Documentation bug on splitting shards -- free disk space requirement?

[Jamshid]

This doc says elasticsearch requires enough free disk space for a second copy of the index when splitting shards.

Split index API | Elasticsearch Guide [8.15] | Elastic

• The node handling the split process must have sufficient free disk space to accommodate a second copy of the existing index.

But this discussion seems to indicate that it depends on how many shards are being created. When splitting a 5 shard index to 20 shards it requires 4x the disk space, right?

Cloned/Split Indexes Take Double Disk Space When Increasing Shards

Currently I have some large index's (100GB - 500GB) that have been allocated 5 shards. I would like to increase these to 10 - 20 shards each. If I use the Split API or Clone API it takes 1-2 minutes for the new index to be created, but the disk space doubles if I set it to 10 shards, and quadruples if I set it to 20 shards. (Replicas are still 0). Is there a way to reduce the disk space? The document counts are the same, Im guessing it has to do with how it links to the physical files at th…

Should I file a github issue to get the documentation fixed? Or am I misunderstanding, maybe the hard links splitting creates makes it seem like it's using more space than it really is?

[DavidTurner]

@Jamshid

Or am I misunderstanding, maybe the hard links splitting creates makes it seem like it's using more space than it really is?

Yeah I think you're misunderstanding. The total size of all the shards will indeed go up by a large amount, but this metric double-counts any files that are hard-linked between shards. The actual disk space needed should only be approximately 2x the size of the original shards.

[Jamshid]

Thanks David. To be clear, splitting shards from 5 => 20 does not even temporarily require 4x the disk space?

[DavidTurner]

Correct