NAME

       zfsconcepts — overview of ZFS concepts

DESCRIPTION

   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.

       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 zpool(8)
       command.

       See zpool(8) for more information on creating and administering pools.

   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.

       Snapshots can have arbitrary names.  Snapshots of volumes can be cloned or  rolled  back,  visibility  is
       determined by the snapdev property of the parent volume.

       File  system  snapshots can be accessed under the .zfs/snapshot directory in the root of the file system.
       Snapshots are  automatically  mounted  on  demand  and  may  be  unmounted  at  regular  intervals.   The
       availability and visibility of the .zfs directory can be controlled by the snapdir property.

   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.

       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.

   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.

       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 origin  property  exposes  this
       dependency, and the destroy command lists any such dependencies, if they exist.

       The  clone  parent-child  dependency  relationship can be reversed by using the promote subcommand.  This
       causes the "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.

   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 /etc/fstab file.  All automatically managed file systems are mounted by ZFS at boot
       time.

       By default, file systems are mounted under /path, where path is the name of the file system  in  the  ZFS
       namespace.  Directories are created and destroyed as needed.

       A  file  system can also have a mount point set in the mountpoint property.  This directory is created as
       needed, and ZFS automatically mounts the file system when the zfs mount -a command  is  invoked  (without
       editing  /etc/fstab).   The  mountpoint  property  can be inherited, so if pool/home has a mount point of
       /export/stuff, then pool/home/user automatically inherits a mount point of /export/stuff/user.

       A file system mountpoint property of none prevents the file system from being mounted.

       If needed, ZFS file systems can also be managed with traditional tools (mount, umount, /etc/fstab).  If a
       file system's mount point is set to 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

       x-systemd.requires=zfs-import.target

       will  ensure  that  the  zfs-import  completes  before  systemd  attempts  mounting  the filesystem.  See
       systemd.mount(5) for details.

   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  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.

       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.

       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.

       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 compression property as a less resource-intensive alternative.

   Block cloning
       Block  cloning  is  a facility that allows a file (or parts of a file) to be "cloned", that is, a shallow
       copy made where the existing data blocks are referenced rather than copied.  Later modifications  to  the
       data  will  cause  a copy of the data block to be taken and that copy modified.  This facility is used to
       implement "reflinks" or "file-level copy-on-write".

       Cloned blocks are tracked in a special on-disk structure called the Block Reference Table (BRT).   Unlike
       deduplication, this table has minimal overhead, so can be enabled at all times.

       Also  unlike  deduplication,  cloning  must  be  requested  by  a user program.  Many common file copying
       programs, including newer versions of /bin/cp,  will  try  to  create  clones  automatically.   Look  for
       "clone", "dedupe" or "reflink" in the documentation for more information.

       There  are  some  limitations  to  block cloning.  Only whole blocks can be cloned, and blocks can not be
       cloned if they are not yet written to disk, or if they are  encrypted,  or  the  source  and  destination
       recordsize  properties  differ.   The  OS  may add additional restrictions; for example, most versions of
       Linux will not allow clones across datasets.

OpenZFS                                          October 6, 2023                                  ZFSCONCEPTS(7)