zpool-features

title: zpool-features(7)

summary: description of ZFS pool features

text:
# zpool-features(7)

## NAME
zpool-features — description of ZFS pool features

## DESCRIPTION
ZFS pool on-disk format versions are specified via “features” which replace the old on-disk format numbers (the last supported on-disk format number is 28). To enable a feature on a pool use the zpool upgrade, or set the feature@feature-name property to enabled. Please also see the Compatibility feature sets section for information on how sets of features may be enabled together.

The pool format does not affect file system version compatibility or the ability to send file systems between pools.

Since most features can be enabled independently of each other, the on-disk format of the pool is specified by the set of all features marked as active on the pool. If the pool was created by another software version this set may include unsupported features.

### Identifying features
Every feature has a GUID of the form com.example:feature-name. The reversed DNS name ensures that the feature's GUID is unique across all ZFS implementations. When unsupported features are encountered on a pool they will be identified by their GUIDs. Refer to the documentation for the ZFS implementation that created the pool for information about those features.

Each supported feature also has a short name. By convention a feature's short name is the portion of its GUID which follows the ‘:’ (i.e. com.example:feature-name would have the short name feature-name), however a feature's short name may differ across ZFS implementations if following the convention would result in name conflicts.

### Feature states
Features can be in one of three states:

active

This feature's on-disk format changes are in effect on the pool. Support for this feature is required to import the pool in read-write mode. If this feature is not read-only compatible, support is also required to import the pool in read-only mode (see Read-only compatibility).

enabled

An administrator has marked this feature as enabled on the pool, but the feature's on-disk format changes have not been made yet. The pool can still be imported by software that does not support this feature, but changes may be made to the on-disk format at any time which will move the feature to the active state. Some features may support returning to the enabled state after becoming active. See feature-specific documentation for details.

disabled

This feature's on-disk format changes have not been made and will not be made unless an administrator moves the feature to the enabled state. Features cannot be disabled once they have been enabled.

The state of supported features is exposed through pool properties of the form feature@short-name.

### Read-only compatibility
Some features may make on-disk format changes that do not interfere with other software's ability to read from the pool. These features are referred to as “read-only compatible”. If all unsupported features on a pool are read-only compatible, the pool can be imported in read-only mode by setting the readonly property during import (see zpool-import(8) for details on importing pools).

### Unsupported features
For each unsupported feature enabled on an imported pool, a pool property named unsupported@feature-name will indicate why the import was allowed despite the unsupported feature. Possible values for this property are:

inactive

The feature is in the enabled state and therefore the pool's on-disk format is still compatible with software that does not support this feature.

readonly

The feature is read-only compatible and the pool has been imported in read-only mode.

### Feature dependencies
Some features depend on other features being enabled in order to function. Enabling a feature will automatically enable any features it depends on.

### Compatibility feature sets
It is sometimes necessary for a pool to maintain compatibility with a specific on-disk format, by enabling and disabling particular features. The compatibility feature facilitates this by allowing feature sets to be read from text files. When set to off (the default), compatibility feature sets are disabled (i.e. all features are enabled); when set to legacy, no features are enabled. When set to a comma-separated list of filenames (each filename may either be an absolute path, or relative to /etc/zfs/compatibility.d or /usr/share/zfs/compatibility.d), the lists of requested features are read from those files, separated by whitespace and/or commas. Only features present in all files are enabled.

Simple sanity checks are applied to the files: they must be between 1 B and 16 KiB in size, and must end with a newline character.

The requested features are applied when a pool is created using zpool create -o compatibility=… and controls which features are enabled when using zpool upgrade. zpool status will not show a warning about disabled features which are not part of the requested feature set.

The special value legacy prevents any features from being enabled, either via zpool upgrade or zpool set feature@feature-name=enabled. This setting also prevents pools from being upgraded to newer on-disk versions. This is a safety measure to prevent new features from being accidentally enabled, breaking compatibility.

By convention, compatibility files in /usr/share/zfs/compatibility.d are provided by the distribution, and include feature sets supported by important versions of popular distributions, and feature sets commonly supported at the start of each year. Compatibility files in /etc/zfs/compatibility.d, if present, will take precedence over files with the same name in /usr/share/zfs/compatibility.d.

If an unrecognized feature is found in these files, an error message will be shown. If the unrecognized feature is in a file in /etc/zfs/compatibility.d, this is treated as an error and processing will stop. If the unrecognized feature is under /usr/share/zfs/compatibility.d, this is treated as a warning and processing will continue. This difference is to allow distributions to include features which might not be recognized by the currently-installed binaries.

Compatibility files may include comments: any text from ‘#’ to the end of the line is ignored.

Example:

```
example# cat /usr/share/zfs/compatibility.d/grub2
# Features which are supported by GRUB2 versions from v2.12 onwards.
allocation_classes
async_destroy
block_cloning
bookmarks
device_rebuild
embedded_data
empty_bpobj
enabled_txg
extensible_dataset
filesystem_limits
hole_birth
large_blocks
livelist
log_spacemap
lz4_compress
project_quota
resilver_defer
spacemap_histogram
spacemap_v2
userobj_accounting
zilsaxattr
zpool_checkpoint

example# cat /usr/share/zfs/compatibility.d/grub2-2.06
# Features which are supported by GRUB2 versions prior to v2.12.
#
# GRUB is not able to detect ZFS pool if snapshot of top level boot pool
# is created. This issue is observed with GRUB versions before v2.12 if
# extensible_dataset feature is enabled on ZFS boot pool.
#
# This file lists all read-only compatible features except
# extensible_dataset and any other feature that depends on it.
#
allocation_classes
async_destroy
block_cloning
device_rebuild
embedded_data
empty_bpobj
enabled_txg
hole_birth
log_spacemap
lz4_compress
resilver_defer
spacemap_histogram
spacemap_v2
zpool_checkpoint

example# zpool create -o compatibility=grub2 bootpool vdev
```

See zpool-create(8) and zpool-upgrade(8) for more information on how these commands are affected by feature sets.

## FEATURES
The following features are supported on this system:

allocation_classes

GUID

org.zfsonlinux:allocation_classes

READ-ONLY COMPATIBLE

yes

This feature enables support for separate allocation classes.

This feature becomes active when a dedicated allocation class vdev (dedup or special) is created with the zpool create or zpool add commands. With device removal, it can be returned to the enabled state if all the dedicated allocation class vdevs are removed.

async_destroy

GUID

com.delphix:async_destroy

READ-ONLY COMPATIBLE

yes

Destroying a file system requires traversing all of its data in order to return its used space to the pool. Without async_destroy, the file system is not fully removed until all space has been reclaimed. If the destroy operation is interrupted by a reboot or power outage, the next attempt to open the pool will need to complete the destroy operation synchronously.

