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 .
Space can be guaranteed to a file system by using reservations. For information
about reservations, see . This model is very similar to the NFS model, where multiple
directories are mounted from the same file system (consider
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
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
not necessarily guarantee that the space usage information is updated immediately.
File system snapshots are inexpensive and easy to create in ZFS. Most likely, snapshots will be common in most ZFS environments. For information about ZFS snapshots, see.
The presence of snapshots can cause some unexpected behavior when you attempt to free space. Typically, given appropriate permissions, you can remove a file from a full file system, and this action results in more space becoming available in the file system. However, if the file to be removed exists in a snapshot of the file system, then no space is gained from the file deletion. The blocks used by the file continue to be referenced from the snapshot.
As a result, the file deletion can consume more disk space, because a new version of the directory needs to be created to reflect the new state of the namespace. This behavior means that you can get an unexpected ENOSPC or EDQUOT when attempting to remove a file.