ZFS can hit “No space left on device” (ENOSPC) errors if the pool fills up. But unlike btrfs’s infamous ENOSPC woes, ZFS was designed to handle these situations much more gracefully. ZFS actually keeps a bit of “slop space” reserved. So, as you approach full, it stops writes early and gives you a chance to clean things up, instead of running into unpredictable issues or impossible snapshot removals like btrfs sometimes does. You can even tweak how much safety space ZFS reserves, though most users don’t need to touch it.

[https://news.ycombinator.com/item?id=11696657]

When you run out of space in ZFS, you get a clear error for write attempts, but the system doesn’t end up fragmented beyond repair or force you into tricky multi-step recovery processes. Freeing up space (by deleting files or snapshots, or expanding the pool) typically makes things happy again.

[https://bobcares.com/blog/zfs-no-space-left-on-device-how-to...]