Some filesystems may require allocating metadata to delete a file. AFIK it's a non issue with traditional Berkeley-style systems, since metadata and data come from a separate pools. Notably ZFS has this problem.
> GlobalReserve is an artificial and internal emergency space. It is used e.g. when the filesystem is full. Its total size is dynamic based on the filesystem size, usually not larger than 512MiB, used may fluctuate.
This hasn't been a problem you should be able to hit in ZFS in a long time.
It reserves a percent of your pool's total space precisely to avoid having 0 actual free space and only allows using space from that amount if the operation is a net gain on free space.
You're misunderstanding. See the sibling thread where p_l says that this problem has been resolved, and any further occurrence would be treated as a bug. Setting the quota is only done now to reduce fragmentation (ZFS's fragmentation avoidance requires sufficient free space to be effective).
According to rincebrain, the "disk too full to delete files" was fixed "shortly after the fork" which means "shortly after 2012." My information was quite out of date.
