freebsd-dev/man/man8/zfsprops.8

.\"
.\" CDDL HEADER START
.\"
.\" The contents of this file are subject to the terms of the
.\" Common Development and Distribution License (the "License").
.\" You may not use this file except in compliance with the License.
.\"
.\" You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
.\" or http://www.opensolaris.org/os/licensing.
.\" See the License for the specific language governing permissions
.\" and limitations under the License.
.\"
.\" When distributing Covered Code, include this CDDL HEADER in each
.\" file and include the License file at usr/src/OPENSOLARIS.LICENSE.
.\" If applicable, add the following below this CDDL HEADER, with the
.\" fields enclosed by brackets "[]" replaced with your own identifying
.\" information: Portions Copyright [yyyy] [name of copyright owner]
.\"
.\" CDDL HEADER END
.\"
.\"
.\" Copyright (c) 2009 Sun Microsystems, Inc. All Rights Reserved.
.\" Copyright 2011 Joshua M. Clulow <josh@sysmgr.org>
.\" Copyright (c) 2011, 2019 by Delphix. All rights reserved.
.\" Copyright (c) 2011, Pawel Jakub Dawidek <pjd@FreeBSD.org>
.\" Copyright (c) 2012, Glen Barber <gjb@FreeBSD.org>
.\" Copyright (c) 2012, Bryan Drewery <bdrewery@FreeBSD.org>
.\" Copyright (c) 2013, Steven Hartland <smh@FreeBSD.org>
.\" Copyright (c) 2013 by Saso Kiselkov. All rights reserved.
.\" Copyright (c) 2014, Joyent, Inc. All rights reserved.
.\" Copyright (c) 2014 by Adam Stevko. All rights reserved.
.\" Copyright (c) 2014 Integros [integros.com]
.\" Copyright (c) 2016 Nexenta Systems, Inc. All Rights Reserved.
.\" Copyright (c) 2014, Xin LI <delphij@FreeBSD.org>
.\" Copyright (c) 2014-2015, The FreeBSD Foundation, All Rights Reserved.
.\" Copyright 2019 Richard Laager. All rights reserved.
.\" Copyright 2018 Nexenta Systems, Inc.
.\" Copyright 2019 Joyent, Inc.
.\"
.Dd January 30, 2020
.Dt ZFSPROPS 8
.Os Linux
.Sh NAME
.Nm zfsprops
.Nd Native properties and user-defined of ZFS datasets.
.Sh DESCRIPTION
Properties are divided into two types, native properties and user-defined
.Po or
.Qq user
.Pc
properties.
Native properties either export internal statistics or control ZFS behavior.
In addition, native properties are either editable or read-only.
User properties have no effect on ZFS behavior, but you can use them to annotate
datasets in a way that is meaningful in your environment.
For more information about user properties, see the
.Sx User Properties
section, below.
.Ss Native Properties
Every dataset has a set of properties that export statistics about the dataset
as well as control various behaviors.
Properties are inherited from the parent unless overridden by the child.
Some properties apply only to certain types of datasets
.Pq file systems, volumes, or snapshots .
.Pp
The values of numeric properties can be specified using human-readable suffixes
.Po for example,
.Sy k ,
.Sy KB ,
.Sy M ,
.Sy Gb ,
and so forth, up to
.Sy Z
for zettabyte
.Pc .
The following are all valid
.Pq and equal
specifications:
.Li 1536M, 1.5g, 1.50GB .
.Pp
The values of non-numeric properties are case sensitive and must be lowercase,
except for
.Sy mountpoint ,
.Sy sharenfs ,
and
.Sy sharesmb .
.Pp
The following native properties consist of read-only statistics about the
dataset.
These properties can be neither set, nor inherited.
Native properties apply to all dataset types unless otherwise noted.
.Bl -tag -width "usedbyrefreservation"
.It Sy available
The amount of space available to the dataset and all its children, assuming that
there is no other activity in the pool.
Because space is shared within a pool, availability can be limited by any number
of factors, including physical pool size, quotas, reservations, or other
datasets within the pool.
.Pp
This property can also be referred to by its shortened column name,
.Sy avail .
.It Sy compressratio
For non-snapshots, the compression ratio achieved for the
.Sy used
space of this dataset, expressed as a multiplier.
The
.Sy used
property includes descendant datasets, and, for clones, does not include the
space shared with the origin snapshot.
For snapshots, the
.Sy compressratio
is the same as the
.Sy refcompressratio
property.
Compression can be turned on by running:
.Nm zfs Cm set Sy compression Ns = Ns Sy on Ar dataset .
The default value is
.Sy off .
.It Sy createtxg
The transaction group (txg) in which the dataset was created. Bookmarks have
the same
.Sy createtxg
as the snapshot they are initially tied to. This property is suitable for
ordering a list of snapshots, e.g. for incremental send and receive.
.It Sy creation
The time this dataset was created.
.It Sy clones
For snapshots, this property is a comma-separated list of filesystems or volumes
which are clones of this snapshot.
The clones'
.Sy origin
property is this snapshot.
If the
.Sy clones
property is not empty, then this snapshot can not be destroyed
.Po even with the
.Fl r
or
.Fl f
options
.Pc .
The roles of origin and clone can be swapped by promoting the clone with the
.Nm zfs Cm promote
command.
.It Sy defer_destroy
This property is
.Sy on
if the snapshot has been marked for deferred destroy by using the
.Nm zfs Cm destroy Fl d
command.
Otherwise, the property is
.Sy off .
.It Sy encryptionroot
For encrypted datasets, indicates where the dataset is currently inheriting its
encryption key from. Loading or unloading a key for the
.Sy encryptionroot
will implicitly load / unload the key for any inheriting datasets (see
.Nm zfs Cm load-key
and
.Nm zfs Cm unload-key
for details).
Clones will always share an
encryption key with their origin. See the
.Em Encryption
section of
.Xr zfs-load-key 8
for details.
.It Sy filesystem_count
The total number of filesystems and volumes that exist under this location in
the dataset tree.
This value is only available when a
.Sy filesystem_limit
has been set somewhere in the tree under which the dataset resides.
.It Sy keystatus
Indicates if an encryption key is currently loaded into ZFS. The possible
values are
.Sy none ,
.Sy available ,
and
.Sy unavailable .
See
.Nm zfs Cm load-key
and
.Nm zfs Cm unload-key .
.It Sy guid
The 64 bit GUID of this dataset or bookmark which does not change over its
entire lifetime. When a snapshot is sent to another pool, the received
snapshot has the same GUID. Thus, the
.Sy guid
is suitable to identify a snapshot across pools.
.It Sy logicalreferenced
The amount of space that is
.Qq logically
accessible by this dataset.
See the
.Sy referenced
property.
The logical space ignores the effect of the
.Sy compression
and
.Sy copies
properties, giving a quantity closer to the amount of data that applications
see.
However, it does include space consumed by metadata.
.Pp
This property can also be referred to by its shortened column name,
.Sy lrefer .
.It Sy logicalused
The amount of space that is
.Qq logically
consumed by this dataset and all its descendents.
See the
.Sy used
property.
The logical space ignores the effect of the
.Sy compression
and
.Sy copies
properties, giving a quantity closer to the amount of data that applications
see.
However, it does include space consumed by metadata.
.Pp
This property can also be referred to by its shortened column name,
.Sy lused .
.It Sy mounted
For file systems, indicates whether the file system is currently mounted.
This property can be either
.Sy yes
or
.Sy no .
