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From Oleksandr Shulgin <oleksandr.shul...@zalando.de>
Subject Re: New token allocation and adding a new DC
Date Wed, 17 Jan 2018 17:26:30 GMT
On Wed, Jan 17, 2018 at 4:21 AM, kurt greaves <kurt@instaclustr.com> wrote:

> I believe you are able to get away with just altering the keyspace to
> include both DC's even before the DC exists, and then adding your nodes to
> that new DC using the algorithm. Note you'll probably want to take the
> opportunity to reduce the number of vnodes to something reasonable. Based
> off memory from previous testing you can get a good token balance with 16
> vnodes if you have at least 6 nodes per rack (with RF=3 and 3 racks).
>

Alexander, Kurt,

Thank you for the suggestions.

None of them did work in the end, unfortunately:

1. Using auto_bootstrap=false always results in random token allocation,
ignoring the allocate_tokens_for_keyspace option.

The token allocation option is only considered if shouldBootstrap() returns
true:
https://github.com/apache/cassandra/blob/cassandra-3.0.15/src/java/org/apache/cassandra/service/StorageService.java#L790
if (shouldBootstrap()) {
https://github.com/apache/cassandra/blob/cassandra-3.0.15/src/java/org/apache/cassandra/service/StorageService.java#L842
  BootStrapper.getBootstrapTokens()  (the only place in code using the
token allocation option)
https://github.com/apache/cassandra/blob/cassandra-3.0.15/src/java/org/apache/cassandra/service/StorageService.java#L901
else { ...

2. Using auto_bootstrap=true and allocate_tokens_for_keyspace=data_ks gives
us balanced range ownership on the new empty DC.  The problem though, is
that rebuilding of an already bootstrapped node doesn't work: the node
believes that it already has all the data.

We are going to proceed by manually assigning a small number of tokens to
the nodes in new DC with auto_bootstrap=false and only use the automatic
token allocation when we need to scale it out.  This seems to be the only
supported way to use it anyway.

Regards,
--
Alex

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