Today we allow to shrink to 1 shard but that might not be possible due to
too many document or a single shard doesn't meet the requirements for the index.
The logic can be expanded to N shards if the source index shards is a multiple of N.
This guarantees that there are not hotspots created due to different number of shards
being shrunk into one.
Today we use `index.routing.allocation.include._id` to filter the allocation
for the shrink target index. That has the sideeffect that the user has to
delete that setting / change it once the primary has been recovered (shrink is done)
This PR adds a dedicated filter that can only be set internally that only filters
allocation for unassigned shards.
This adds a low level primitive operations to shrink an existing
index into a new index with a single shard. This primitive expects
all shards of the source index to allocated on a single node. Once the target index is initializing on the shrink node it takes a snapshot of the source index shards and copies all files into the target indices data folder. An [optimization](https://issues.apache.org/jira/browse/LUCENE-7300) coming in Lucene 6.1 will also allow for optional constant time copy if hard-links are supported by the filesystem. All mappings are merged into the new indexes metadata once the snapshots have been taken on the merge node.
To shrink an existing index all shards must be moved to a single node (one instance of each shard) and the index must be read-only:
```BASH
$ curl -XPUT 'http://localhost:9200/logs/_settings' -d '{
"settings" : {
"index.routing.allocation.require._name" : "shrink_node_name",
"index.blocks.write" : true
}
}
```
once all shards are started on the shrink node. the new index can be created via:
```BASH
$ curl -XPUT 'http://localhost:9200/logs/_shrink/logs_single_shard' -d '{
"settings" : {
"index.codec" : "best_compression",
"index.number_of_replicas" : 1
}
}'
```
This API will perform all needed check before the new index is created and selects the shrink node based on the allocation of the source index. This call returns immediately, to monitor shrink progress the recovery API should be used since all copy operations are reflected in the recovery API with byte copy progress etc.
The shrink operation does not modify the source index, if a shrink operation should
be canceled or if the shrink failed, the target index can simply be deleted and
all resources are released.
Glen Smith
Christoph Wurm
Simon Willnauer
Clinton Gormley