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|Oracle Solaris ZFS Administration Guide Oracle Solaris 10 8/11 Information Library|
ZFS is based on the 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 disk 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 disk 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. A specified amount of disk 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 preallocate much of their metadata. As a result, at file system creation time, an immediate space cost 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 disk 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 called pending changes. The amount of disk 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 disk space usage information is updated immediately.
For additional details about ZFS disk space consumption as reported by the du and df commands, see:
File system snapshots are inexpensive and easy to create in ZFS. Snapshots are common in most ZFS environments. For information about ZFS snapshots, see Chapter 7, Working With Oracle Solaris ZFS Snapshots and Clones.
The presence of snapshots can cause some unexpected behavior when you attempt to free disk space. Typically, given appropriate permissions, you can remove a file from a full file system, and this action results in more disk space becoming available in the file system. However, if the file to be removed exists in a snapshot of the file system, then no disk 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 receive an unexpected ENOSPC or EDQUOT error when attempting to remove a file.