When async_destroy is enabled, the file system's data will be reclaimed by a background process, allowing the destroy operation to complete without traversing the entire file system. The background process is able to resume interrupted destroys after the pool has been opened, eliminating the need to finish interrupted destroys as part of the open operation. The amount of space remaining to be reclaimed by the background process is available through the freeing property.

This feature is only active while freeing is non-zero.

blake3

GUID

org.openzfs:blake3

DEPENDENCIES

extensible_dataset

READ-ONLY COMPATIBLE

no

This feature enables the use of the BLAKE3 hash algorithm for checksum and dedup. BLAKE3 is a secure hash algorithm focused on high performance.

When the blake3 feature is set to enabled, the administrator can turn on the blake3 checksum on any dataset using zfs set checksum=blake3 dset (see zfs-set(8)). This feature becomes active once a checksum property has been set to blake3, and will return to being enabled once all filesystems that have ever had their checksum set to blake3 are destroyed.

block_cloning

GUID

com.fudosecurity:block_cloning

READ-ONLY COMPATIBLE

yes

When this feature is enabled ZFS will use block cloning for operations like copy_file_range(2). Block cloning allows to create multiple references to a single block. It is much faster than copying the data (as the actual data is neither read nor written) and takes no additional space. Blocks can be cloned across datasets under some conditions (like equal recordsize, the same master encryption key, etc.). ZFS tries its best to clone across datasets including encrypted ones. This is limited for various (nontrivial) reasons depending on the OS and/or ZFS internals.

This feature becomes active when first block is cloned. When the last cloned block is freed, it goes back to the enabled state.

block_cloning_endian

GUID

com.truenas:block_cloning_endian

READ-ONLY COMPATIBLE

yes

This feature corrects ZAP entry endianness issues in the Block Reference Table (BRT) used by block cloning. During the original block cloning implementation, BRT ZAP entries were mistakenly stored as arrays of 8 single-byte entries instead of single 8-byte entries, making pools non-endian-safe.

This feature is activated when the first BRT ZAP is created (that way ensuring compatibility with existing pools). When active, new BRT entries are stored in the correct endian-safe format. The feature becomes inactive when all BRT ZAPs are destroyed.

bookmarks

GUID

com.delphix:bookmarks

DEPENDENCIES

extensible_dataset

READ-ONLY COMPATIBLE

yes

This feature enables use of the zfs bookmark command.

This feature is active while any bookmarks exist in the pool. All bookmarks in the pool can be listed by running zfs list -t bookmark -r poolname.

bookmark_v2

GUID

com.datto:bookmark_v2

DEPENDENCIES

bookmark, extensible_dataset

READ-ONLY COMPATIBLE

no

This feature enables the creation and management of larger bookmarks which are needed for other features in ZFS.

This feature becomes active when a v2 bookmark is created and will be returned to the enabled state when all v2 bookmarks are destroyed.

bookmark_written

GUID

com.delphix:bookmark_written

DEPENDENCIES

bookmark, extensible_dataset, bookmark_v2

READ-ONLY COMPATIBLE

no

This feature enables additional bookmark accounting fields, enabling the written#bookmark property (space written since a bookmark) and estimates of send stream sizes for incrementals from bookmarks.

This feature becomes active when a bookmark is created and will be returned to the enabled state when all bookmarks with these fields are destroyed.

device_rebuild

GUID

org.openzfs:device_rebuild

READ-ONLY COMPATIBLE

yes

This feature enables the ability for the zpool attach and zpool replace commands to perform sequential reconstruction (instead of healing reconstruction) when resilvering.

Sequential reconstruction resilvers a device in LBA order without immediately verifying the checksums. Once complete, a scrub is started, which then verifies the checksums. This approach allows full redundancy to be restored to the pool in the minimum amount of time. This two-phase approach will take longer than a healing resilver when the time to verify the checksums is included. However, unless there is additional pool damage, no checksum errors should be reported by the scrub. This feature is incompatible with raidz configurations. This feature becomes active while a sequential resilver is in progress, and returns to enabled when the resilver completes.

device_removal

GUID

com.delphix:device_removal

READ-ONLY COMPATIBLE

no

This feature enables the zpool remove command to remove top-level vdevs, evacuating them to reduce the total size of the pool.

This feature becomes active when the zpool remove command is used on a top-level vdev, and will never return to being enabled.

draid

GUID

org.openzfs:draid

READ-ONLY COMPATIBLE

no

This feature enables use of the draid vdev type. dRAID is a variant of RAID-Z which provides integrated distributed hot spares that allow faster resilvering while retaining the benefits of RAID-Z. Data, parity, and spare space are organized in redundancy groups and distributed evenly over all of the devices.

This feature becomes active when creating a pool which uses the draid vdev type, or when adding a new draid vdev to an existing pool.

dynamic_gang_header

GUID

com.klarasystems:dynamic_gang_header

READ-ONLY COMPATIBLE

no

This feature enables larger gang headers based on the sector size of the pool. When enabled, gang headers will use the entire space allocated for them, instead of always restricting themselves to 512 bytes. This can reduce the need for nested gang trees in extreme fragmentation scenarios.

This feature becomes active when a gang header is written that is larger than 512 bytes. This feature is not enabled by zpool-upgrade(8). Instead, it must be manually enabled, or be part of a compatibility file.

edonr

GUID

org.illumos:edonr

DEPENDENCIES

extensible_dataset

READ-ONLY COMPATIBLE

no

This feature enables the use of the Edon-R hash algorithm for checksum, including for nopwrite (if compression is also enabled, an overwrite of a block whose checksum matches the data being written will be ignored). In an abundance of caution, Edon-R requires verification when used with dedup: zfs set dedup=edonr,verify (see zfs-set(8)).

Edon-R is a very high-performance hash algorithm that was part of the NIST SHA-3 competition. It provides extremely high hash performance (over 350% faster than SHA-256), but was not selected because of its unsuitability as a general purpose secure hash algorithm. This implementation utilizes the new salted checksumming functionality in ZFS, which means that the checksum is pre-seeded with a secret 256-bit random key (stored on the pool) before being fed the data block to be checksummed. Thus the produced checksums are unique to a given pool, preventing hash collision attacks on systems with dedup.

