Don't worry if, for some reason, you are forced to cancel migration application. This migration tool is designed to continue from previously noted checkpoint.
Data migration consumes disk space, and if there's lower than 20% available disk space on your server, migration application is going to terminate itself. All data previously migrated are not going to be affected.
Migration has concept of copying data from the newest dates and goes straight back to the past. That way you're sure that even if migration is not able to transfer all data from Postgres - you'll have the newest data securely transferred. However, if you're able to provide additional disk space you could start migration application again and copy the rest of the data to Elasticsearch without any problems.