bd7efa908a
This change adds a new `_split` API that allows to split indices into a new index with a power of two more shards that the source index. This API works alongside the `_shrink` API but doesn't require any shard relocation before indices can be split. The split operation is conceptually an inverse `_shrink` operation since we initialize the index with a _syntetic_ number of routing shards that are used for the consistent hashing at index time. Compared to indices created with earlier versions this might produce slightly different shard distributions but has no impact on the per-index backwards compatibility. For now, the user is required to prepare an index to be splittable by setting the `index.number_of_routing_shards` at index creation time. The setting allows the user to prepare the index to be splittable in factors of `index.number_of_routing_shards` ie. if the index is created with `index.number_of_routing_shards: 16` and `index.number_of_shards: 2` it can be split into `4, 8, 16` shards. This is an intermediate step until we can make this the default. This also allows us to safely backport this change to 6.x. The `_split` operation is implemented internally as a DeleteByQuery on the lucene level that is executed while the primary shards execute their initial recovery. Subsequent merges that are triggered due to this operation will not be executed immediately. All merges will be deferred unti the shards are started and will then be throttled accordingly. This change is intended for the 6.1 feature release but will not support pre-6.1 indices to be split unless these indices have been shrunk before. In that case these indices can be split backwards into their original number of shards. |
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