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https://issues.apache.org/jira/browse/CASSANDRA-4658?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=13454159#comment-13454159
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Nick Bailey commented on CASSANDRA-4658:
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We could potentially say that rack awareness and vnodes are incompatible. Which
will essentially mean that cassandra can't guarantee that losing a single rack
won't bring down an entire replica set. That makes the problem slightly easier,
but still requires some sort of clear upgrade path for people on NTS with
racks. Changing the replication strategy or changing the snitch to report
different racks won't cause data to be streamed to any new owners. So we would
need to come up with a mechanism for updating without data loss.
> NTS/RackAwareness doesn't work with vnodes
> ------------------------------------------
>
> 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: 1.2.0
>
>
> 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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