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Split the old shard [1] This will split the shard in two on the existing node. Note you need enough disk space. https://solr1/admin/collections?action=SPLITSHARD&collection=name&shard=shardID&async=9999
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Poll for completion [2] https://solr1/admin/collections?action=REQUESTSTATUS&requestid=9999
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Move one of the replicas to the other host (solr2) [3] https://solr1/admin/collections?action=MOVEREPLICA&collection=name&shard=shardID&replica=replicaID&sourceNode=solr1:8983_solr&targetNode=solr2:8983_solr&async=8888
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Poll for completion [2] https://solr1/admin/collections?action=REQUESTSTATUS&requestid=8888
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Remove old, inactive replica [4] https://solr1/admin/collections?action=DELETEREPLICA&collection=name&shard=shardID&replica=replicaID
[1] https://solr.apache.org/guide/8_4/shard-management.html#splitshard
[2] https://solr.apache.org/guide/8_4/collections-api.html#requeststatus
[3] https://solr.apache.org/guide/8_4/replica-management.html#movereplica
[4] https://solr.apache.org/guide/8_4/replica-management.html#deletereplica