.It Sy objsetid
A unique identifier for this dataset within the pool. Unlike the dataset's
.Sy guid
, the
.Sy objsetid
of a dataset is not transferred to other pools when the snapshot is copied
with a send/receive operation.
The
.Sy objsetid
can be reused (for a new dataset) after the dataset is deleted.
.It Sy origin
For cloned file systems or volumes, the snapshot from which the clone was
created.
See also the
.Sy clones
property.
.It Sy receive_resume_token
For filesystems or volumes which have saved partially-completed state from
.Sy zfs receive -s ,
this opaque token can be provided to
.Sy zfs send -t
to resume and complete the
.Sy zfs receive .
.It Sy redact_snaps
For bookmarks, this is the list of snapshot guids the bookmark contains a redaction
list for.
For snapshots, this is the list of snapshot guids the snapshot is redacted with
respect to.
.It Sy referenced
The amount of data that is accessible by this dataset, which may or may not be
shared with other datasets in the pool.
When a snapshot or clone is created, it initially references the same amount of
space as the file system or snapshot it was created from, since its contents are
identical.
.Pp
This property can also be referred to by its shortened column name,
.Sy refer .
.It Sy refcompressratio
The compression ratio achieved for the
.Sy referenced
space of this dataset, expressed as a multiplier.
See also the
.Sy compressratio
property.
.It Sy snapshot_count
The total number of snapshots that exist under this location in the dataset
tree.
This value is only available when a
.Sy snapshot_limit
has been set somewhere in the tree under which the dataset resides.
.It Sy type
The type of dataset:
.Sy filesystem ,
.Sy volume ,
or
.Sy snapshot .
.It Sy used
The amount of space consumed by this dataset and all its descendents.
This is the value that is checked against this dataset's quota and reservation.
The space used does not include this dataset's reservation, but does take into
account the reservations of any descendent datasets.
The amount of space that a dataset consumes from its parent, as well as the
amount of space that is freed if this dataset is recursively destroyed, is the
greater of its space used and its reservation.
.Pp
The used space of a snapshot
.Po see the
.Em Snapshots
section of
.Xr zfsconcepts 8
.Pc
is space that is referenced exclusively by this snapshot.
If this snapshot is destroyed, the amount of
.Sy used
space will be freed.
Space that is shared by multiple snapshots isn't accounted for in this metric.
When a snapshot is destroyed, space that was previously shared with this
snapshot can become unique to snapshots adjacent to it, thus changing the used
space of those snapshots.
The used space of the latest snapshot can also be affected by changes in the
file system.
Note that the
.Sy used
space of a snapshot is a subset of the
.Sy written
space of the snapshot.
.Pp
The amount of space used, available, or referenced does not take into account
pending changes.
Pending changes are generally accounted for within a few seconds.
Committing a change to a disk using
.Xr fsync 2
or
.Dv O_SYNC
does not necessarily guarantee that the space usage information is updated
immediately.
.It Sy usedby*
The
.Sy usedby*
properties decompose the
.Sy used
properties into the various reasons that space is used.
Specifically,
.Sy used No =
.Sy usedbychildren No +
.Sy usedbydataset No +
.Sy usedbyrefreservation No +
.Sy usedbysnapshots .
These properties are only available for datasets created on
.Nm zpool
.Qo version 13 Qc
pools.
.It Sy usedbychildren
The amount of space used by children of this dataset, which would be freed if
all the dataset's children were destroyed.
.It Sy usedbydataset
The amount of space used by this dataset itself, which would be freed if the
dataset were destroyed
.Po after first removing any
.Sy refreservation
and destroying any necessary snapshots or descendents
.Pc .
.It Sy usedbyrefreservation
The amount of space used by a
.Sy refreservation
set on this dataset, which would be freed if the
.Sy refreservation
was removed.
.It Sy usedbysnapshots
The amount of space consumed by snapshots of this dataset.
In particular, it is the amount of space that would be freed if all of this
dataset's snapshots were destroyed.
Note that this is not simply the sum of the snapshots'
.Sy used
properties because space can be shared by multiple snapshots.
.It Sy userused Ns @ Ns Em user
The amount of space consumed by the specified user in this dataset.
Space is charged to the owner of each file, as displayed by
.Nm ls Fl l .
The amount of space charged is displayed by
.Nm du
and
.Nm ls Fl s .
See the
.Nm zfs Cm userspace
subcommand for more information.
.Pp
Unprivileged users can access only their own space usage.
The root user, or a user who has been granted the
.Sy userused
privilege with
.Nm zfs Cm allow ,
can access everyone's usage.
.Pp
The
.Sy userused Ns @ Ns Em ...
properties are not displayed by
.Nm zfs Cm get Sy all .
The user's name must be appended after the @ symbol, using one of the following
forms:
.Bl -bullet -width ""
.It
.Em POSIX name
.Po for example,
.Sy joe
.Pc
.It
.Em POSIX numeric ID
.Po for example,
.Sy 789
.Pc
.It
.Em SID name
.Po for example,
.Sy joe.smith@mydomain
.Pc
.It
.Em SID numeric ID
.Po for example,
.Sy S-1-123-456-789
.Pc
.El
.Pp
Files created on Linux always have POSIX owners.
.It Sy userobjused Ns @ Ns Em user
The
.Sy userobjused
property is similar to
.Sy userused
but instead it counts the number of objects consumed by a user. This property
counts all objects allocated on behalf of the user, it may differ from the
results of system tools such as
.Nm df Fl i .
.Pp
When the property
.Sy xattr=on
is set on a file system additional objects will be created per-file to store
extended attributes. These additional objects are reflected in the
.Sy userobjused
value and are counted against the user's
.Sy userobjquota .
When a file system is configured to use
.Sy xattr=sa
no additional internal objects are normally required.
.It Sy userrefs
This property is set to the number of user holds on this snapshot.
User holds are set by using the
.Nm zfs Cm hold
command.
.It Sy groupused Ns @ Ns Em group
The amount of space consumed by the specified group in this dataset.
Space is charged to the group of each file, as displayed by
.Nm ls Fl l .
See the
.Sy userused Ns @ Ns Em user
property for more information.
.Pp
Unprivileged users can only access their own groups' space usage.
The root user, or a user who has been granted the
.Sy groupused
privilege with
.Nm zfs Cm allow ,
can access all groups' usage.
.It Sy groupobjused Ns @ Ns Em group
The number of objects consumed by the specified group in this dataset.
Multiple objects may be charged to the group for each file when extended
attributes are in use. See the
.Sy userobjused Ns @ Ns Em user
property for more information.
.Pp
Unprivileged users can only access their own groups' space usage.
The root user, or a user who has been granted the
.Sy groupobjused
privilege with
.Nm zfs Cm allow ,
can access all groups' usage.
.It Sy projectused Ns @ Ns Em project
The amount of space consumed by the specified project in this dataset. Project
is identified via the project identifier (ID) that is object-based numeral
attribute. An object can inherit the project ID from its parent object (if the
parent has the flag of inherit project ID that can be set and changed via
.Nm chattr Fl /+P
or
.Nm zfs project Fl s )
when being created. The privileged user can set and change object's project
ID via
.Nm chattr Fl p
or
.Nm zfs project Fl s
anytime. Space is charged to the project of each file, as displayed by
.Nm lsattr Fl p
or
.Nm zfs project .
See the
.Sy userused Ns @ Ns Em user
property for more information.
.Pp
The root user, or a user who has been granted the
.Sy projectused
privilege with
.Nm zfs allow ,
can access all projects' usage.