When the edonr feature is set to enabled, the administrator can turn on the edonr checksum on any dataset using zfs set checksum=edonr dset (see zfs-set(8)). This feature becomes active once a checksum property has been set to edonr, and will return to being enabled once all filesystems that have ever had their checksum set to edonr are destroyed.

embedded_data

GUID

com.delphix:embedded_data

READ-ONLY COMPATIBLE

no

This feature improves the performance and compression ratio of highly-compressible blocks. Blocks whose contents can compress to 112 bytes or smaller can take advantage of this feature.

When this feature is enabled, the contents of highly-compressible blocks are stored in the block “pointer” itself (a misnomer in this case, as it contains the compressed data, rather than a pointer to its location on disk). Thus the space of the block (one sector, typically 512 B or 4 KiB) is saved, and no additional I/O is needed to read and write the data block. This feature becomes active as soon as it is enabled and will never return to being enabled.

empty_bpobj

GUID

com.delphix:empty_bpobj

READ-ONLY COMPATIBLE

yes

This feature increases the performance of creating and using a large number of snapshots of a single filesystem or volume, and also reduces the disk space required.

When there are many snapshots, each snapshot uses many Block Pointer Objects (bpobjs) to track blocks associated with that snapshot. However, in common use cases, most of these bpobjs are empty. This feature allows us to create each bpobj on-demand, thus eliminating the empty bpobjs.

This feature is active while there are any filesystems, volumes, or snapshots which were created after enabling this feature.

enabled_txg

GUID

com.delphix:enabled_txg

READ-ONLY COMPATIBLE

yes

Once this feature is enabled, ZFS records the transaction group number in which new features are enabled. This has no user-visible impact, but other features may depend on this feature.

This feature becomes active as soon as it is enabled and will never return to being enabled.

encryption

GUID

com.datto:encryption

DEPENDENCIES

bookmark_v2, extensible_dataset

READ-ONLY COMPATIBLE

no

This feature enables the creation and management of natively encrypted datasets.

This feature becomes active when an encrypted dataset is created and will be returned to the enabled state when all datasets that use this feature are destroyed.

fast_dedup

GUID

com.klarasystems:fast_dedup

READ-ONLY COMPATIBLE

yes

This feature allows more advanced deduplication features to be enabled on new dedup tables.

This feature will be active when the first deduplicated block is written after a new dedup table is created (i.e. after a new pool creation, or new checksum used on a dataset with dedup enabled). It will be returned to the enabled state when all deduplicated blocks using it are freed.

extensible_dataset

GUID

com.delphix:extensible_dataset

READ-ONLY COMPATIBLE

no

This feature allows more flexible use of internal ZFS data structures, and exists for other features to depend on.

This feature will be active when the first dependent feature uses it, and will be returned to the enabled state when all datasets that use this feature are destroyed.

filesystem_limits

GUID

com.joyent:filesystem_limits

DEPENDENCIES

extensible_dataset

READ-ONLY COMPATIBLE

yes

This feature enables filesystem and snapshot limits. These limits can be used to control how many filesystems and/or snapshots can be created at the point in the tree on which the limits are set.

This feature is active once either of the limit properties has been set on a dataset and will never return to being enabled.

head_errlog

GUID

com.delphix:head_errlog

READ-ONLY COMPATIBLE

no

This feature enables the upgraded version of errlog, which required an on-disk error log format change. Now the error log of each head dataset is stored separately in the zap object and keyed by the head id. With this feature enabled, every dataset affected by an error block is listed in the output of zpool status. In case of encrypted filesystems with unloaded keys we are unable to check their snapshots or clones for errors and these will not be reported. An "access denied" error will be reported.

This feature becomes active as soon as it is enabled and will never return to being enabled.

hole_birth

GUID

com.delphix:hole_birth

DEPENDENCIES

enabled_txg

READ-ONLY COMPATIBLE

no

This feature has/had bugs, the result of which is that, if you do a zfs send -i (or -R, since it uses -i) from an affected dataset, the receiving party will not see any checksum or other errors, but the resulting destination snapshot will not match the source. Its use by zfs send -i has been disabled by default (see send_holes_without_birth_time in zfs(4)).

This feature improves performance of incremental sends (zfs send -i) and receives for objects with many holes. The most common case of hole-filled objects is zvols.

An incremental send stream from snapshot A to snapshot B contains information about every block that changed between A and B. Blocks which did not change between those snapshots can be identified and omitted from the stream using a piece of metadata called the “block birth time”, but birth times are not recorded for holes (blocks filled only with zeroes). Since holes created after A cannot be distinguished from holes created before A, information about every hole in the entire filesystem or zvol is included in the send stream.

For workloads where holes are rare this is not a problem. However, when incrementally replicating filesystems or zvols with many holes (for example a zvol formatted with another filesystem) a lot of time will be spent sending and receiving unnecessary information about holes that already exist on the receiving side.

Once the hole_birth feature has been enabled the block birth times of all new holes will be recorded. Incremental sends between snapshots created after this feature is enabled will use this new metadata to avoid sending information about holes that already exist on the receiving side.

This feature becomes active as soon as it is enabled and will never return to being enabled.

large_blocks

GUID

org.open-zfs:large_blocks

DEPENDENCIES

extensible_dataset

READ-ONLY COMPATIBLE

no

This feature allows the record size on a dataset to be set larger than 128 KiB.

This feature becomes active once a dataset contains a file with a block size larger than 128 KiB, and will return to being enabled once all filesystems that have ever had their recordsize larger than 128 KiB are destroyed.

large_dnode

GUID

org.zfsonlinux:large_dnode

DEPENDENCIES

extensible_dataset

READ-ONLY COMPATIBLE

no

This feature allows the size of dnodes in a dataset to be set larger than 512 B. This feature becomes active once a dataset contains an object with a dnode larger than 512 B, which occurs as a result of setting the dnodesize dataset property to a value other than legacy. The feature will return to being enabled once all filesystems that have ever contained a dnode larger than 512 B are destroyed. Large dnodes allow more data to be stored in the bonus buffer, thus potentially improving performance by avoiding the use of spill blocks.

large_microzap

GUID

com.klarasystems:large_microzap

DEPENDENCIES

extensible_dataset, large_blocks

READ-ONLY COMPATIBLE

yes

This feature allows "micro" ZAPs to grow larger than 128 KiB without being upgraded to "fat" ZAPs.

This feature becomes active the first time a micro ZAP grows larger than 128KiB. It will only be returned to the enabled state when all datasets that ever had a large micro ZAP are destroyed.

