> In contrast ext3 introduced a hash table to quickly look up file metadata in huge directories. Ah, but it has its own consequences. ext4 doesn't dynamically

> In contrast ext3 introduced a hash table to quickly look up file metadata in huge directories. Ah, but it has its own consequences. ext4 doesn't dynamically reclaim inodes or garbage collect these directory hash tables, so if you create a directory and cycle a bunch of files through it, like millions and billions of files many times over, you can get into a case where you have sizable portions of your drive as free space, but you get ENOSPC errors because adding files to the directory tries to insert them into the hash table, which fails, because the hash table takes up so much space it causes the drive to be actually full. Oh, don't try actually doing directory operations without the index, either. Something like `du -sh` without that index will take upwards of 10+ minutes on a fast virtio SSD to account for like 10 gigabytes of files. I recently had a 1TB ext4 drive that got into this state where it had 150GB free but the directory hash indexes were fucking massive, because billions of files had gone through the filesystem and it was basically hosed because even turning off the index and then turning it back on does not drop the hash table index, it only literally turns off the index use in the read path and leaves the index as is. I couldn't find a way to drop and clear the hash indicies. Yeah. Apparently you can get into a similar state if you exhaust the inode count because, again, there is no dynamic inode reclamation. In this case, I had the `gecko-dev` Git repository, which is absolutely massive, and I was extracting many copies of it onto my drive as part of some testing automation. So that added up fast. It was on a separate drive from my main /home mount, at least. The lesson I have learned from this is to mostly just use XFS instead of ext4, I guess. Which I was already doing on all my servers, so a bit funny I learned that lesson only on my own personal machine.