FreeBSD/Linux Kernel Cross Reference
sys/contrib/openzfs/man/man7/zfsconcepts.7

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    .Dd June 30, 2019
    .Dt ZFSCONCEPTS 7
    .Os
    .
    .Sh NAME
    .Nm zfsconcepts
    .Nd overview of ZFS concepts
    .
    .Sh DESCRIPTION
    .Ss ZFS File System Hierarchy
    A ZFS storage pool is a logical collection of devices that provide space for
    datasets.
    A storage pool is also the root of the ZFS file system hierarchy.
    .Pp
    The root of the pool can be accessed as a file system, such as mounting and
    unmounting, taking snapshots, and setting properties.
    The physical storage characteristics, however, are managed by the
    .Xr zpool 8
    command.
    .Pp
    See
    .Xr zpool 8
    for more information on creating and administering pools.
    .Ss Snapshots
    A snapshot is a read-only copy of a file system or volume.
    Snapshots can be created extremely quickly, and initially consume no additional
    space within the pool.
    As data within the active dataset changes, the snapshot consumes more data than
    would otherwise be shared with the active dataset.
    .Pp
    Snapshots can have arbitrary names.
    Snapshots of volumes can be cloned or rolled back, visibility is determined
    by the
    .Sy snapdev
    property of the parent volume.
    .Pp
    File system snapshots can be accessed under the
    .Pa .zfs/snapshot
    directory in the root of the file system.
    Snapshots are automatically mounted on demand and may be unmounted at regular
    intervals.
    The visibility of the
    .Pa .zfs
    directory can be controlled by the
    .Sy snapdir
    property.
    .Ss Bookmarks
    A bookmark is like a snapshot, a read-only copy of a file system or volume.
    Bookmarks can be created extremely quickly, compared to snapshots, and they
    consume no additional space within the pool.
    Bookmarks can also have arbitrary names, much like snapshots.
    .Pp
    Unlike snapshots, bookmarks can not be accessed through the filesystem in any
    way.
    From a storage standpoint a bookmark just provides a way to reference
    when a snapshot was created as a distinct object.
    Bookmarks are initially tied to a snapshot, not the filesystem or volume,
    and they will survive if the snapshot itself is destroyed.
    Since they are very light weight there's little incentive to destroy them.
    .Ss Clones
    A clone is a writable volume or file system whose initial contents are the same
    as another dataset.
    As with snapshots, creating a clone is nearly instantaneous, and initially
    consumes no additional space.
    .Pp
    Clones can only be created from a snapshot.
    When a snapshot is cloned, it creates an implicit dependency between the parent
    and child.
   Even though the clone is created somewhere else in the dataset hierarchy, the
   original snapshot cannot be destroyed as long as a clone exists.
   The
   .Sy origin
   property exposes this dependency, and the
   .Cm destroy
   command lists any such dependencies, if they exist.
   .Pp
   The clone parent-child dependency relationship can be reversed by using the
   .Cm promote
   subcommand.
   This causes the
   .Qq origin
   file system to become a clone of the specified file system, which makes it
   possible to destroy the file system that the clone was created from.
   .Ss "Mount Points"
   Creating a ZFS file system is a simple operation, so the number of file systems
   per system is likely to be numerous.
   To cope with this, ZFS automatically manages mounting and unmounting file
   systems without the need to edit the
   .Pa /etc/fstab
   file.
   All automatically managed file systems are mounted by ZFS at boot time.
   .Pp
   By default, file systems are mounted under
   .Pa /path ,
   where
   .Ar path
   is the name of the file system in the ZFS namespace.
   Directories are created and destroyed as needed.
   .Pp
   A file system can also have a mount point set in the
   .Sy mountpoint
   property.
   This directory is created as needed, and ZFS automatically mounts the file
   system when the
   .Nm zfs Cm mount Fl a
   command is invoked
   .Po without editing
   .Pa /etc/fstab
   .Pc .
   The
   .Sy mountpoint
   property can be inherited, so if
   .Em pool/home
   has a mount point of
   .Pa /export/stuff ,
   then
   .Em pool/home/user
   automatically inherits a mount point of
   .Pa /export/stuff/user .
   .Pp
   A file system
   .Sy mountpoint
   property of
   .Sy none
   prevents the file system from being mounted.
   .Pp
   If needed, ZFS file systems can also be managed with traditional tools
   .Po
   .Nm mount ,
   .Nm umount ,
   .Pa /etc/fstab
   .Pc .
   If a file system's mount point is set to
   .Sy legacy ,
   ZFS makes no attempt to manage the file system, and the administrator is
   responsible for mounting and unmounting the file system.
   Because pools must
   be imported before a legacy mount can succeed, administrators should ensure
   that legacy mounts are only attempted after the zpool import process
   finishes at boot time.
   For example, on machines using systemd, the mount option
   .Pp
   .Nm x-systemd.requires=zfs-import.target
   .Pp
   will ensure that the zfs-import completes before systemd attempts mounting
   the filesystem.
   See
   .Xr systemd.mount 5
   for details.
   .Ss Deduplication
   Deduplication is the process for removing redundant data at the block level,
   reducing the total amount of data stored.
   If a file system has the
   .Sy dedup
   property enabled, duplicate data blocks are removed synchronously.
   The result
   is that only unique data is stored and common components are shared among files.
   .Pp
   Deduplicating data is a very resource-intensive operation.
   It is generally recommended that you have at least 1.25 GiB of RAM
   per 1 TiB of storage when you enable deduplication.
   Calculating the exact requirement depends heavily
   on the type of data stored in the pool.
   .Pp
   Enabling deduplication on an improperly-designed system can result in
   performance issues (slow I/O and administrative operations).
   It can potentially lead to problems importing a pool due to memory exhaustion.
   Deduplication can consume significant processing power (CPU) and memory as well
   as generate additional disk I/O.
   .Pp
   Before creating a pool with deduplication enabled, ensure that you have planned
   your hardware requirements appropriately and implemented appropriate recovery
   practices, such as regular backups.
   Consider using the
   .Sy compression
   property as a less resource-intensive alternative.

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