Introducing ZFS Properties
Properties are the main mechanism that you use to control the behavior
of file systems, volumes, snapshots, and clones. Unless stated otherwise,
the properties defined in this
section apply to all the dataset types.
Properties are divided into two types, native properties and user defined
properties. Native properties either export internal statistics or control
ZFS file system behavior. In addition, native properties are either settable
or read-only. User properties have no effect on ZFS file system behavior,
but you can use them to annotate datasets in a way that is meaningful in your
environment. For more information on user properties, see ZFS User Properties.
Most settable properties are also inheritable. An inheritable property
is a property that, when set on a parent, is propagated down to all of its
descendents.
All inheritable properties have an associated source. The source indicates
how a property was obtained. The source of a property can have the following
values:
-
local
-
A local source indicates that the property
was explicitly set on the dataset by using the zfs set command
as described in Setting ZFS Properties.
-
inherited from dataset-name
-
A value of inherited from dataset-name means that the property was inherited from the named ancestor.
-
default
-
A value of default means that the property
setting was not inherited or set locally. This source is a result of no ancestor
having the property as source local.
The following table identifies both read-only and settable native ZFS
file system properties. Read-only native properties are identified as such.
All other native properties listed in this table are settable. For information
about user properties, see ZFS User Properties.
Table 6–1 ZFS Native Property Descriptions
|
Property Name
|
Type
|
Default Value
|
Description
|
|
aclinherit
|
String
|
secure
|
Controls how ACL entries are inherited when files and directories are
created. The values are discard, noallow, secure, and passthrough. For a description of
these values, see ACL Property Modes.
|
|
aclmode
|
String
|
groupmask
|
Controls how an ACL entry is modified during a chmod operation.
The values are discard, groupmask, and passthrough. For a description of these values, see ACL Property Modes.
|
|
atime
|
Boolean
|
on
|
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.
|
|
available
|
Number
|
N/A
|
Read-only property that identifies the amount of space available to
the dataset and all its children, assuming no other activity in the pool.
Because space is shared within a pool, available space can be limited by
various factors including physical pool size, quotas, reservations, or other
datasets within the pool.
This property can also be referenced by its shortened column name, avail.
For more information about space accounting, see ZFS Space Accounting.
|
|
canmount
|
Boolean
|
on
|
Controls whether the given file system can be mounted with the zfs mount command. This property can be set on any
file system and the property itself is not inheritable. However, when this
property is set to off, a mountpoint can be inherited to
descendent file systems, but the file system itself is never mounted.
When the noauto option is set, 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 zfs mount-a command or unmounted by the zfs unmount-a command.
For more information, see The canmount Property.
|
|
checksum
|
String
|
on
|
Controls the checksum used to verify data integrity. The default value
is on, which automatically selects an appropriate algorithm,
currently fletcher4. The values are on,
off, fletcher2, fletcher4, and sha256.
A value of off disables integrity checking on user data.
A value of off is not recommended.
|
|
compression
|
String
|
off
|
Enables or disables compression for this dataset. The values are on, off, and lzjb, gzip,
or gzip-N. Currently, setting
this property to lzjb, gzip, or gzip-N has the same effect as setting this
property to on. The default value is off.
Enabling compression on a file system with existing data only compresses new
data. Existing data remains uncompressed.
This property can also be referred to by its shortened column name, compress.
|
|
compressratio
|
Number
|
N/A
|
Read-only property that identifies the compression ratio achieved for
this dataset, expressed as a multiplier. Compression can be turned on by running zfs set compression=on dataset.
Calculated from the logical size of all files and the amount of referenced
physical data. Includes explicit savings through the use of the compression property.
|
|
copies
|
Number
|
1
|
Sets the number of copies of user data per file system. Available values
are 1, 2 or 3. These copies are in addition to any pool-level redundancy.
Space used by multiple copies of user data is charged to the corresponding
file and dataset and counts against quotas and reservations. In addition,
the used property is updated when multiple copies are enabled.
