Suddenly I was getting "no space left on device" messages, but no filesystem
was actually full. I found that the btrfs (and its subvolumes) are 68% full,
but have no more inodes left! At the moment the system cannot even do
TAB-completion on the bash comamndline ("-bash: cannot create temp file for
here-document: No space left of device").
All the btrfs subvolumes show "Inodes:0 IUsed:0 IFree:0". This isn't actually
helpful when finding the filesystem that has too many inodes, it seems.
When the problem occurred I was writing to a different big btrfs filesystem
located on a different device. Did I hit some global limit of btrfs, or what is
happening?