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https://issues.apache.org/jira/browse/LUCENE-9234?page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel&focusedCommentId=17042087#comment-17042087
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Adrien Grand commented on LUCENE-9234:
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I think this option is appealing because it doesn't require direct trade-offs
from the users, but it definitely has a big maintenance/test cost.
Thanks [~tflobbe] for checking!
> Keep write support for old codecs?
> ----------------------------------
>
> Key: LUCENE-9234
> URL: https://issues.apache.org/jira/browse/LUCENE-9234
> Project: Lucene - Core
> Issue Type: Wish
> Reporter: Adrien Grand
> Priority: Minor
>
> Currenty we maintain read/write support for the latest codec in lucene/core,
> and read-only support for codecs of previous versions (up to \{N-1\}.0}) in
> lucene/backward-codecs. We often keep write support in test-framework for
> testing purposes only.
> This raises challenges for Elasticsearch with regard to rolling upgrades: we
> have some users who index very large amounts of data on clusters that are
> quite large, so that rolling upgrades take significant time. Meanwhile,
> several indices may be created.
> Allocating indices when the cluster has nodes of different versions requires
> care as Lucene indices created on nodes with a newer version cannot be read
> by the nodes running the older version. It is possible to force primary
> replicas to be allocated on the older nodes, but this brings other problems
> like availability, uneven disk usage across nodes, or moving a lot of data
> around.
> If Lucene could write data using the minimum version that exists in the
> cluster, this would avoid this problem as the written data could be read by
> any node of the cluster. I understand this change would not come for free,
> especially when it comes to testing as we'd need to make sure that older
> Lucene versions can read indices created by this "compatibility mode".
> I'd be curious to understand whether this is a problem for Solr too, if not
> how this problem is being handled, and maybe whether there are other problems
> that you have encountered that would also benefit from the ability to write
> data with an older format.
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