Consider setting this property when the file system is created because changing
this property on an existing file system only affects newly written data.
|
|
creation
|
String
|
N/A
|
Read-only property that identifies the date and time that this dataset
was created.
|
|
devices
|
Boolean
|
on
|
Controls the ability to open device files in the file system.
|
|
exec
|
Boolean
|
on
|
Controls whether programs within this file system are allowed to be
executed. Also, when set to off, mmap(2) calls
with PROT_EXEC are disallowed.
|
|
mounted
|
boolean
|
N/A
|
Read-only property that indicates whether this file system, clone, or
snapshot is currently mounted. This property does not apply to volumes. Value
can be either yes or no.
|
|
mountpoint
|
String
|
N/A
|
Controls the mount point used for this file system. When the 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 legacy, then they remain unmounted. Otherwise, they are automatically
remounted in the new location if the property was previously legacy or 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.
For more information about using this property, see Managing ZFS Mount Points.
|
|
primarycache
|
String
|
off
|
Controls what is cached in the ARC. Possible values are all, none, and metadata. If set to all,
both user data and metadata are cached. If set to none,
neither user data nor metadata is cached. If set to metadata,
only metadata is cached. The default is all.
|
|
origin
|
String
|
N/A
|
Read-only property for cloned file systems or volumes that identifies
the snapshot from which the clone was created. The origin cannot be destroyed
(even with the -r or -f options) as long as
a clone exists.
Non-cloned file systems have an origin of none.
|
|
quota
|
Number (or none)
|
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, including
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. Quotas cannot
be set on volumes, as the volsize property acts as an implicit
quota.
For information about setting quotas, see Setting Quotas on ZFS File Systems.
|
|
readonly
|
Boolean
|
off
|
Controls whether this dataset can be modified. When set to on,
no modifications can be made to the dataset.
This property can also be referred to by its shortened column name, rdonly.
|
|
recordsize
|
Number
|
128K
|
Specifies a suggested block size for files in the file system.
This property can also be referred to by its shortened column name, recsize. For a detailed description, see The recordsize Property.
|
|
referenced
|
Number
|
N/A
|
Read-only property that identifies the amount of data accessible by
this dataset, which might or might 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, because
its contents are identical.
This property can also be referred to by its shortened column name, refer.
|
|
refquota
|
Number (or none)
|
none
|
Sets the amount of space that 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, such as snapshots and clones.
|
|
refreservation
|
Number (or none)
|
none
|
Sets the minimum amount of space that is guaranteed to a dataset, not
including descendents, such as snapshots and clones. When the amount of space
that is used is below this value, the dataset is treated as if it were taking
up the amount of space specified by refreservation. The
refreservation reservation is accounted for in the parent
datasets' space used, and counts against the parent datasets' quotas and
reservations.
If refreservation is set, a snapshot is only allowed
if enough free pool space is available outside of this reservation to accommodate
the current number of referenced bytes in the dataset.
This property can also be referred to by its shortened column name, refreserv.
|
|
reservation
|
Number (or none)
|
none
|
The minimum amount of space guaranteed to a dataset and its descendents.
When the amount of space used is below this value, the dataset is treated
as if it were using 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.
This property can also be referred to by its shortened column name, reserv.
For more information, see Setting Reservations on ZFS File Systems.
|
|
secondarycache
|
String
|
off
|
Controls what is cached in the L2ARC. Possible values are all, none, and metadata. If set to all,
both user data and metadata are cached. If set to none,
neither user data nor metadata is cached. If set to metadata,
only metadata is cached. The default is all.
|
|
setuid
|
Boolean
|
on
|
Controls whether the setuid bit is honored in the
file system.
|
|
sharenfs
|
String
|
off
|
Controls whether the file system is available over NFS, and what options
are used. If set to on, the zfs share command
is invoked with no options. Otherwise, the zfs share command
is invoked with options equivalent to the contents of this property. If set
to off, the file system is managed by using the legacy share and unshare commands and the dfstab file.
For more information on sharing ZFS file systems, see Sharing and Unsharing ZFS File Systems.
|
|
snapdir
|
String
|
hidden
|
Controls whether the .zfs directory is hidden or
visible in the root of the file system. For more information on using snapshots,
see Overview of ZFS Snapshots.
|
|
type
|
String
|
N/A
|
Read-only property that identifies the dataset type as filesystem (file
system or clone), volume, or snapshot.
|
|
used
|
Number
|
N/A
|
Read-only property that identifies the amount of space consumed by the
dataset and all its descendents.
