I wish ZFS supported per-user reservations, not just per-user quotas
ZFS supports a variety of ways to control space usage in filesystems. You can set a quota or a reservation on a filesystem, and you can set disk space and object count quotas on users, groups, and 'projects' in a filesystem. However, if you look at this list you'll notice an omission; you can't set a reservation for users, groups, or projects in a filesystem. There are some situations (at least in our world) where this would be convenient to have.
The most common case that comes up is that we have a bunch of people in a single filesystem, some of whom may fill up the filesystem by accident in the course of their work and others (such as professors) who we always want to be able to use some additional space so they can keep working. This is the ideal situation for a positive reservation instead of a negative quota, since what we want to put a limit on is the pool of space used by a group of people.
(The real ZFS answer is to put people who need reservations in their own filesystems because filesystems are cheap. But moving people from one filesystem to another is often rather disruptive and not trivial to coordinate, so often it doesn't get seriously contemplated until actual problems happen.)
OpenZFS has supported 'project quotas' since version 0.8.0, as covered in zfs-project(8) and zfs-projectspace(8). Project quotas can be used to give a single person (or group of people) a reservation in a filesystem, by putting their directories into a new project and then putting a project quota limit on the default project. However, you can't use this to give two people each a reservation of their own without putting quotas on each of them too, which is potentially (very) undesirable.
(ZFS project quotas appear to be in the current version of Illumos but I'm not sure when they appeared. It may have been added to the tree in August of 2019, per issue #11479.)
I don't have any personal experience with project quotas. Our Ubuntu ZFS fileservers are still running Ubuntu 18.04, which is too old to support them, and even once we upgrade to 22.04 we probably won't try it because of the various challenges of administering and managing them.
PS: Since ZFS supports project quotas, it also supports tracking
space usage by 'project'. Here 'project' is basically 'whatever you
want to tag with some unique identifier', which means that you could
go through and tag every top level directory in a filesystem with
a separate project ID so you could easily get reports on how much
space is in use in each of them. Ordinary people probably just use
PPS: I think it would be reasonable to require the filesystem to have a reservation that was at least as big as the sum of all of the user reservations in it (or the user, group, and project ones if you wanted to support all of those).