.It Sy projectobjused Ns @ Ns Em project
The
.Sy projectobjused
is similar to
.Sy projectused
but instead it counts the number of objects consumed by project. When the
property
.Sy xattr=on
is set on a fileset, ZFS will create additional objects per-file to store
extended attributes. These additional objects are reflected in the
.Sy projectobjused
value and are counted against the project's
.Sy projectobjquota .
When a filesystem is configured to use
.Sy xattr=sa
no additional internal objects are required. See the
.Sy userobjused Ns @ Ns Em user
property for more information.
.Pp
The root user, or a user who has been granted the
.Sy projectobjused
privilege with
.Nm zfs allow ,
can access all projects' objects usage.
.It Sy volblocksize
For volumes, specifies the block size of the volume.
The
.Sy blocksize
cannot be changed once the volume has been written, so it should be set at
volume creation time.
The default
.Sy blocksize
for volumes is 8 Kbytes.
Any power of 2 from 512 bytes to 128 Kbytes is valid.
.Pp
This property can also be referred to by its shortened column name,
.Sy volblock .
.It Sy written
The amount of space
.Sy referenced
by this dataset, that was written since the previous snapshot
.Pq i.e. that is not referenced by the previous snapshot .
.It Sy written Ns @ Ns Em snapshot
The amount of
.Sy referenced
space written to this dataset since the specified snapshot.
This is the space that is referenced by this dataset but was not referenced by
the specified snapshot.
.Pp
The
.Em snapshot
may be specified as a short snapshot name
.Po just the part after the
.Sy @
.Pc ,
in which case it will be interpreted as a snapshot in the same filesystem as
this dataset.
The
.Em snapshot
may be a full snapshot name
.Po Em filesystem Ns @ Ns Em snapshot Pc ,
which for clones may be a snapshot in the origin's filesystem
.Pq or the origin of the origin's filesystem, etc.
.El
.Pp
The following native properties can be used to change the behavior of a ZFS
dataset.
.Bl -tag -width ""
.It Xo
.Sy aclinherit Ns = Ns Sy discard Ns | Ns Sy noallow Ns | Ns
.Sy restricted Ns | Ns Sy passthrough Ns | Ns Sy passthrough-x
.Xc
Controls how ACEs are inherited when files and directories are created.
.Bl -tag -width "passthrough-x"
.It Sy discard
does not inherit any ACEs.
.It Sy noallow
only inherits inheritable ACEs that specify
.Qq deny
permissions.
.It Sy restricted
default, removes the
.Sy write_acl
and
.Sy write_owner
permissions when the ACE is inherited.
.It Sy passthrough
inherits all inheritable ACEs without any modifications.
.It Sy passthrough-x
same meaning as
.Sy passthrough ,
except that the
.Sy owner@ ,
.Sy group@ ,
and
.Sy everyone@
ACEs inherit the execute permission only if the file creation mode also requests
the execute bit.
.El
.Pp
When the property value is set to
.Sy passthrough ,
files are created with a mode determined by the inheritable ACEs.
If no inheritable ACEs exist that affect the mode, then the mode is set in
accordance to the requested mode from the application.
.Pp
The
.Sy aclinherit
property does not apply to POSIX ACLs.
.It Xo
.Sy aclmode Ns = Ns Sy discard Ns | Ns Sy groupmask Ns | Ns
.Sy passthrough Ns | Ns Sy restricted Ns
.Xc
Controls how an ACL is modified during chmod(2) and how inherited ACEs
are modified by the file creation mode.
.Bl -tag -width "passthrough"
.It Sy discard
default, deletes all
.Sy ACEs
except for those representing
the mode of the file or directory requested by
.Xr chmod 2 .
.It Sy groupmask
reduces permissions granted in all
.Sy ALLOW
entries found in the
.Sy ACL
such that they are no greater than the group permissions specified by
.Xr chmod 2 .
.It Sy passthrough
indicates that no changes are made to the
.Tn ACL
other than creating or updating the necessary
.Tn ACL
entries to represent the new mode of the file or directory.
.It Sy restricted
will cause the
.Xr chmod 2
operation to return an error when used on any file or directory which has
a non-trivial
.Tn ACL
whose entries can not be represented by a mode.
.Xr chmod 2
is required to change the set user ID, set group ID, or sticky bits on a file
or directory, as they do not have equivalent
.Tn ACL
entries.
In order to use
.Xr chmod 2
on a file or directory with a non-trivial
.Tn ACL
when
.Sy aclmode
is set to
.Sy restricted ,
you must first remove all
.Tn ACL
entries which do not represent the current mode.
.El
.It Sy acltype Ns = Ns Sy off Ns | Ns Sy noacl Ns | Ns Sy posixacl
Controls whether ACLs are enabled and if so what type of ACL to use.
This property is not visible on FreeBSD yet.
.Bl -tag -width "posixacl"
.It Sy off
default, when a file system has the
.Sy acltype
property set to off then ACLs are disabled.
.It Sy noacl
an alias for
.Sy off
.It Sy posixacl
indicates POSIX ACLs should be used. POSIX ACLs are specific to Linux and are
not functional on other platforms. POSIX ACLs are stored as an extended
attribute and therefore will not overwrite any existing NFSv4 ACLs which
may be set.
.El
.Pp
To obtain the best performance when setting
.Sy posixacl
users are strongly encouraged to set the
.Sy xattr=sa
property. This will result in the POSIX ACL being stored more efficiently on
disk. But as a consequence, all new extended attributes will only be
accessible from OpenZFS implementations which support the
.Sy xattr=sa
property. See the
.Sy xattr
property for more details.
.It Sy atime Ns = Ns Sy on Ns | Ns Sy off
Controls whether the access time for files is updated when they are read.
Turning this property off avoids producing write traffic when reading files and
can result in significant performance gains, though it might confuse mailers
and other similar utilities. The values
.Sy on
and
.Sy off
are equivalent to the
.Sy atime
and
.Sy noatime
mount options. The default value is
.Sy on .
See also
.Sy relatime
below.
.It Sy canmount Ns = Ns Sy on Ns | Ns Sy off Ns | Ns Sy noauto
If this property is set to
.Sy off ,
the file system cannot be mounted, and is ignored by
.Nm zfs Cm mount Fl a .
Setting this property to
.Sy off
is similar to setting the
.Sy mountpoint
property to
.Sy none ,
except that the dataset still has a normal
.Sy mountpoint
property, which can be inherited.
Setting this property to
.Sy off
allows datasets to be used solely as a mechanism to inherit properties.
One example of setting
.Sy canmount Ns = Ns Sy off
is to have two datasets with the same
.Sy mountpoint ,
so that the children of both datasets appear in the same directory, but might
have different inherited characteristics.
.Pp
When set to
.Sy noauto ,
a dataset can only be mounted and unmounted explicitly.
The dataset is not mounted automatically when the dataset is created or
imported, nor is it mounted by the
.Nm zfs Cm mount Fl a
command or unmounted by the
.Nm zfs Cm unmount Fl a
command.
.Pp
This property is not inherited.
.It Xo
.Sy checksum Ns = Ns Sy on Ns | Ns Sy off Ns | Ns Sy fletcher2 Ns | Ns
.Sy fletcher4 Ns | Ns Sy sha256 Ns | Ns Sy noparity Ns | Ns
.Sy sha512 Ns | Ns Sy skein Ns | Ns Sy edonr
.Xc
Controls the checksum used to verify data integrity.
The default value is
.Sy on ,
which automatically selects an appropriate algorithm
.Po currently,
.Sy fletcher4 ,
but this may change in future releases
.Pc .
