5.2.1 Read-Only Properties

Read-only properties are properties that can be retrieved but cannot be set. Read-only properties are not inherited. Some properties are specific to a particular type of dataset; in such cases the particular dataset type is called out in the description.

• available -- 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, this can be limited by any number of factors, including physical pool size, quotas, reservations, or other datasets within the pool.

  This property can also be referred to by its shortened column name, avail.

  For more information about space accounting, see 3.2 Space Accounting.

  This property can also be referred to by its shortened column name, avail.

• creation -- Date and time that this dataset was created.

• mounted -- Indicates whether this filesystem, clone, or snapshot is currently mounted; does not apply to volumes.

• origin -- For cloned filesystems only; the snapshot from which this clone originated. Non-cloned filesystems have an origin of none. The origin cannot be destroyed so long as a clone exists.

• compressratio -- The compression ratio achieved on this 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.

• referenced -- The amount of data 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. This property can also be referred to by its shortened column name, refer.

• type -- Dataset type such as filesystem (filesystem/clone), volume, or snapshot

• used -- The amount of space consumed by this dataset and all its descendants. 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 descendant 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.

  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. 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 3.3 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.

For more information on space accounting: including the used, referenced, and available properties listed above see 3.2 Space Accounting.

5.2.2 Settable Properties

Settable properties are properties whose values can be both retrieved and set. Settable properties are set via the zfs set interface described in 5.4.1 Setting Properties. With the exceptions of quotas and reservations, settable properties are inherited. For more information on quotas and reservations, see 5.6 Quotas and Reservations.

Some settable properties are specific to a particular type of dataset; in such cases the particular dataset type is called out in the description field of the table. If not specifically mentioned, a property applies to all dataset types: filesystems, volumes, clones, and snapshots.

• atime -- Controls whether the access time for files is updated when read. Turning this property off avoids producing write traffic when reading files and can result in significant performance gains, though it may confuse mailers and other similar utilities.

• checksum -- Controls the checksum used to verify data integrity. The value on automatically selects an appropriate algorithm (currently fletcher2, but this may change in future releases). The value off disables integrity checking on user data; this is not recommended.

• compression -- Controls the compression algorithm used for this dataset. The value on automatically selects an appropriate algorithm. There is currently only one algorithm, lzjb, though this may change in future releases.

• devices -- Controls whether device nodes found within this filesystem can be opened.

• exec -- Controls whether programs within this filesystem are allowed to be executed. Also, when set to off, mmap(2) calls with PROT_EXEC will be disallowed.

• mountpoint -- Controls the mount point used for this filesystem. When the mountpoint property is changed for a filesystem, the filesystem 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 on using this property, see 5.5.1 Managing Mount Points.

• quota -- Limits the amount of space a dataset and its descendents can consume. This 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. Quotas cannot be set on volumes, as the volsize property acts as an implicit quota.

  For information about setting quotas, see 5.6.1 Setting Quotas.

• readonly -- Controls whether this dataset can be modified. When set to on, no modifications can be made to the dataset.

• recordsize -- 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. For databases that create very large files but access them 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 filesystem's recordsize only affects files created afterward; existing files are unaffected.

  This property can also be referred to by its shortened column name, recsize.

• reservation -- 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 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. This property can also be referred to by its shortened column name, reserv.

  For more information, see 5.6.2 Setting Reservations).

• sharenfs -- Controls whether the filesystem is shared via NFS, and what options are used. A filesystem with a sharenfs property of off is managed through traditional tools such as share(1M), unshare(1M), and dfstab(4). Otherwise, the filesystem is automatically shared and unshared with the zfs share and zfs unshare commands. If the property is set to on, the share(1M) command is invoked with no options. Otherwise, the share(1M) command is invoked with options equivalent to the contents of this property. When the sharenfs property is changed for a dataset, the dataset and any children are re-shared with the new options, only if the property was previously off, or if they were shared before the property was changed. If the new property is off, the filesystems are unshared.

  For more information on sharing ZFS filesystems, see 5.5.5 Sharing ZFS File Systems.

• setuid -- Controls whether the set-UID bit is respected for the filesystem.

• snapdir -- Controls whether the .zfs directory is hidden or visible in the root of the file system. For more information on using snapshots, see 6.1 ZFS Snapshots.

• volsize -- 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 consumers. A volume which 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 shrinking 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 8.1 Emulated Volumes.

• volblocksize -- For volumes, specifies the block size of the volume. The blocksize cannot be changed once the volume has been written, so it should be set at volume creation time. The default blocksize 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 -- Indicates whether this dataset has been added to a non-global zone. If this is set, then the mount point is not respected in the global zone, and ZFS refuses to mount such a filesystem when asked. When a zone is first installed, this is set for any added filesystems. For more information on using ZFS with zones installed, see 8.2 Using ZFS on a Solaris System With Zones Installed.