ZFS Terminology

This section describes the basic terminology used throughout this book:


A 256-bit hash of the data in a file system block. The checksum capability can range from the simple and fast fletcher2 (the default) to cryptographically strong hashes such as SHA256.


A file system whose initial contents are identical to the contents of a snapshot.

For information about clones, see ZFS Clones.


A generic name for the following ZFS entities: clones, file systems, snapshots, or volumes.

Each dataset is identified by a unique name in the ZFS namespace. Datasets are identified using the following format:



Identifies the name of the storage pool that contains the dataset


Is a slash-delimited path name for the dataset object


Is an optional component that identifies a snapshot of a dataset

For more information about datasets, see Chapter 5, Managing ZFS File Systems.

file system

A dataset that contains a standard POSIX file system.

For more information about file systems, see Chapter 5, Managing ZFS File Systems.


A virtual device that stores identical copies of data on two or more disks. If any disk in a mirror fails, any other disk in that mirror can provide the same data.


A logical group of devices describing the layout and physical characteristics of the available storage. Space for datasets is allocated from a pool.

For more information about storage pools, see Chapter 4, Managing ZFS Storage Pools.


A virtual device that stores data and parity on multiple disks, similar to RAID-5. For more information about RAID-Z, see RAID-Z Storage Pool Configuration.


The process of transferring data from one device to another device is known as resilvering. For example, if a mirror component is replaced or taken offline, the data from the up-to-date mirror component is copied to the newly restored mirror component. This process is referred to as mirror resynchronization in traditional volume management products.

For more information about ZFS resilvering, see Viewing Resilvering Status.


A read-only image of a file system or volume at a given point in time.

For more information about snapshots, see ZFS Snapshots.

virtual device

A logical device in a pool, which can be a physical device, a file, or a collection of devices.

For more information about virtual devices, see Virtual Devices in a Storage Pool.


A dataset used to emulate a physical device in order to support legacy file systems.

For more information about emulated volumes, see Emulated Volumes.