Note that even when this feature is enabled, micro ZAPs cannot grow larger than 128 KiB without also changing the zap_micro_max_size module parameter. See zfs(4).

livelist

GUID

com.delphix:livelist

DEPENDENCIES

extensible_dataset

READ-ONLY COMPATIBLE

yes

This feature allows clones to be deleted faster than the traditional method when a large number of random/sparse writes have been made to the clone. All blocks allocated and freed after a clone is created are tracked by the the clone's livelist which is referenced during the deletion of the clone. The feature is activated when a clone is created and remains active until all clones have been destroyed.

log_spacemap

GUID

com.delphix:log_spacemap

DEPENDENCIES

com.delphix:spacemap_v2

READ-ONLY COMPATIBLE

yes

This feature improves performance for heavily-fragmented pools, especially when workloads are heavy in random-writes. It does so by logging all the metaslab changes on a single spacemap every TXG instead of scattering multiple writes to all the metaslab spacemaps.

This feature becomes active as soon as it is enabled and will never return to being enabled.

longname

GUID

org.zfsonlinux:longname

DEPENDENCIES

extensible_dataset

READ-ONLY COMPATIBLE

no

This feature allows creating files and directories with name up to 1023 bytes in length. A new dataset property longname is also introduced to toggle longname support for each dataset individually. This property can be disabled even if it contains longname files. In such case, new file cannot be created with longname but existing longname files can still be looked up.

This feature becomes active when a file name greater than 255 is created in a dataset, and returns to being enabled when all such datasets are destroyed.

lz4_compress

GUID

org.illumos:lz4_compress

READ-ONLY COMPATIBLE

no

lz4 is a high-performance real-time compression algorithm that features significantly faster compression and decompression as well as a higher compression ratio than the older lzjb compression. Typically, lz4 compression is approximately 50% faster on compressible data and 200% faster on incompressible data than lzjb. It is also approximately 80% faster on decompression, while giving approximately a 10% better compression ratio.

When the lz4_compress feature is set to enabled, the administrator can turn on lz4 compression on any dataset on the pool using the zfs-set(8) command. All newly written metadata will be compressed with the lz4 algorithm.

This feature becomes active as soon as it is enabled and will never return to being enabled.

multi_vdev_crash_dump

GUID

com.joyent:multi_vdev_crash_dump

READ-ONLY COMPATIBLE

no

This feature allows a dump device to be configured with a pool comprised of multiple vdevs. Those vdevs may be arranged in any mirrored or raidz configuration.

When the multi_vdev_crash_dump feature is set to enabled, the administrator can use dumpadm(8) to configure a dump device on a pool comprised of multiple vdevs.

Under FreeBSD and Linux this feature is unused, but registered for compatibility. New pools created on these systems will have the feature enabled but will never transition to active, as this functionality is not required for crash dump support. Existing pools where this feature is active can be imported.

obsolete_counts

GUID

com.delphix:obsolete_counts

DEPENDENCIES

device_removal

READ-ONLY COMPATIBLE

yes

This feature is an enhancement of device_removal, which will over time reduce the memory used to track removed devices. When indirect blocks are freed or remapped, we note that their part of the indirect mapping is “obsolete” – no longer needed.

This feature becomes active when the zpool remove command is used on a top-level vdev, and will never return to being enabled.

physical_rewrite

GUID

com.truenas:physical_rewrite

DEPENDENCIES

extensible_dataset

READ-ONLY COMPATIBLE

yes

This feature enables physical block rewriting that preserves logical birth times, avoiding unnecessary inclusion of rewritten blocks in incremental zfs send streams. When enabled, the zfs rewrite -P command can be used.

This feature becomes active the first time zfs rewrite -P is used on any dataset, and will return to being enabled once all datasets that have ever used physical rewrite are destroyed.

project_quota

GUID

org.zfsonlinux:project_quota

DEPENDENCIES

extensible_dataset

READ-ONLY COMPATIBLE

yes

This feature allows administrators to account the spaces and objects usage information against the project identifier (ID).

The project ID is an object-based attribute. When upgrading an existing filesystem, objects without a project ID will be assigned a zero project ID. When this feature is enabled, newly created objects inherit their parent directories' project ID if the parent's inherit flag is set (via chattr [+-]P or zfs project -s|-C). Otherwise, the new object's project ID will be zero. An object's project ID can be changed at any time by the owner (or privileged user) via chattr -p prjid or zfs project -p prjid.

This feature will become active as soon as it is enabled and will never return to being disabled. Each filesystem will be upgraded automatically when remounted, or when a new file is created under that filesystem. The upgrade can also be triggered on filesystems via zfs set version=current fs. The upgrade process runs in the background and may take a while to complete for filesystems containing large amounts of files.

raidz_expansion

GUID

org.openzfs:raidz_expansion

DEPENDENCIES

none

READ-ONLY COMPATIBLE

no

This feature enables the zpool attach subcommand to attach a new device to a RAID-Z group, expanding the total amount usable space in the pool. See zpool-attach(8).

redaction_bookmarks

GUID

com.delphix:redaction_bookmarks

DEPENDENCIES

bookmarks, extensible_dataset

READ-ONLY COMPATIBLE

no

This feature enables the use of redacted zfs sends, which create redaction bookmarks storing the list of blocks redacted by the send that created them. For more information about redacted sends, see zfs-send(8).

redacted_datasets

GUID

com.delphix:redacted_datasets

DEPENDENCIES

extensible_dataset

READ-ONLY COMPATIBLE

no

This feature enables the receiving of redacted zfs send streams, which create redacted datasets when received. These datasets are missing some of their blocks, and so cannot be safely mounted, and their contents cannot be safely read. For more information about redacted receives, see zfs-send(8).

redaction_list_spill

GUID

com.delphix:redaction_list_spill

DEPENDENCIES

redaction_bookmarks

READ-ONLY COMPATIBLE

no

This feature enables the redaction list created by zfs redact to store many more entries. It becomes active when a redaction list is created with more than 36 entries, and returns to being enabled when no long redaction lists remain in the pool. For more information about redacted sends, see zfs-send(8).

resilver_defer

GUID

com.datto:resilver_defer

READ-ONLY COMPATIBLE

yes

This feature allows ZFS to postpone new resilvers if an existing one is already in progress. Without this feature, any new resilvers will cause the currently running one to be immediately restarted from the beginning.

This feature becomes active once a resilver has been deferred, and returns to being enabled when the deferred resilver begins.

sha512

GUID

org.illumos:sha512

DEPENDENCIES

extensible_dataset

READ-ONLY COMPATIBLE

no

This feature enables the use of the SHA-512/256 truncated hash algorithm (FIPS 180-4) for checksum and dedup. The native 64-bit arithmetic of SHA-512 provides an approximate 50% performance boost over SHA-256 on 64-bit hardware and is thus a good minimum-change replacement candidate for systems where hash performance is important, but these systems cannot for whatever reason utilize the faster skein and edonr algorithms.