For a detailed description, see The used Property.
|
|
usedbychildren
|
Number
|
off
|
Read-only property that identifies the amount of space that is used
by children of this dataset, which would be freed if all the dataset's children
were destroyed. The property abbreviation is usedchild.
|
|
usedbydataset
|
Number
|
off
|
Read-only property that identifies the amount of space that is used
by this dataset itself, which would be freed if the dataset was destroyed,
after first destroying any snapshots and removing any refreservation.
The property abbreviation is usedds.
|
|
usedbyrefreservation
|
Number
|
off
|
Read-only property that identifies the amount of space that is used
by a refreservation set on this dataset, which would be
freed if the refreservation was removed. The property abbreviation
is usedrefreserv.
|
|
usedbysnapshots
|
Number
|
off
|
Read-only property that identifies the amount of space that is 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' used properties,
because space can be shared by multiple snapshots. The property abbreviation
is usedsnap.
|
|
volsize
|
Number
|
N/A
|
For volumes, specifies the logical size of the volume.
For a detailed description, see The volsize Property.
|
|
volblocksize
|
Number
|
8 Kbytes
|
For volumes, specifies the block size of the volume. The block size
cannot be changed once the volume has been written, so set the block size
at volume creation time. The default block size for volumes is 8 Kbytes. Any
power of 2 from 512 bytes to 128 Kbytes is valid.
This property can also be referred to by its shortened column name, volblock.
|
|
zoned
|
Boolean
|
N/A
|
Indicates whether this dataset has been added to a non-global zone.
If this property is set, then the mount point is not honored in the global
zone, and ZFS cannot mount such a file system when requested. When a zone
is first installed, this property is set for any added file systems.
For more information about using ZFS with zones installed, see Using ZFS on a Solaris System With Zones Installed.
|
|
xattr
|
Boolean
|
on
|
Indicates whether extended attributes are enabled or disabled for this
file system. The default value is on.
|
ZFS Read-Only Native Properties
Read-only native properties are properties that can be retrieved but
cannot be set. Read-only native properties are not inherited. Some native
properties are specific to a particular type of dataset. In such cases, the
particular dataset type is mentioned in the description in Table 6–1.
The read-only native properties are listed here and are described in Table 6–1.
For more information on space accounting, including the used, referenced, and available properties, see ZFS Space Accounting.
The used Property
The amount of space consumed by this dataset and all its descendents.
This value is checked against the dataset's quota and reservation. The space
used does not include the dataset's reservation, but does consider the reservation
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 the dataset is
recursively destroyed, is the greater of its space used and its reservation.
When snapshots are created, their space is initially shared between
the snapshot and the file system, and possibly with previous snapshots. As
the file system changes, space that was previously shared becomes unique to
the snapshot, and counted in the snapshot's space used. The space that is
used by a snapshot accounts for its unique data. Additionally, deleting snapshots
can increase the amount of space unique to (and used by) other snapshots.
For more information about snapshots and space issues, see Out of Space Behavior.
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 fsync(3c) or O_SYNC does not necessarily guarantee that the space usage information
will be updated immediately.
The usedbychildren, usedbydataset, usedbyrefreservation, and usedbysnapshots property
information can be displayed with the zfs list -o
space command. These properties break down the used property
into space that is consumed by descendents. For more information, see Table 6–1.
Settable ZFS Native Properties
Settable native properties are properties whose values can be both retrieved
and set. Settable native properties are set by using the zfs set command,
as described in Setting ZFS Properties or
by using the zfs create command as described in Creating a ZFS File System. With the exceptions
of quotas and reservations, settable native properties are inherited. For
more information about quotas and reservations, see Setting ZFS Quotas and Reservations.
Some settable native properties are specific to a particular type of
dataset. In such cases, the particular dataset type is mentioned in the description
in Table 6–1. If not specifically
mentioned, a property applies to all dataset types: file systems, volumes,
clones, and snapshots.
The settable properties are listed here and are described in Table 6–1.
-
aclinherit
For a detailed
description, see ACL Property Modes.
-
aclmode
For a detailed description,
see ACL Property Modes.
