I think this problem has been present for ~3 years. I've had this experience every time I've tried to migrate files across 4 production installs at three different companies. 2 of these installs were "pristine", meaning we never did any trickery or writing of our own extensions.
- Dry run shows that some files are going to fail
- Actually migrating the files shows even more failures that weren't present in the dry run
- Nothing ever explains what these files are (or were, some files don't exist anymore) or what the relative risk / loss you're incurring equates to
Being a seasoned file migration error encounterer, I've come to find that the majority of files fall into these categories:
- Pholio thumbnails
- Files with manually set strange policies
- Files that were not manually uploaded by users (eg, files that some other phabricator application made for some reason)
The failures encountered are mostly harmless / have managed to fix things up without a lot of pain involved. However, it's still terrifying to not really know what's happening when you migrate files, why things failed, what the files are/were etc.
I've migrated files as follows and always had these problems:
- Disk -> DB (twice)
- DB -> S3
- Disk -> S3
It's a relatively small sample size I suppose (4 migrations), but seems pretty consistent. I have db dumps and files from the most recent migration I can make mostly available to Phacility for the purposes of debugging if you like.