When the sha512 feature is set to enabled, the administrator can turn on the sha512 checksum on any dataset using zfs set checksum=sha512 dset (see zfs-set(8)). This feature becomes active once a checksum property has been set to sha512, and will return to being enabled once all filesystems that have ever had their checksum set to sha512 are destroyed.

skein

GUID

org.illumos:skein

DEPENDENCIES

extensible_dataset

READ-ONLY COMPATIBLE

no

This feature enables the use of the Skein hash algorithm for checksum and dedup. Skein is a high-performance secure hash algorithm that was a finalist in the NIST SHA-3 competition. It provides a very high security margin and high performance on 64-bit hardware (80% faster than SHA-256). This implementation also utilizes the new salted checksumming functionality in ZFS, which means that the checksum is pre-seeded with a secret 256-bit random key (stored on the pool) before being fed the data block to be checksummed. Thus the produced checksums are unique to a given pool, preventing hash collision attacks on systems with dedup.

When the skein feature is set to enabled, the administrator can turn on the skein checksum on any dataset using zfs set checksum=skein dset (see zfs-set(8)). This feature becomes active once a checksum property has been set to skein, and will return to being enabled once all filesystems that have ever had their checksum set to skein are destroyed.

spacemap_histogram

GUID

com.delphix:spacemap_histogram

READ-ONLY COMPATIBLE

yes

This features allows ZFS to maintain more information about how free space is organized within the pool. If this feature is enabled, it will be activated when a new space map object is created, or an existing space map is upgraded to the new format, and never returns back to being enabled.

spacemap_v2

GUID

com.delphix:spacemap_v2

READ-ONLY COMPATIBLE

yes

This feature enables the use of the new space map encoding which consists of two words (instead of one) whenever it is advantageous. The new encoding allows space maps to represent large regions of space more efficiently on-disk while also increasing their maximum addressable offset.

This feature becomes active once it is enabled, and never returns back to being enabled.

userobj_accounting

GUID

org.zfsonlinux:userobj_accounting

DEPENDENCIES

extensible_dataset

READ-ONLY COMPATIBLE

yes

This feature allows administrators to account the object usage information by user and group.

This feature becomes active as soon as it is enabled and will never return to being enabled. Each filesystem will be upgraded automatically when remounted, or when a new file is created under that filesystem. The upgrade can also be triggered on filesystems via zfs set version=current fs. The upgrade process runs in the background and may take a while to complete for filesystems containing large amounts of files.

vdev_zaps_v2

GUID

com.klarasystems:vdev_zaps_v2

READ-ONLY COMPATIBLE

no

This feature creates a ZAP object for the root vdev.

This feature becomes active after the next zpool import or zpool reguid. Properties can be retrieved or set on the root vdev using zpool get and zpool set with root as the vdev name which is an alias for root-0.

zilsaxattr

GUID

org.openzfs:zilsaxattr

DEPENDENCIES

extensible_dataset

READ-ONLY COMPATIBLE

yes

This feature enables xattr=sa extended attribute logging in the ZIL. If enabled, extended attribute changes (both xattrdir=dir and xattr=sa) are guaranteed to be durable if either the dataset had sync=always set at the time the changes were made, or sync(2) is called on the dataset after the changes were made.

This feature becomes active when a ZIL is created for at least one dataset and will be returned to the enabled state when it is destroyed for all datasets that use this feature.

zpool_checkpoint

GUID

com.delphix:zpool_checkpoint

READ-ONLY COMPATIBLE

yes

This feature enables the zpool checkpoint command that can checkpoint the state of the pool at the time it was issued and later rewind back to it or discard it.

This feature becomes active when the zpool checkpoint command is used to checkpoint the pool. The feature will only return back to being enabled when the pool is rewound or the checkpoint has been discarded.

zstd_compress

GUID

org.freebsd:zstd_compress

DEPENDENCIES

extensible_dataset

READ-ONLY COMPATIBLE

no

zstd is a high-performance compression algorithm that features a combination of high compression ratios and high speed. Compared to gzip, zstd offers slightly better compression at much higher speeds. Compared to lz4, zstd offers much better compression while being only modestly slower. Typically, zstd compression speed ranges from 250 to 500 MB/s per thread and decompression speed is over 1 GB/s per thread.

When the zstd feature is set to enabled, the administrator can turn on zstd compression of any dataset using zfs set compress=zstd dset (see zfs-set(8)). This feature becomes active once a compress property has been set to zstd, and will return to being enabled once all filesystems that have ever had their compress property set to zstd are destroyed.

## SEE ALSO
zfs(8), zpool(8)


zpoolconcepts

package: zfs-utils

title: zpoolconcepts(7)

summary: overview of ZFS storage pools

text:
# zpoolconcepts(7)

## NAME
zpoolconcepts — overview of ZFS storage pools

## DESCRIPTION

### Virtual Devices (vdevs)
A "virtual device" describes a single device or a collection of devices, organized according to certain performance and fault characteristics. The following virtual devices are supported:

disk

A block device, typically located under /dev. ZFS can use individual slices or partitions, though the recommended mode of operation is to use whole disks. A disk can be specified by a full path, or it can be a shorthand name (the relative portion of the path under /dev). A whole disk can be specified by omitting the slice or partition designation. For example, sda is equivalent to /dev/sda. When given a whole disk, ZFS automatically labels the disk, if necessary.

file

A regular file. The use of files as a backing store is strongly discouraged. It is designed primarily for experimental purposes, as the fault tolerance of a file is only as good as the file system on which it resides. A file must be specified by a full path.

mirror

A mirror of two or more devices. Data is replicated in an identical fashion across all components of a mirror. A mirror with N disks of size X can hold X bytes and can withstand N-1 devices failing, without losing data.

raidz, raidz1, raidz2, raidz3

A distributed-parity layout, similar to RAID-5/6, with improved distribution of parity, and which does not suffer from the RAID-5/6 "write hole", (in which data and parity become inconsistent after a power loss). Data and parity is striped across all disks within a raidz group, though not necessarily in a consistent stripe width. A raidz group can have single, double, or triple parity, meaning that the raidz group can sustain one, two, or three failures, respectively, without losing any data. The raidz1 vdev type specifies a single-parity raidz group; the raidz2 vdev type specifies a double-parity raidz group; and the raidz3 vdev type specifies a triple-parity raidz group. The raidz vdev type is an alias for raidz1.

