Quotas

Filesystems can have usage quotas enforced

Soft limits can be exceeded, but
User is warned for each subsequent write
Cannot exceed soft quota for more than a week

Hard limits can never be exceeded

Quotas are on a per-user basis

Root is not subject to filesystem quotas

User-space tools available to view quota status and violators

At filesystem creation time, a percentage of the filesystem is reserved for root, regardless of quotas