Major Description
SplitShardCmd is currently quite complex.
Shard splitting occurs on active shards, which are still being updated, so the splitting has to involve several carefully orchestrated steps, making sure that new sub-shard placeholders are properly created and visible, and then also applying buffered updates to the split leaders and performing recovery on sub-shard replicas.
This process could be simplified in cases where collections are not actively being updated or can tolerate a little downtime - we could put the shard "offline", ie. disable writing while the splitting is in progress (in order to avoid users' confusion we should disable writing to the whole collection).
The actual index splitting could perhaps be improved to use HardLinkCopyDirectoryWrapper for creating a copy of the index by hard-linking existing index segments, and then applying deletes to the documents that don't belong in a sub-shard. However, the resulting index slices that replicas would have to pull would be the same size as the whole shard.