A raidz group with N disks of size X with P parity disks can hold approximately (N-P)*X bytes and can withstand P devices failing without losing data. The minimum number of devices in a raidz group is one more than the number of parity disks. The recommended number is between 3 and 9 to help increase performance.

draid, draid1, draid2, draid3

A variant of raidz that provides integrated distributed hot spares, allowing for faster resilvering, while retaining the benefits of raidz. A dRAID vdev is constructed from multiple internal raidz groups, each with D data devices and P parity devices. These groups are distributed over all of the children in order to fully utilize the available disk performance. Unlike raidz, dRAID uses a fixed stripe width (padding as necessary with zeros) to allow fully sequential resilvering. This fixed stripe width significantly affects both usable capacity and IOPS. For example, with the default D=8 and 4 KiB disk sectors the minimum allocation size is 32 KiB. If using compression, this relatively large allocation size can reduce the effective compression ratio. When using ZFS volumes (zvols) and dRAID, the default of the volblocksize property is increased to account for the allocation size. If a dRAID pool will hold a significant amount of small blocks, it is recommended to also add a mirrored special vdev to store those blocks.

In regards to I/O, performance is similar to raidz since, for any read, all D data disks must be accessed. Delivered random IOPS can be reasonably approximated as floor((N-S)/(D+P))*single_drive_IOPS.

Like raidz, a dRAID can have single-, double-, or triple-parity. The draid1, draid2, and draid3 types can be used to specify the parity level. The draid vdev type is an alias for draid1.

A dRAID with N disks of size X, D data disks per redundancy group, P parity level, and S distributed hot spares can hold approximately (N-S)*(D/(D+P))*X bytes and can withstand P devices failing without losing data.

draid[parity][:datad][:childrenc][:sparess]

A non-default dRAID configuration can be specified by appending one or more of the following optional arguments to the draid keyword: parity

The parity level (1-3).

data

The number of data devices per redundancy group. In general, a smaller value of D will increase IOPS, improve the compression ratio, and speed up resilvering at the expense of total usable capacity. Defaults to 8, unless N-P-S is less than 8.

children

The expected number of children. Useful as a cross-check when listing a large number of devices. An error is returned when the provided number of children differs.

spares

The number of distributed hot spares. Defaults to zero.

spare

A pseudo-vdev which keeps track of available hot spares for a pool. For more information, see the Hot Spares section.

log

A separate intent log device. If more than one log device is specified, then writes are load-balanced between devices. Log devices can be mirrored. However, raidz vdev types are not supported for the intent log. For more information, see the Intent Log section.

dedup

A device solely dedicated for deduplication tables. The redundancy of this device should match the redundancy of the other normal devices in the pool. If more than one dedup device is specified, then allocations are load-balanced between those devices.

special

A device dedicated solely for allocating various kinds of internal metadata, and optionally small file blocks. The redundancy of this device should match the redundancy of the other normal devices in the pool. If more than one special device is specified, then allocations are load-balanced between those devices. For more information on special allocations, see the Special Allocation Class section.

cache

A device used to cache storage pool data. A cache device cannot be configured as a mirror or raidz group. For more information, see the Cache Devices section.

Virtual devices cannot be nested arbitrarily. A mirror, raidz or draid virtual device can only be created with files or disks. Mirrors of mirrors or other such combinations are not allowed.

A pool can have any number of virtual devices at the top of the configuration (known as "root vdevs"). Data is dynamically distributed across all top-level devices to balance data among devices. As new virtual devices are added, ZFS automatically places data on the newly available devices.

Virtual devices are specified one at a time on the command line, separated by whitespace. Keywords like mirror and raidz are used to distinguish where a group ends and another begins. For example, the following creates a pool with two root vdevs, each a mirror of two disks:

\# zpool create mypool mirror sda sdb mirror sdc sdd

### Device Failure and Recovery
ZFS supports a rich set of mechanisms for handling device failure and data corruption. All metadata and data is checksummed, and ZFS automatically repairs bad data from a good copy, when corruption is detected.

In order to take advantage of these features, a pool must make use of some form of redundancy, using either mirrored or raidz groups. While ZFS supports running in a non-redundant configuration, where each root vdev is simply a disk or file, this is strongly discouraged. A single case of bit corruption can render some or all of your data unavailable.

A pool's health status is described by one of three states: online, degraded, or faulted. An online pool has all devices operating normally. A degraded pool is one in which one or more devices have failed, but the data is still available due to a redundant configuration. A faulted pool has corrupted metadata, or one or more faulted devices, and insufficient replicas to continue functioning.

The health of the top-level vdev, such as a mirror or raidz device, is potentially impacted by the state of its associated vdevs or component devices. A top-level vdev or component device is in one of the following states:

DEGRADED

One or more top-level vdevs is in the degraded state because one or more component devices are offline. Sufficient replicas exist to continue functioning. One or more component devices is in the degraded or faulted state, but sufficient replicas exist to continue functioning. The underlying conditions are as follows:

- The number of checksum errors or slow I/Os exceeds acceptable levels and the device is degraded as an indication that something may be wrong. ZFS continues to use the device as necessary.

- The number of I/O errors exceeds acceptable levels. The device could not be marked as faulted because there are insufficient replicas to continue functioning.

FAULTED

One or more top-level vdevs is in the faulted state because one or more component devices are offline. Insufficient replicas exist to continue functioning. One or more component devices is in the faulted state, and insufficient replicas exist to continue functioning. The underlying conditions are as follows:

- The device could be opened, but the contents did not match expected values.

- The number of I/O errors exceeds acceptable levels and the device is faulted to prevent further use of the device.

OFFLINE

The device was explicitly taken offline by the zpool offline command.

ONLINE

The device is online and functioning.

REMOVED

The device was physically removed while the system was running. Device removal detection is hardware-dependent and may not be supported on all platforms.

UNAVAIL

The device could not be opened. If a pool is imported when a device was unavailable, then the device will be identified by a unique identifier instead of its path since the path was never correct in the first place.

Checksum errors represent events where a disk returned data that was expected to be correct, but was not. In other words, these are instances of silent data corruption. The checksum errors are reported in zpool status and zpool events. When a block is stored redundantly, a damaged block may be reconstructed (e.g. from raidz parity or a mirrored copy). In this case, ZFS reports the checksum error against the disks that contained damaged data. If a block is unable to be reconstructed (e.g. due to 3 disks being damaged in a raidz2 group), it is not possible to determine which disks were silently corrupted. In this case, checksum errors are reported for all disks on which the block is stored.

If a device is removed and later re-attached to the system, ZFS attempts to bring the device online automatically. Device attachment detection is hardware-dependent and might not be supported on all platforms.

