ZFS is based on a concept of pooled storage. Unlike typical file systems, which are mapped to physical storage, all ZFS file systems in a pool share the available storage in the pool. So, the available space reported by utilities such as df might change even when the file system is inactive, as other file systems in the pool consume or release space. Note that the maximum file system size can be limited by using quotas. For information about quotas, see Setting Quotas on ZFS File Systems. Space can be guaranteed to a file system by using reservations. For information about reservations, see Setting Reservations on ZFS File Systems. This model is very similar to the NFS model, where multiple directories are mounted from the same file system (consider /home).

All metadata in ZFS is allocated dynamically. Most other file systems pre-allocate much of their metadata. As a result, an immediate space cost at file system creation for this metadata is required. This behavior also means that the total number of files supported by the file systems is predetermined. Because ZFS allocates its metadata as it needs it, no initial space cost is required, and the number of files is limited only by the available space. The output from the df -g command must be interpreted differently for ZFS than other file systems. The total files reported is only an estimate based on the amount of storage that is available in the pool.

ZFS is a transactional file system. Most file system modifications are bundled into transaction groups and committed to disk asynchronously. Until these modifications are committed to disk, they are termed pending changes. The amount of space used, available, and referenced by a file or file system does not consider pending changes. Pending changes are generally accounted for within a few seconds. Even committing a change to disk by using fsync(3c) or O_SYNC does not necessarily guarantee that the space usage information is updated immediately.