Deprecate and remove Multiple Data Paths

[rjernst]

Multiple Data Paths (MDP) is a pseudo-software-RAID-0 feature within Elasticsearch allowing multiple paths to be specified in the path.data setting (which usually point to different disks). Although it has been used in the past as a simple way to run a multi-disk setups, it has long been a source of user complaints due to confusing or unintuitive behavior. Additionally, the implementation is complex, and not well-tested nor maintained, with practically no benefit over spanning the data path filesystem across multiple drives and/or running one node for each data path.

We have long advised against using MDP, and are now ready to deprecate and remove it. This is a meta-issue to track that work.

• Deprecate MDP in 7.13
• Document migration path Document migration path off multiple data paths #71871
• Remove documentation from 8.0
• Block MDP in 8.0
• Remove MDP from 8.0

[elasticmachine]

Pinging @elastic/es-core-infra (Team:Core/Infra)

[willemdh]

@rjernst
What? This is imho a serious regression, which makes me kind of depressed, as this would imply we are going to have to redeploy all our nodes using mdp..... Why remove this handy feature?