I'm migrating a site from drupal 7 to 8. It has many many media files. Something like 6TB worth. And adding daily. So i want to do the db migration on my machine (not the webserver), work out kinks and modify custom code to work w/ d8, then do the real thing on the live machine over a slow weekend when i have all the steps worked out.
In the migration process, i've gotten to drush migrate-upgrade d7_node, and it (on initial run, with only a token few files in /sites/default/files) created a handful of db entries in file_managed. So i sshfs mounted the live files directory read only from my machine and re-ran migrate-upgrade d7_node from the restored db. Now it doesn't create anything in the file_managed table, so i suspect it's not working, but it gives no errors, and some grepping doesn't readily yield the code that does this work. I wouldn't mind hacking for this part only.
Hence the question. And what can i do alternatively to avoid copying 6TB of files for the preparatory migration?