Snapshot ,restore and free disk space

When I create a snapshot, it prompts NoSuchException: source_log_2023_06][0]][backup_repo2:snapshot_name/tS8r2NxSRV29Qoj_pZI0Ig] failed to snapshot shard.
And this info:
low disk watermark [85%] exceeded on [9kkzFDY6Re6hOwmPdHOxLg][localhost][data/nodes/0] free: 10.4gb[14.9%], replicas will not be assigned to this node。
So I want to know how much disk space is required to create a snapshot in elasticsearch7.8? Can it be modified?

A snapshot is a byte-for-byte copy of the primary shards in the cluster. So however big those shards are, that's how much space you need for a snapshot.

This version is very old, you should upgrade as a matter of urgency.

ok, thank you. This project is very old and it is not convenient to upgrade.

I still want to know why the snapshot creation fails when the disk still has 15% free space, which is enough to save the Elasticsearch data?

Don't know, sorry, I don't have a dev env for such old versions any more with which to investigate.