MR v2 commit algorithm should be deprecated and not the default

The v2 MR commit algorithm moves files from the task attempt dir into the dest dir on task commit -one by one

It is therefore not atomic

1. if a task commit fails partway through and another task attempt commits -unless exactly the same filenames are used, output of the first attempt may be included in the final result
2. if a worker partitions partway through task commit, and then continues after another attempt has committed, it may partially overwrite the output -even when the filenames are the same

Both MR and spark assume that task commits are atomic. Either they need to consider that this is not the case, we add a way to probe for a committer supporting atomic task commit, and the engines both add handling for task commit failures (probably fail job)

Better: we remove this as the default, maybe also warn when it is being used