Andy Bristol 7e3cd6a019 reindex: automatically choose the number of slices (#26030)
In reindex APIs, when using the `slices` parameter to choose the number of slices, adds the option to specify `slices` as "auto" which will choose a reasonable number of slices. It uses the number of shards in the source index, up to a ceiling. If there is more than one source index, it uses the smallest number of shards among them.

This gives users an easy way to use slicing in these APIs without having to make decisions about how to configure it, as it provides a good-enough configuration for them out of the box. This may become the default behavior for these APIs in the future.
2017-08-11 08:25:25 -07:00
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