For my 2c, I haven't touch BTRFS since using it as the primary storage in a main storage server at work. We had a 6TB system reporting 3TB free (back when this

For my 2c, I haven't touch BTRFS since using it as the primary storage in a main storage server at work. We had a 6TB system reporting 3TB free (back when this was a lot of storage), yet every write operation (including deletes) was failing with an error indicating the disk was full. This was years ago, so I'm a little fuzzy on the details, but after some really painful downtime and troubleshooting, it turns out that BTRFS marks blocks as either metadata or data blocks. Once a block is marked, it won't be unmarked even if empty. So because this was a high churn system all of the blocks ended up marked as data and there was none available when more metadata was needed. The solution was running `balance` with some specific flags. My understanding is that this is partially resolved now (empty blocks are automatically freed now), but not completely (partially empty blocks are never compacted without a balance, so you can still have a bunch of half-empty data blocks and no room for metadata), so one is a lot less likely to run into similar issues, but it was scarring enough to me that I've never run a BTRFS system again.