The value
.Sy off
disables integrity checking on user data.
The value
.Sy noparity
not only disables integrity but also disables maintaining parity for user data.
This setting is used internally by a dump device residing on a RAID-Z pool and
should not be used by any other dataset.
Disabling checksums is
.Sy NOT
a recommended practice.
.Pp
The
.Sy sha512 ,
.Sy skein ,
and
.Sy edonr
checksum algorithms require enabling the appropriate features on the pool.
These pool features are not supported by GRUB and must not be used on the
pool if GRUB needs to access the pool (e.g. for /boot).
FreeBSD does not support the
.Sy edonr
algorithm.
.Pp
Please see
.Xr zpool-features 5
for more information on these algorithms.
.Pp
Changing this property affects only newly-written data.
.It Xo
.Sy compression Ns = Ns Sy on Ns | Ns Sy off Ns | Ns Sy gzip Ns | Ns
.Sy gzip- Ns Em N Ns | Ns Sy lz4 Ns | Ns Sy lzjb Ns | Ns Sy zle
.Xc
Controls the compression algorithm used for this dataset.
.Pp
Setting compression to
.Sy on
indicates that the current default compression algorithm should be used.
The default balances compression and decompression speed, with compression ratio
and is expected to work well on a wide variety of workloads.
Unlike all other settings for this property,
.Sy on
does not select a fixed compression type.
As new compression algorithms are added to ZFS and enabled on a pool, the
default compression algorithm may change.
The current default compression algorithm is either
.Sy lzjb
or, if the
.Sy lz4_compress
feature is enabled,
.Sy lz4 .
.Pp
The
.Sy lz4
compression algorithm is a high-performance replacement for the
.Sy lzjb
algorithm.
It features significantly faster compression and decompression, as well as a
moderately higher compression ratio than
.Sy lzjb ,
but can only be used on pools with the
.Sy lz4_compress
feature set to
.Sy enabled .
See
.Xr zpool-features 5
for details on ZFS feature flags and the
.Sy lz4_compress
feature.
.Pp
The
.Sy lzjb
compression algorithm is optimized for performance while providing decent data
compression.
.Pp
The
.Sy gzip
compression algorithm uses the same compression as the
.Xr gzip 1
command.
You can specify the
.Sy gzip
level by using the value
.Sy gzip- Ns Em N ,
where
.Em N
is an integer from 1
.Pq fastest
to 9
.Pq best compression ratio .
Currently,
.Sy gzip
is equivalent to
.Sy gzip-6
.Po which is also the default for
.Xr gzip 1
.Pc .
.Pp
The
.Sy zle
compression algorithm compresses runs of zeros.
.Pp
This property can also be referred to by its shortened column name
.Sy compress .
Changing this property affects only newly-written data.
.Pp
When any setting except
.Sy off
is selected, compression will explicitly check for blocks consisting of only
zeroes (the NUL byte).  When a zero-filled block is detected, it is stored as
a hole and not compressed using the indicated compression algorithm.
.Pp
Any block being compressed must be no larger than 7/8 of its original size
after compression, otherwise the compression will not be considered worthwhile
and the block saved uncompressed. Note that when the logical block is less than
8 times the disk sector size this effectively reduces the necessary compression
ratio; for example 8k blocks on disks with 4k disk sectors must compress to 1/2
or less of their original size.
.It Xo
.Sy context Ns = Ns Sy none Ns | Ns
.Em SELinux_User:SElinux_Role:Selinux_Type:Sensitivity_Level
.Xc
This flag sets the SELinux context for all files in the file system under
a mount point for that file system. See
.Xr selinux 8
for more information.
.It Xo
.Sy fscontext Ns = Ns Sy none Ns | Ns
.Em SELinux_User:SElinux_Role:Selinux_Type:Sensitivity_Level
.Xc
This flag sets the SELinux context for the file system file system being
mounted. See
.Xr selinux 8
for more information.
.It Xo
.Sy defcontext Ns = Ns Sy none Ns | Ns
.Em SELinux_User:SElinux_Role:Selinux_Type:Sensitivity_Level
.Xc
This flag sets the SELinux default context for unlabeled files. See
.Xr selinux 8
for more information.
.It Xo
.Sy rootcontext Ns = Ns Sy none Ns | Ns
.Em SELinux_User:SElinux_Role:Selinux_Type:Sensitivity_Level
.Xc
This flag sets the SELinux context for the root inode of the file system. See
.Xr selinux 8
for more information.
.It Sy copies Ns = Ns Sy 1 Ns | Ns Sy 2 Ns | Ns Sy 3
Controls the number of copies of data stored for this dataset.
These copies are in addition to any redundancy provided by the pool, for
example, mirroring or RAID-Z.
The copies are stored on different disks, if possible.
The space used by multiple copies is charged to the associated file and dataset,
changing the
.Sy used
property and counting against quotas and reservations.
.Pp
Changing this property only affects newly-written data.
Therefore, set this property at file system creation time by using the
.Fl o Sy copies Ns = Ns Ar N
option.
.Pp
Remember that ZFS will not import a pool with a missing top-level vdev. Do
.Sy NOT
create, for example a two-disk striped pool and set
.Sy copies=2
on some datasets thinking you have setup redundancy for them. When a disk
fails you will not be able to import the pool and will have lost all of your
data.
.Pp
Encrypted datasets may not have
.Sy copies Ns = Ns Em 3
since the implementation stores some encryption metadata where the third copy
would normally be.
.It Sy devices Ns = Ns Sy on Ns | Ns Sy off
Controls whether device nodes can be opened on this file system.
The default value is
.Sy on .
The values
.Sy on
and
.Sy off
are equivalent to the
.Sy dev
and
.Sy nodev
mount options.
.It Xo
.Sy dedup Ns = Ns Sy off Ns | Ns Sy on Ns | Ns Sy verify Ns | Ns
.Sy sha256[,verify] Ns | Ns Sy sha512[,verify] Ns | Ns Sy skein[,verify] Ns | Ns
.Sy edonr,verify
.Xc
Configures deduplication for a dataset. The default value is
.Sy off .
The default deduplication checksum is
.Sy sha256
(this may change in the future). When
.Sy dedup
is enabled, the checksum defined here overrides the
.Sy checksum
property. Setting the value to
.Sy verify
has the same effect as the setting
.Sy sha256,verify.
.Pp
If set to
.Sy verify ,
ZFS will do a byte-to-byte comparison in case of two blocks having the same
signature to make sure the block contents are identical. Specifying
.Sy verify
is mandatory for the
.Sy edonr
algorithm.
.Pp
Unless necessary, deduplication should NOT be enabled on a system. See the
.Em Deduplication
section of
.Xr zfsconcepts 8 .
.It Xo
.Sy dnodesize Ns = Ns Sy legacy Ns | Ns Sy auto Ns | Ns Sy 1k Ns | Ns
.Sy 2k Ns | Ns Sy 4k Ns | Ns Sy 8k Ns | Ns Sy 16k
.Xc
Specifies a compatibility mode or literal value for the size of dnodes in the
file system. The default value is
.Sy legacy .
Setting this property to a value other than
.Sy legacy
requires the large_dnode pool feature to be enabled.
.Pp
Consider setting
.Sy dnodesize
to
.Sy auto
if the dataset uses the
.Sy xattr=sa
property setting and the workload makes heavy use of extended attributes. This
may be applicable to SELinux-enabled systems, Lustre servers, and Samba
servers, for example. Literal values are supported for cases where the optimal
size is known in advance and for performance testing.
