Indexing performance degraded after elastic 2.3 index upgraded to 5.6

Elastic Stack Elasticsearch
1

I had a cluster which was running in Elastic 2.3 for a while, we had plans to upgrade to 5.6 and to 6 eventually.
We upgraded the cluster to 5.6 using the full cluster restart method.

After the upgrade the index that was created in 2.3 was restored in 5.6.

There is a performance degrade in indexing speed for the index that was created in 2.3 and running in 5.6.
The index mapping had few "string" data types.I read that this data type is not used in 5.6, but is supported for the indices created in 2.3. But can this be a reason for slow indexing speed ?

I have created a new index with 5.6[ keyword data type] and try to load documents at the same rate and this works absolutely fine.

Is there anything else that we have to take into consideration that might effect 2.3 index running in 5.6 ?

Does index size anyway effect indexing speed in 5.6.The same index is still doing good in 2.3

2

Sorry that I cannot directly answer your 'why' questions, but I'd say that it isn't worth asking.

I think the only reason new ES versions support old version indices is purely so you can reindex them once you've upgraded.

  • running in 2.3
  • upgrade to 5.6, continuing to run the original 2.3 indices
  • create analogous 5.6 mapping (only changes being syntax related, like string -> text/keyword, etc)
  • Reindex directly from 2.3 indices to the new 5.6 mapping, to never have to deal with the old 2.3 indices again

See https://www.elastic.co/guide/en/elasticsearch/reference/current/docs-reindex.html

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