-
atime
-
canmount
-
checksum
-
compression
-
copies
-
devices
-
exec
-
mountpoint
-
primarycache
-
quota
-
readonly
-
recordsize
For a detailed
description, see The recordsize Property.
-
refquota
-
refreservation
-
reservation
-
secondarycache
-
sharenfs
-
setuid
-
snapdir
-
volsize
For a detailed description,
see The volsize Property.
-
volblocksize
-
zoned
The canmount Property
If this property is set to off, the file system cannot be mounted by
using the zfs mount or zfs mount -a commands.
Setting this property is similar to setting the mountpoint property
to none, except that the dataset still has a normal mountpoint property that can be inherited. For example, you can
set this property to off, establish inheritable properties for descendent
file systems, but the file system itself is never mounted nor is it accessible to users. In
this case, the parent file system with this property set to off is serving
as a container so that you can set attributes on the
container, but the container itself is never accessible.
In the following example, userpool is created and
the canmount property is set to off. Mount points for
descendent user file systems are set to one common mount point, /export/home. Properties that are set on the parent file system are inherited
by descendent file systems, but the parent file system itself is never mounted.
# zpool create userpool mirror c0t5d0 c1t6d0
# zfs set canmount=off userpool
# zfs set mountpoint=/export/home userpool
# zfs set compression=on userpool
# zfs create userpool/user1
# zfs create userpool/user2
# zfs mount
userpool/user1 /export/home/user1
userpool/user2 /export/home/user2
|
Setting the canmount property to noauto means
that the dataset can only be mounted explicitly, not automatically. This setting
is used by the Solaris upgrade software so that only those datasets belonging
to the active boot environment (BE) are mounted at boot time.
The recordsize Property
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 adjust block sizes according
to internal algorithms optimized for typical access patterns. For databases
that create very large files but access the files in small random chunks,
these algorithms may be suboptimal. Specifying a 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. The size specified
must be a power of two greater than or equal to 512 and less than or equal
to 128 Kbytes. Changing the file system's recordsize only
affects files created afterward. Existing files are unaffected.
This property can also be referred to by its shortened column name, recsize.
The volsize Property
The logical size of the volume. By default, creating a volume establishes
a reservation for the same amount. Any changes to volsize are
reflected in an equivalent change to the reservation. These checks are used
to prevent unexpected behavior for users. A volume that contains less space
than it claims is available can result 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, particularly when you shrink the
size. Extreme care should be used when adjusting the volume size.
Though not recommended, you can create a sparse volume by specifying
the -s flag to zfs create -V, or by changing
the reservation once the volume has been created. A sparse volume is
defined as a volume where the reservation is not equal to the volume size.
For a sparse volume, changes to volsize are not reflected
in the reservation.
For more information about using volumes, see ZFS Volumes.
ZFS User Properties
In addition to the standard native properties, ZFS supports arbitrary
user properties. User properties have no effect on ZFS behavior, but you can
use them to annotate datasets with information that is meaningful in your
environment.
User property names must conform to the following characteristics:
-
Contain a colon (':') character to distinguish them from native
properties.
-
Contain lowercase letters, numbers, and the following punctuation
characters: ':', '+','.', '_'.
-
Maximum user property name is 256 characters.
The expected convention is that the property name is divided into the
following two components but this namespace is not enforced by ZFS:
When making programmatic use of user properties, use a reversed DNS
domain name for the module component of property
names to reduce the chance that two independently-developed packages will
use the same property name for different purposes. Property names that begin
with "com.sun." are reserved for use by Sun Microsystems.
The values of user properties have the following characteristics:
For example:
# zfs set dept:users=finance userpool/user1
# zfs set dept:users=general userpool/user2
# zfs set dept:users=itops userpool/user3
|
All of the commands that operate on properties, such as zfs
list, zfs get, zfs set, and
so on, can be used to manipulate both native properties and user properties.
For example:
zfs get -r dept:users userpool
NAME PROPERTY VALUE SOURCE
userpool dept:users all local
userpool/user1 dept:users finance local
userpool/user2 dept:users general local
userpool/user3 dept:users itops local
|
To clear a user property, use the zfs inherit command.
For example:
# zfs inherit -r dept:users userpool
|
If the property is not defined in any parent dataset, it is removed
entirely.
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