.Pp
Leave
.Sy dnodesize
set to
.Sy legacy
if you need to receive a send stream of this dataset on a pool that doesn't
enable the large_dnode feature, or if you need to import this pool on a system
that doesn't support the large_dnode feature.
.Pp
This property can also be referred to by its shortened column name,
.Sy dnsize .
.It Xo
.Sy encryption Ns = Ns Sy off Ns | Ns Sy on Ns | Ns Sy aes-128-ccm Ns | Ns
.Sy aes-192-ccm Ns | Ns Sy aes-256-ccm Ns | Ns Sy aes-128-gcm Ns | Ns
.Sy aes-192-gcm Ns | Ns Sy aes-256-gcm
.Xc
Controls the encryption cipher suite (block cipher, key length, and mode) used
for this dataset. Requires the
.Sy encryption
feature to be enabled on the pool.
Requires a
.Sy keyformat
to be set at dataset creation time.
.Pp
Selecting
.Sy encryption Ns = Ns Sy on
when creating a dataset indicates that the default encryption suite will be
selected, which is currently
.Sy aes-256-gcm .
In order to provide consistent data protection, encryption must be specified at
dataset creation time and it cannot be changed afterwards.
.Pp
For more details and caveats about encryption see the
.Sy Encryption
section.
.It Sy keyformat Ns = Ns Sy raw Ns | Ns Sy hex Ns | Ns Sy passphrase
Controls what format the user's encryption key will be provided as. This
property is only set when the dataset is encrypted.
.Pp
Raw keys and hex keys must be 32 bytes long (regardless of the chosen
encryption suite) and must be randomly generated. A raw key can be generated
with the following command:
.Bd -literal
# dd if=/dev/urandom of=/path/to/output/key bs=32 count=1
.Ed
.Pp
Passphrases must be between 8 and 512 bytes long and will be processed through
PBKDF2 before being used (see the
.Sy pbkdf2iters
property). Even though the
encryption suite cannot be changed after dataset creation, the keyformat can be
with
.Nm zfs Cm change-key .
.It Xo
.Sy keylocation Ns = Ns Sy prompt Ns | Ns Sy file:// Ns Em </absolute/file/path>
.Xc
Controls where the user's encryption key will be loaded from by default for
commands such as
.Nm zfs Cm load-key
and
.Nm zfs Cm mount Cm -l .
This property is only set for encrypted datasets which are encryption roots. If
unspecified, the default is
.Sy prompt.
.Pp
Even though the encryption suite cannot be changed after dataset creation, the
keylocation can be with either
.Nm zfs Cm set
or
.Nm zfs Cm change-key .
If
.Sy prompt
is selected ZFS will ask for the key at the command prompt when it is required
to access the encrypted data (see
.Nm zfs Cm load-key
for details). This setting will also allow the key to be passed in via STDIN,
but users should be careful not to place keys which should be kept secret on
the command line. If a file URI is selected, the key will be loaded from the
specified absolute file path.
.It Sy pbkdf2iters Ns = Ns Ar iterations
Controls the number of PBKDF2 iterations that a
.Sy passphrase
encryption key should be run through when processing it into an encryption key.
This property is only defined when encryption is enabled and a keyformat of
.Sy passphrase
is selected. The goal of PBKDF2 is to significantly increase the
computational difficulty needed to brute force a user's passphrase. This is
accomplished by forcing the attacker to run each passphrase through a
computationally expensive hashing function many times before they arrive at the
resulting key. A user who actually knows the passphrase will only have to pay
this cost once. As CPUs become better at processing, this number should be
raised to ensure that a brute force attack is still not possible. The current
default is
.Sy 350000
and the minimum is
.Sy 100000 .
This property may be changed with
.Nm zfs Cm change-key .
.It Sy exec Ns = Ns Sy on Ns | Ns Sy off
Controls whether processes can be executed from within this file system.
The default value is
.Sy on .
The values
.Sy on
and
.Sy off
are equivalent to the
.Sy exec
and
.Sy noexec
mount options.
.It Sy filesystem_limit Ns = Ns Em count Ns | Ns Sy none
Limits the number of filesystems and volumes that can exist under this point in
the dataset tree.
The limit is not enforced if the user is allowed to change the limit.
Setting a
.Sy filesystem_limit
to
.Sy on
a descendent of a filesystem that already has a
.Sy filesystem_limit
does not override the ancestor's
.Sy filesystem_limit ,
but rather imposes an additional limit.
This feature must be enabled to be used
.Po see
.Xr zpool-features 5
.Pc .
.It Sy special_small_blocks Ns = Ns Em size
This value represents the threshold block size for including small file
blocks into the special allocation class. Blocks smaller than or equal to this
value will be assigned to the special allocation class while greater blocks
will be assigned to the regular class. Valid values are zero or a power of two
from 512B up to 1M. The default size is 0 which means no small file blocks
will be allocated in the special class.
.Pp
Before setting this property, a special class vdev must be added to the
pool. See
.Xr zpool 8
for more details on the special allocation class.
.It Sy mountpoint Ns = Ns Pa path Ns | Ns Sy none Ns | Ns Sy legacy
Controls the mount point used for this file system.
See the
.Em Mount Points
section of
.Xr zfsconcepts 8
for more information on how this property is used.
.Pp
When the
.Sy mountpoint
property is changed for a file system, the file system and any children that
inherit the mount point are unmounted.
If the new value is
.Sy legacy ,
then they remain unmounted.
Otherwise, they are automatically remounted in the new location if the property
was previously
.Sy legacy
or
.Sy none ,
or if they were mounted before the property was changed.
In addition, any shared file systems are unshared and shared in the new
location.
.It Sy nbmand Ns = Ns Sy on Ns | Ns Sy off
Controls whether the file system should be mounted with
.Sy nbmand
.Pq Non Blocking mandatory locks .
This is used for SMB clients.
Changes to this property only take effect when the file system is umounted and
remounted.
See
.Xr mount 8
for more information on
.Sy nbmand
mounts. This property is not used on Linux.
.It Sy overlay Ns = Ns Sy on Ns | Ns Sy off
Allow mounting on a busy directory or a directory which already contains
files or directories.
This is the default mount behavior for Linux and FreeBSD file systems.
On these platforms the property is
.Sy on
by default.
Set to
.Sy off
to disable overlay mounts for consistency with OpenZFS on other platforms.
.It Sy primarycache Ns = Ns Sy all Ns | Ns Sy none Ns | Ns Sy metadata
Controls what is cached in the primary cache
.Pq ARC .
If this property is set to
.Sy all ,
then both user data and metadata is cached.
If this property is set to
.Sy none ,
then neither user data nor metadata is cached.
If this property is set to
.Sy metadata ,
then only metadata is cached.
The default value is
.Sy all .
.It Sy quota Ns = Ns Em size Ns | Ns Sy none
Limits the amount of space a dataset and its descendents can consume.
This property enforces a hard limit on the amount of space used.
This includes all space consumed by descendents, including file systems and
snapshots.
Setting a quota on a descendent of a dataset that already has a quota does not
override the ancestor's quota, but rather imposes an additional limit.
.Pp
Quotas cannot be set on volumes, as the
.Sy volsize
property acts as an implicit quota.
.It Sy snapshot_limit Ns = Ns Em count Ns | Ns Sy none
Limits the number of snapshots that can be created on a dataset and its
descendents.
Setting a
.Sy snapshot_limit
on a descendent of a dataset that already has a
.Sy snapshot_limit
does not override the ancestor's
.Sy snapshot_limit ,
but rather imposes an additional limit.