### Hot Spares
ZFS allows devices to be associated with pools as "hot spares". These devices are not actively used in the pool. But, when an active device fails, it is automatically replaced by a hot spare. To create a pool with hot spares, specify a spare vdev with any number of devices. For example,

\# zpool create pool mirror sda sdb spare sdc sdd

Spares can be shared across multiple pools, and can be added with the zpool add command and removed with the zpool remove command. Once a spare replacement is initiated, a new spare vdev is created within the configuration that will remain there until the original device is replaced. At this point, the hot spare becomes available again, if another device fails.

If a pool has a shared spare that is currently being used, the pool cannot be exported, since other pools may use this shared spare, which may lead to potential data corruption.

Shared spares add some risk. If the pools are imported on different hosts, and both pools suffer a device failure at the same time, both could attempt to use the spare at the same time. This may not be detected, resulting in data corruption.

An in-progress spare replacement can be canceled by detaching the hot spare. If the original faulted device is detached, then the hot spare assumes its place in the configuration, and is removed from the spare list of all active pools.

The draid vdev type provides distributed hot spares. These hot spares are named after the dRAID vdev they're a part of (draid1-2-3 specifies spare 3 of vdev 2, which is a single parity dRAID) and may only be used by that dRAID vdev. Otherwise, they behave the same as normal hot spares.

Spares cannot replace log devices.

### Intent Log
The ZFS Intent Log (ZIL) satisfies POSIX requirements for synchronous transactions. For instance, databases often require their transactions to be on stable storage devices when returning from a system call. NFS and other applications can also use fsync(2) to ensure data stability. By default, the intent log is allocated from blocks within the main pool. However, it might be possible to get better performance using separate intent log devices such as NVRAM or a dedicated disk. For example:

\# zpool create pool sda sdb log sdc

Multiple log devices can also be specified, and they can be mirrored. See the EXAMPLES section for an example of mirroring multiple log devices.

### Cache Devices
Devices can be added to a storage pool as "cache devices". These devices provide an additional layer of caching between main memory and disk. For read-heavy workloads, where the working set size is much larger than what can be cached in main memory, using cache devices allows much more of this working set to be served from low latency media. Using cache devices provides the greatest performance improvement for random read-workloads of mostly static content.

To create a pool with cache devices, specify a cache vdev with any number of devices. For example:

\# zpool create pool sda sdb cache sdc sdd

Cache devices cannot be mirrored or part of a raidz configuration. If a read error is encountered on a cache device, that read I/O is reissued to the original storage pool device, which might be part of a mirrored or raidz configuration.

The content of the cache devices is persistent across reboots and restored asynchronously when importing the pool in L2ARC (persistent L2ARC). This can be disabled by setting l2arc_rebuild_enabled=0. For cache devices smaller than 1 GiB, ZFS does not write the metadata structures required for rebuilding the L2ARC, to conserve space. This can be changed with l2arc_rebuild_blocks_min_l2size. The cache device header (512 B) is updated even if no metadata structures are written. Setting l2arc_headroom=0 will result in scanning the full-length ARC lists for cacheable content to be written in L2ARC (persistent ARC). If a cache device is added with zpool add, its label and header will be overwritten and its contents will not be restored in L2ARC, even if the device was previously part of the pool. If a cache device is onlined with zpool online, its contents will be restored in L2ARC. This is useful in case of memory pressure, where the contents of the cache device are not fully restored in L2ARC. The user can off- and online the cache device when there is less memory pressure, to fully restore its contents to L2ARC.

### Pool checkpoint
Before starting critical procedures that include destructive actions (like zfs destroy), an administrator can checkpoint the pool's state and, in the case of a mistake or failure, rewind the entire pool back to the checkpoint. Otherwise, the checkpoint can be discarded when the procedure has completed successfully.

A pool checkpoint can be thought of as a pool-wide snapshot and should be used with care as it contains every part of the pool's state, from properties to vdev configuration. Thus, certain operations are not allowed while a pool has a checkpoint. Specifically, vdev removal/attach/detach, mirror splitting, and changing the pool's GUID. Adding a new vdev is supported, but in the case of a rewind it will have to be added again. Finally, users of this feature should keep in mind that scrubs in a pool that has a checkpoint do not repair checkpointed data.

To create a checkpoint for a pool:

\# zpool checkpoint pool

To later rewind to its checkpointed state, you need to first export it and then rewind it during import:

\# zpool export pool

\# zpool import --rewind-to-checkpoint pool

Note that rewinding to a checkpoint will permanently discard it. Once the pool has been successfully imported with the above rewind command, you cannot rewind to the same checkpoint.

To discard the checkpoint from a pool:

\# zpool checkpoint -d pool

Dataset reservations (controlled by the reservation and refreservation properties) may be unenforceable while a checkpoint exists, because the checkpoint is allowed to consume the dataset's reservation. Finally, data that is part of the checkpoint but has been freed in the current state of the pool won't be scanned during a scrub.

### Special Allocation Class
Allocations in the special class are dedicated to specific block types. By default, this includes all metadata, the indirect blocks of user data, intent log (in absence of separate log device), and deduplication tables. The class can also be provisioned to accept small file blocks or zvol blocks on a per dataset granularity.

A pool must always have at least one normal (non-dedup/-special) vdev before other devices can be assigned to the special class. If the special class becomes full, then allocations intended for it will spill back into the normal class.

Deduplication tables can be excluded from the special class by unsetting the zfs_ddt_data_is_special ZFS module parameter.

Inclusion of small file or zvol blocks in the special class is opt-in. Each dataset can control the size of small file blocks allowed in the special class by setting the special_small_blocks property to nonzero. See zfsprops(7) for more info on this property.


zpoolprops

title: zpoolprops(7)

summary: properties of ZFS storage pools

text:
# zpoolprops(7)

## NAME
zpoolprops — properties of ZFS storage pools

## DESCRIPTION
Each pool has several properties associated with it. Some properties are read-only statistics while others are configurable and change the behavior of the pool.

User properties have no effect on ZFS behavior. Use them to annotate pools in a way that is meaningful in your environment. For more information about user properties, see the User Properties section.

