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https://issues.apache.org/jira/browse/CASSANDRA-18093?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17643413#comment-17643413
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Branimir Lambov commented on CASSANDRA-18093:
---------------------------------------------
This is an example of the allocation {{generatetokens}} is currently
constructing for the second nodes in a rack:
{code}Replicated node load in rack=0 after allocating node 3: max 1.33 min 0.56
stddev 0.0370.
Replicated node load in rack=1 after allocating node 4: max 1.67 min 0.70
stddev 0.0288.
Replicated node load in rack=2 after allocating node 5: max 1.16 min 0.84
stddev 0.0064.
{code} and this is what it should be {code}Replicated node load in rack=0 after
allocating node 3: max 1.06 min 0.94 stddev 0.0052.
Replicated node load in rack=1 after allocating node 4: max 1.06 min 0.94
stddev 0.0052.
Replicated node load in rack=2 after allocating node 5: max 1.06 min 0.94
stddev 0.0052.
{code}
The second result was generated after applying this patch:{code}diff --git
a/src/java/org/apache/cassandra/dht/tokenallocator/TokenAllocation.java
b/src/java/org/apache/cassandra/dht/tokenallocator/TokenAllocation.java
index 184332907b..84ccb95ff4 100644
--- a/src/java/org/apache/cassandra/dht/tokenallocator/TokenAllocation.java
+++ b/src/java/org/apache/cassandra/dht/tokenallocator/TokenAllocation.java
@@ -239,7 +239,8 @@ public class TokenAllocation
private StrategyAdapter getOrCreateStrategy(String dc, String rack)
{
- return strategyByRackDc.computeIfAbsent(dc, k -> new
HashMap<>()).computeIfAbsent(rack, k -> createStrategy(dc, rack));
+ return createStrategy(dc, rack);
+// return strategyByRackDc.computeIfAbsent(dc, k -> new
HashMap<>()).computeIfAbsent(rack, k -> createStrategy(dc, rack));
}
private StrategyAdapter createStrategy(String dc, String rack)
@@ -281,8 +282,11 @@ public class TokenAllocation
// group by rack
return createStrategy(strategy.snitch, dc, null, replicas, true);
}
- else if (racks == 1)
+ else if (racks == 1 ||
topology.getDatacenterEndpoints().get(dc).size() < replicas)
{
+ // One rack, each node treated as separate.
+ // This is also used as a fallback when number of nodes is lower
than the replication factor
+ // (where allocation is done randomly).
return createStrategy(strategy.snitch, dc, null, replicas, false);
}
{code}
> `generatetokens` uses the wrong allocation strategy
> ---------------------------------------------------
>
> Key: CASSANDRA-18093
> URL: https://issues.apache.org/jira/browse/CASSANDRA-18093
> Project: Cassandra
> Issue Type: Bug
> Reporter: Branimir Lambov
> Priority: Normal
>
> When the number of racks is the same as the replication factor, token
> allocation should use `NoReplicationTokenAllocator`. Because `generatetokens`
> caches the allocation strategy
> [here|https://github.com/apache/cassandra/commit/2346ed8241022882e77433e283ab8ce3004d12b0#diff-a93e5fa817f4b6d64484e0c28391b76e1760f51860de7f4f036470766ff5090cR225],
> it uses one that was created before all the racks are defined and does not
> switch to the correct one.
> This has the effect of constructing very poor allocations for the racks = RF
> case, especially for the first couple of nodes per rack.
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