The limit is not enforced if the user is allowed to change the limit.
For example, this means that recursive snapshots taken from the global zone are
counted against each delegated dataset within a zone.
This feature must be enabled to be used
.Po see
.Xr zpool-features 5
.Pc .
.It Sy userquota@ Ns Em user Ns = Ns Em size Ns | Ns Sy none
Limits the amount of space consumed by the specified user.
User space consumption is identified by the
.Sy userspace@ Ns Em user
property.
.Pp
Enforcement of user quotas may be delayed by several seconds.
This delay means that a user might exceed their quota before the system notices
that they are over quota and begins to refuse additional writes with the
.Er EDQUOT
error message.
See the
.Nm zfs Cm userspace
subcommand for more information.
.Pp
Unprivileged users can only access their own groups' space usage.
The root user, or a user who has been granted the
.Sy userquota
privilege with
.Nm zfs Cm allow ,
can get and set everyone's quota.
.Pp
This property is not available on volumes, on file systems before version 4, or
on pools before version 15.
The
.Sy userquota@ Ns Em ...
properties are not displayed by
.Nm zfs Cm get Sy all .
The user's name must be appended after the
.Sy @
symbol, using one of the following forms:
.Bl -bullet
.It
.Em POSIX name
.Po for example,
.Sy joe
.Pc
.It
.Em POSIX numeric ID
.Po for example,
.Sy 789
.Pc
.It
.Em SID name
.Po for example,
.Sy joe.smith@mydomain
.Pc
.It
.Em SID numeric ID
.Po for example,
.Sy S-1-123-456-789
.Pc
.El
.Pp
Files created on Linux always have POSIX owners.
.It Sy userobjquota@ Ns Em user Ns = Ns Em size Ns | Ns Sy none
The
.Sy userobjquota
is similar to
.Sy userquota
but it limits the number of objects a user can create. Please refer to
.Sy userobjused
for more information about how objects are counted.
.It Sy groupquota@ Ns Em group Ns = Ns Em size Ns | Ns Sy none
Limits the amount of space consumed by the specified group.
Group space consumption is identified by the
.Sy groupused@ Ns Em group
property.
.Pp
Unprivileged users can access only their own groups' space usage.
The root user, or a user who has been granted the
.Sy groupquota
privilege with
.Nm zfs Cm allow ,
can get and set all groups' quotas.
.It Sy groupobjquota@ Ns Em group Ns = Ns Em size Ns | Ns Sy none
The
.Sy groupobjquota
is similar to
.Sy groupquota
but it limits number of objects a group can consume. Please refer to
.Sy userobjused
for more information about how objects are counted.
.It Sy projectquota@ Ns Em project Ns = Ns Em size Ns | Ns Sy none
Limits the amount of space consumed by the specified project. Project
space consumption is identified by the
.Sy projectused@ Ns Em project
property. Please refer to
.Sy projectused
for more information about how project is identified and set/changed.
.Pp
The root user, or a user who has been granted the
.Sy projectquota
privilege with
.Nm zfs allow ,
can access all projects' quota.
.It Sy projectobjquota@ Ns Em project Ns = Ns Em size Ns | Ns Sy none
The
.Sy projectobjquota
is similar to
.Sy projectquota
but it limits number of objects a project can consume. Please refer to
.Sy userobjused
for more information about how objects are counted.
.It Sy readonly Ns = Ns Sy on Ns | Ns Sy off
Controls whether this dataset can be modified.
The default value is
.Sy off .
The values
.Sy on
and
.Sy off
are equivalent to the
.Sy ro
and
.Sy rw
mount options.
.Pp
This property can also be referred to by its shortened column name,
.Sy rdonly .
.It Sy recordsize Ns = Ns Em size
Specifies a suggested block size for files in the file system.
This property is designed solely for use with database workloads that access
files in fixed-size records.
ZFS automatically tunes block sizes according to internal algorithms optimized
for typical access patterns.
.Pp
For databases that create very large files but access them in small random
chunks, these algorithms may be suboptimal.
Specifying a
.Sy recordsize
greater than or equal to the record size of the database can result in
significant performance gains.
Use of this property for general purpose file systems is strongly discouraged,
and may adversely affect performance.
.Pp
The size specified must be a power of two greater than or equal to 512 and less
than or equal to 128 Kbytes.
If the
.Sy large_blocks
feature is enabled on the pool, the size may be up to 1 Mbyte.
See
.Xr zpool-features 5
for details on ZFS feature flags.
.Pp
Changing the file system's
.Sy recordsize
affects only files created afterward; existing files are unaffected.
.Pp
This property can also be referred to by its shortened column name,
.Sy recsize .
.It Sy redundant_metadata Ns = Ns Sy all Ns | Ns Sy most
Controls what types of metadata are stored redundantly.
ZFS stores an extra copy of metadata, so that if a single block is corrupted,
the amount of user data lost is limited.
This extra copy is in addition to any redundancy provided at the pool level
.Pq e.g. by mirroring or RAID-Z ,
and is in addition to an extra copy specified by the
.Sy copies
property
.Pq up to a total of 3 copies .
For example if the pool is mirrored,
.Sy copies Ns = Ns 2 ,
and
.Sy redundant_metadata Ns = Ns Sy most ,
then ZFS stores 6 copies of most metadata, and 4 copies of data and some
metadata.
.Pp
When set to
.Sy all ,
ZFS stores an extra copy of all metadata.
If a single on-disk block is corrupt, at worst a single block of user data
.Po which is
.Sy recordsize
bytes long
.Pc
can be lost.
.Pp
When set to
.Sy most ,
ZFS stores an extra copy of most types of metadata.
This can improve performance of random writes, because less metadata must be
written.
In practice, at worst about 100 blocks
.Po of
.Sy recordsize
bytes each
.Pc
of user data can be lost if a single on-disk block is corrupt.
The exact behavior of which metadata blocks are stored redundantly may change in
future releases.
.Pp
The default value is
.Sy all .
.It Sy refquota Ns = Ns Em size Ns | Ns Sy none
Limits the amount of space a dataset can consume.
This property enforces a hard limit on the amount of space used.
This hard limit does not include space used by descendents, including file
systems and snapshots.
.It Sy refreservation Ns = Ns Em size Ns | Ns Sy none Ns | Ns Sy auto
The minimum amount of space guaranteed to a dataset, not including its
descendents.
When the amount of space used is below this value, the dataset is treated as if
it were taking up the amount of space specified by
.Sy refreservation .
The
.Sy refreservation
reservation is accounted for in the parent datasets' space used, and counts
against the parent datasets' quotas and reservations.
.Pp
If
.Sy refreservation
is set, a snapshot is only allowed if there is enough free pool space outside of
this reservation to accommodate the current number of
.Qq referenced
bytes in the dataset.
.Pp
If
.Sy refreservation
is set to
.Sy auto ,
a volume is thick provisioned
.Po or
.Qq not sparse
.Pc .
.Sy refreservation Ns = Ns Sy auto
is only supported on volumes.
See
.Sy volsize
in the
.Sx Native Properties
section for more information about sparse volumes.
.Pp
This property can also be referred to by its shortened column name,
.Sy refreserv .
.It Sy relatime Ns = Ns Sy on Ns | Ns Sy off
Controls the manner in which the access time is updated when
.Sy atime=on
is set. Turning this property on causes the access time to be updated relative
to the modify or change time. Access time is only updated if the previous
access time was earlier than the current modify or change time or if the
existing access time hasn't been updated within the past 24 hours. The default
value is
.Sy off .
