doesn't look like the current vnode placement strategy will work with people using NTS and multiple racks.
> For reasons also described on CASSANDRA-3810, using racks and NTS requires tokens to alternate racks around the ring in order to get an even distribution of data. The current solution for upgrading/placing vnodes won't take this into account and will likely generate some hotspots around the ring. 
> Not sure what the best solution is. The two immediately obvious approaches appear to be quite complicated at first.
> * Fixing NTS to remove the requirement for rack ordering
> ** No idea how this would be accomplished
> ** Presents challenges for people upgrading. Would need to deprecate NTS for a new strategy that replaces it, then have a clear upgrade path to that strategy which would need to be in a pre 1.2 release.
> * Changing vnode placement strategy
> ** Ordering vnodes would require quite a bit of additional logic. Adding a new node or rack would also need to maintain ordering which could cause enough data movement to remove any benefits vnodes have already.
> ** We could potentially adjust vnode token placement to offset any imbalances caused by NTS but this would require a fairly intelligent mechanism for determining vnode placement. 

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