The values
.Sy on
and
.Sy off
are equivalent to the
.Sy relatime
and
.Sy norelatime
mount options.
.It Sy reservation Ns = Ns Em size Ns | Ns Sy none
The minimum amount of space guaranteed to a dataset and its descendants.
When the amount of space used is below this value, the dataset is treated as if
it were taking up the amount of space specified by its reservation.
Reservations are accounted for in the parent datasets' space used, and count
against the parent datasets' quotas and reservations.
.Pp
This property can also be referred to by its shortened column name,
.Sy reserv .
.It Sy secondarycache Ns = Ns Sy all Ns | Ns Sy none Ns | Ns Sy metadata
Controls what is cached in the secondary cache
.Pq L2ARC .
If this property is set to
.Sy all ,
then both user data and metadata is cached.
If this property is set to
.Sy none ,
then neither user data nor metadata is cached.
If this property is set to
.Sy metadata ,
then only metadata is cached.
The default value is
.Sy all .
.It Sy setuid Ns = Ns Sy on Ns | Ns Sy off
Controls whether the setuid bit is respected for the file system.
The default value is
.Sy on .
The values
.Sy on
and
.Sy off
are equivalent to the
.Sy suid
and
.Sy nosuid
mount options.
.It Sy sharesmb Ns = Ns Sy on Ns | Ns Sy off Ns | Ns Em opts
Controls whether the file system is shared by using
.Sy Samba USERSHARES
and what options are to be used. Otherwise, the file system is automatically
shared and unshared with the
.Nm zfs Cm share
and
.Nm zfs Cm unshare
commands. If the property is set to on, the
.Xr net 8
command is invoked to create a
.Sy USERSHARE .
.Pp
Because SMB shares requires a resource name, a unique resource name is
constructed from the dataset name. The constructed name is a copy of the
dataset name except that the characters in the dataset name, which would be
invalid in the resource name, are replaced with underscore (_) characters.
Linux does not currently support additional options which might be available
on Solaris.
.Pp
If the
.Sy sharesmb
property is set to
.Sy off ,
the file systems are unshared.
.Pp
The share is created with the ACL (Access Control List) "Everyone:F" ("F"
stands for "full permissions", ie. read and write permissions) and no guest
access (which means Samba must be able to authenticate a real user, system
passwd/shadow, LDAP or smbpasswd based) by default. This means that any
additional access control (disallow specific user specific access etc) must
be done on the underlying file system.
.It Sy sharenfs Ns = Ns Sy on Ns | Ns Sy off Ns | Ns Em opts
Controls whether the file system is shared via NFS, and what options are to be
used.
A file system with a
.Sy sharenfs
property of
.Sy off
is managed with the
.Xr exportfs 8
command and entries in the
.Em /etc/exports
file.
Otherwise, the file system is automatically shared and unshared with the
.Nm zfs Cm share
and
.Nm zfs Cm unshare
commands.
If the property is set to
.Sy on ,
the dataset is shared using the default options:
.Pp
.Em sec=sys,rw,crossmnt,no_subtree_check
.Pp
See
.Xr exports 5
for the meaning of the default options. Otherwise, the
.Xr exportfs 8
command is invoked with options equivalent to the contents of this property.
.Pp
When the
.Sy sharenfs
property is changed for a dataset, the dataset and any children inheriting the
property are re-shared with the new options, only if the property was previously
.Sy off ,
or if they were shared before the property was changed.
If the new property is
.Sy off ,
the file systems are unshared.
.It Sy logbias Ns = Ns Sy latency Ns | Ns Sy throughput
Provide a hint to ZFS about handling of synchronous requests in this dataset.
If
.Sy logbias
is set to
.Sy latency
.Pq the default ,
ZFS will use pool log devices
.Pq if configured
to handle the requests at low latency.
If
.Sy logbias
is set to
.Sy throughput ,
ZFS will not use configured pool log devices.
ZFS will instead optimize synchronous operations for global pool throughput and
efficient use of resources.
.It Sy snapdev Ns = Ns Sy hidden Ns | Ns Sy visible
Controls whether the volume snapshot devices under
.Em /dev/zvol/<pool>
are hidden or visible. The default value is
.Sy hidden .
.It Sy snapdir Ns = Ns Sy hidden Ns | Ns Sy visible
Controls whether the
.Pa .zfs
directory is hidden or visible in the root of the file system as discussed in
the
.Em Snapshots
section of
.Xr zfsconcepts 8 .
The default value is
.Sy hidden .
.It Sy sync Ns = Ns Sy standard Ns | Ns Sy always Ns | Ns Sy disabled
Controls the behavior of synchronous requests
.Pq e.g. fsync, O_DSYNC .
.Sy standard
is the
.Tn POSIX
specified behavior of ensuring all synchronous requests are written to stable
storage and all devices are flushed to ensure data is not cached by device
controllers
.Pq this is the default .
.Sy always
causes every file system transaction to be written and flushed before its
system call returns.
This has a large performance penalty.
.Sy disabled
disables synchronous requests.
File system transactions are only committed to stable storage periodically.
This option will give the highest performance.
However, it is very dangerous as ZFS would be ignoring the synchronous
transaction demands of applications such as databases or NFS.
Administrators should only use this option when the risks are understood.
.It Sy version Ns = Ns Em N Ns | Ns Sy current
The on-disk version of this file system, which is independent of the pool
version.
This property can only be set to later supported versions.
See the
.Nm zfs Cm upgrade
command.
.It Sy volsize Ns = Ns Em size
For volumes, specifies the logical size of the volume.
By default, creating a volume establishes a reservation of equal size.
For storage pools with a version number of 9 or higher, a
.Sy refreservation
is set instead.
Any changes to
.Sy volsize
are reflected in an equivalent change to the reservation
.Po or
.Sy refreservation
.Pc .
The
.Sy volsize
can only be set to a multiple of
.Sy volblocksize ,
and cannot be zero.
.Pp
The reservation is kept equal to the volume's logical size to prevent unexpected
behavior for consumers.
Without the reservation, the volume could run out of space, resulting in
undefined behavior or data corruption, depending on how the volume is used.
These effects can also occur when the volume size is changed while it is in use
.Pq particularly when shrinking the size .
Extreme care should be used when adjusting the volume size.
.Pp
Though not recommended, a
.Qq sparse volume
.Po also known as
.Qq thin provisioned
.Pc
can be created by specifying the
.Fl s
option to the
.Nm zfs Cm create Fl V
command, or by changing the value of the
.Sy refreservation
property
.Po or
.Sy reservation
property on pool version 8 or earlier
.Pc
after the volume has been created.
A
.Qq sparse volume
is a volume where the value of
.Sy refreservation
is less than the size of the volume plus the space required to store its
metadata.
Consequently, writes to a sparse volume can fail with
.Er ENOSPC
when the pool is low on space.
For a sparse volume, changes to
.Sy volsize
are not reflected in the
.Sy refreservation.
A volume that is not sparse is said to be
.Qq thick provisioned .
A sparse volume can become thick provisioned by setting
.Sy refreservation
to
.Sy auto .
.It Sy volmode Ns = Ns Cm default | full | geom | dev | none
This property specifies how volumes should be exposed to the OS.
Setting it to
.Sy full
exposes volumes as fully fledged block devices, providing maximal
functionality. The value
.Sy geom
is just an alias for
.Sy full
and is kept for compatibility.
Setting it to
.Sy dev
hides its partitions.
Volumes with property set to
.Sy none
are not exposed outside ZFS, but can be snapshotted, cloned, replicated, etc,
that can be suitable for backup purposes.
Value
.Sy default
means that volumes exposition is controlled by system-wide tunable
.Va zvol_volmode ,
where
.Sy full ,
.Sy dev
and
.Sy none
are encoded as 1, 2 and 3 respectively.
The default values is
.Sy full .
.It Sy vscan Ns = Ns Sy on Ns | Ns Sy off
Controls whether regular files should be scanned for viruses when a file is
opened and closed.
In addition to enabling this property, the virus scan service must also be
enabled for virus scanning to occur.
The default value is
.Sy off .
This property is not used on Linux.
.It Sy xattr Ns = Ns Sy on Ns | Ns Sy off Ns | Ns Sy sa
Controls whether extended attributes are enabled for this file system. Two
styles of extended attributes are supported either directory based or system
attribute based.
.Pp
The default value of
.Sy on
enables directory based extended attributes. This style of extended attribute
imposes no practical limit on either the size or number of attributes which
can be set on a file. Although under Linux the
.Xr getxattr 2
and
.Xr setxattr 2
system calls limit the maximum size to 64K. This is the most compatible
style of extended attribute and is supported by all OpenZFS implementations.
.Pp
System attribute based xattrs can be enabled by setting the value to
.Sy sa .
The key advantage of this type of xattr is improved performance. Storing
extended attributes as system attributes significantly decreases the amount of
disk IO required. Up to 64K of data may be stored per-file in the space
reserved for system attributes. If there is not enough space available for
an extended attribute then it will be automatically written as a directory
based xattr. System attribute based extended attributes are not accessible
on platforms which do not support the
.Sy xattr=sa
feature.
.Pp
The use of system attribute based xattrs is strongly encouraged for users of
SELinux or POSIX ACLs. Both of these features heavily rely of extended
attributes and benefit significantly from the reduced access time.
.Pp
The values
.Sy on
and
.Sy off
are equivalent to the
.Sy xattr
and
.Sy noxattr
mount options.
.It Sy jailed Ns = Ns Cm off | on
Controls whether the dataset is managed from a jail. See the
.Qq Sx Jails
section in
.Xr zfs 8
for more information. Jails are a FreeBSD feature and are not relevant on
other platforms. The default value is
.Cm off .
.It Sy zoned Ns = Ns Sy on Ns | Ns Sy off
Controls whether the dataset is managed from a non-global zone. Zones are a
Solaris feature and are not relevant on other platforms. The default value is
.Sy off .
.El
.Pp
The following three properties cannot be changed after the file system is
created, and therefore, should be set when the file system is created.
If the properties are not set with the
.Nm zfs Cm create
or
.Nm zpool Cm create
commands, these properties are inherited from the parent dataset.
If the parent dataset lacks these properties due to having been created prior to
these features being supported, the new file system will have the default values
for these properties.
.Bl -tag -width ""
.It Xo
.Sy casesensitivity Ns = Ns Sy sensitive Ns | Ns
.Sy insensitive Ns | Ns Sy mixed
.Xc
Indicates whether the file name matching algorithm used by the file system
should be case-sensitive, case-insensitive, or allow a combination of both
styles of matching.
The default value for the
.Sy casesensitivity
property is
.Sy sensitive .
Traditionally,
.Ux
and
.Tn POSIX
file systems have case-sensitive file names.
.Pp
The
.Sy mixed
value for the
.Sy casesensitivity
property indicates that the file system can support requests for both
case-sensitive and case-insensitive matching behavior.
Currently, case-insensitive matching behavior on a file system that supports
mixed behavior is limited to the SMB server product.
For more information about the
.Sy mixed
value behavior, see the "ZFS Administration Guide".
.It Xo
.Sy normalization Ns = Ns Sy none Ns | Ns Sy formC Ns | Ns
.Sy formD Ns | Ns Sy formKC Ns | Ns Sy formKD
.Xc
Indicates whether the file system should perform a
.Sy unicode
normalization of file names whenever two file names are compared, and which
normalization algorithm should be used.
File names are always stored unmodified, names are normalized as part of any
comparison process.
If this property is set to a legal value other than
.Sy none ,
and the
.Sy utf8only
property was left unspecified, the
.Sy utf8only
property is automatically set to
.Sy on .
The default value of the
.Sy normalization
property is
.Sy none .
This property cannot be changed after the file system is created.
.It Sy utf8only Ns = Ns Sy on Ns | Ns Sy off
Indicates whether the file system should reject file names that include
characters that are not present in the
.Sy UTF-8
character code set.
If this property is explicitly set to
.Sy off ,
the normalization property must either not be explicitly set or be set to
.Sy none .
The default value for the
.Sy utf8only
property is
.Sy off .
This property cannot be changed after the file system is created.
.El
.Pp
The
.Sy casesensitivity ,
.Sy normalization ,
and
.Sy utf8only
properties are also new permissions that can be assigned to non-privileged users
by using the ZFS delegated administration feature.
.Ss "Temporary Mount Point Properties"
When a file system is mounted, either through
.Xr mount 8
for legacy mounts or the
.Nm zfs Cm mount
command for normal file systems, its mount options are set according to its
properties.
The correlation between properties and mount options is as follows:
.Bd -literal
    PROPERTY                MOUNT OPTION
    atime                   atime/noatime
    canmount                auto/noauto
    devices                 dev/nodev
    exec                    exec/noexec
    readonly                ro/rw
    relatime                relatime/norelatime
    setuid                  suid/nosuid
    xattr                   xattr/noxattr
.Ed
.Pp
In addition, these options can be set on a per-mount basis using the
.Fl o
option, without affecting the property that is stored on disk.
The values specified on the command line override the values stored in the
dataset.
The
.Sy nosuid
option is an alias for
.Sy nodevices Ns \&, Ns Sy nosetuid .
These properties are reported as
.Qq temporary
by the
.Nm zfs Cm get
command.
If the properties are changed while the dataset is mounted, the new setting
overrides any temporary settings.
.Ss "User Properties"
In addition to the standard native properties, ZFS supports arbitrary user
properties.
User properties have no effect on ZFS behavior, but applications or
administrators can use them to annotate datasets
.Pq file systems, volumes, and snapshots .
.Pp
User property names must contain a colon
.Pq Qq Sy \&:
character to distinguish them from native properties.
They may contain lowercase letters, numbers, and the following punctuation
characters: colon
.Pq Qq Sy \&: ,
dash
.Pq Qq Sy - ,
period
.Pq Qq Sy \&. ,
and underscore
.Pq Qq Sy _ .
The expected convention is that the property name is divided into two portions
such as
.Em module Ns \&: Ns Em property ,
but this namespace is not enforced by ZFS.
User property names can be at most 256 characters, and cannot begin with a dash
.Pq Qq Sy - .
.Pp
When making programmatic use of user properties, it is strongly suggested to use
a reversed
.Sy DNS
domain name for the
.Em module
component of property names to reduce the chance that two
independently-developed packages use the same property name for different
purposes.
.Pp
The values of user properties are arbitrary strings, are always inherited, and
are never validated.
All of the commands that operate on properties
.Po Nm zfs Cm list ,
.Nm zfs Cm get ,
.Nm zfs Cm set ,
and so forth
.Pc
can be used to manipulate both native properties and user properties.
Use the
.Nm zfs Cm inherit
command to clear a user property.
If the property is not defined in any parent dataset, it is removed entirely.
Property values are limited to 8192 bytes.