The following are read-only properties:

allocated

Amount of storage used within the pool. See fragmentation and free for more information.

bcloneratio

The ratio of the total amount of storage that would be required to store all the cloned blocks without cloning to the actual storage used. The bcloneratio property is calculated as: ((bclonesaved + bcloneused) * 100) / bcloneused

bclonesaved

The amount of additional storage that would be required if block cloning was not used.

bcloneused

The amount of storage used by cloned blocks.

capacity

Percentage of pool space used. This property can also be referred to by its shortened column name, cap.

dedupcached

Total size of the deduplication table currently loaded into the ARC. See zpool-prefetch(8).

dedup_table_size

Total on-disk size of the deduplication table.

expandsize

Amount of uninitialized space within the pool or device that can be used to increase the total capacity of the pool. On whole-disk vdevs, this is the space beyond the end of the GPT – typically occurring when a LUN is dynamically expanded or a disk replaced with a larger one. On partition vdevs, this is the space appended to the partition after it was added to the pool – most likely by resizing it in-place. The space can be claimed for the pool by bringing it online with autoexpand=on or using zpool online -e.

fragmentation

The amount of fragmentation in the pool. As the amount of space allocated increases, it becomes more difficult to locate free space. This may result in lower write performance compared to pools with more unfragmented free space.

free

The amount of free space available in the pool. By contrast, the zfs(8) available property describes how much new data can be written to ZFS filesystems/volumes. The zpool free property is not generally useful for this purpose, and can be substantially more than the zfs available space. This discrepancy is due to several factors, including raidz parity; zfs reservation, quota, refreservation, and refquota properties; and space set aside by spa_slop_shift (see zfs(4) for more information).

freeing

After a file system or snapshot is destroyed, the space it was using is returned to the pool asynchronously. freeing is the amount of space remaining to be reclaimed. Over time freeing will decrease while free increases.

guid

A unique identifier for the pool.

health

The current health of the pool. Health can be one of ONLINE, DEGRADED, FAULTED, OFFLINE, REMOVED, UNAVAIL.

last_scrubbed_txg

Indicates the transaction group (TXG) up to which the most recent scrub operation has checked and repaired the dataset. This provides insight into the data integrity status of their pool at a specific point in time. zpool-scrub(8) can utilize this property to scan only data that has changed since the last scrub completed, when given the -C flag. This property is not updated when performing an error scrub with the -e flag.

leaked

Space not released while freeing due to corruption, now permanently leaked into the pool.

load_guid

A unique identifier for the pool. Unlike the guid property, this identifier is generated every time we load the pool (i.e. does not persist across imports/exports) and never changes while the pool is loaded (even if a reguid operation takes place).

size

Total size of the storage pool.

unsupported@guid

Information about unsupported features that are enabled on the pool. See zpool-features(7) for details.

The space usage properties report actual physical space available to the storage pool. The physical space can be different from the total amount of space that any contained datasets can actually use. The amount of space used in a raidz configuration depends on the characteristics of the data being written. In addition, ZFS reserves some space for internal accounting that the zfs(8) command takes into account, but the zpoolprops command does not. For non-full pools of a reasonable size, these effects should be invisible. For small pools, or pools that are close to being completely full, these discrepancies may become more noticeable.

The following property can be set at creation time and import time:

altroot

Alternate root directory. If set, this directory is prepended to any mount points within the pool. This can be used when examining an unknown pool where the mount points cannot be trusted, or in an alternate boot environment, where the typical paths are not valid. altroot is not a persistent property. It is valid only while the system is up. Setting altroot defaults to using cachefile=none, though this may be overridden using an explicit setting.

The following property can be set only at import time:

readonly=on|off

If set to on, the pool will be imported in read-only mode. This property can also be referred to by its shortened column name, rdonly.

The following properties can be set at creation time and import time, and later changed with the zpool set command:

ashift=ashift

Pool sector size exponent, to the power of 2 (internally referred to as ashift). Values from 9 to 16, inclusive, are valid; also, the value 0 (the default) means to auto-detect using the kernel's block layer and a ZFS internal exception list. I/O operations will be aligned to the specified size boundaries. Additionally, the minimum (disk) write size will be set to the specified size, so this represents a space/performance trade-off. For optimal performance, the pool sector size should be greater than or equal to the sector size of the underlying disks. The typical case for setting this property is when performance is important and the underlying disks use 4KiB sectors but report 512B sectors to the OS (for compatibility reasons); in that case, set ashift=12 (which is 1<<12 = 4096). When set, this property is used as the default hint value in subsequent vdev operations (add, attach and replace). Changing this value will not modify any existing vdev, not even on disk replacement; however it can be used, for instance, to replace a dying 512B sectors disk with a newer 4KiB sectors device: this will probably result in bad performance but at the same time could prevent loss of data.

autoexpand=on|off

Controls automatic pool expansion when the underlying LUN is grown. If set to on, the pool will be resized according to the size of the expanded device. If the device is part of a mirror or raidz then all devices within that mirror/raidz group must be expanded before the new space is made available to the pool. The default behavior is off. This property can also be referred to by its shortened column name, expand.

autoreplace=on|off

Controls automatic device replacement. If set to off, device replacement must be initiated by the administrator by using the zpool replace command. If set to on, any new device, found in the same physical location as a device that previously belonged to the pool, is automatically formatted and replaced. The default behavior is off. This property can also be referred to by its shortened column name, replace. Autoreplace can also be used with virtual disks (like device mapper) provided that you use the /dev/disk/by-vdev paths setup by vdev_id.conf. See the vdev_id(8) manual page for more details. Autoreplace and autoonline require the ZFS Event Daemon be configured and running. See the zed(8) manual page for more details.

autotrim=on|off

When set to on space which has been recently freed, and is no longer allocated by the pool, will be periodically trimmed. This allows block device vdevs which support BLKDISCARD, such as SSDs, or file vdevs on which the underlying file system supports hole-punching, to reclaim unused blocks. The default value for this property is off. Automatic TRIM does not immediately reclaim blocks after a free. Instead, it will optimistically delay allowing smaller ranges to be aggregated into a few larger ones. These can then be issued more efficiently to the storage. TRIM on L2ARC devices is enabled by setting l2arc_trim_ahead > 0.

Be aware that automatic trimming of recently freed data blocks can put significant stress on the underlying storage devices. This will vary depending of how well the specific device handles these commands. For lower-end devices it is often possible to achieve most of the benefits of automatic trimming by running an on-demand (manual) TRIM periodically using the zpool trim command.

bootfs=(unset)|pool[/dataset]

Identifies the default bootable dataset for the root pool. This property is expected to be set mainly by the installation and upgrade programs. Not all Linux distribution boot processes use the bootfs property.

cachefile=path|none

Controls the location of where the pool configuration is cached. Discovering all pools on system startup requires a cached copy of the configuration data that is stored on the root file system. All pools in this cache are automatically imported when the system boots. Some environments, such as install and clustering, need to cache this information in a different location so that pools are not automatically imported. Setting this property caches the pool configuration in a different location that can later be imported with zpool import -c. Setting it to the value none creates a temporary pool that is never cached, and the "" (empty string) uses the default location. Multiple pools can share the same cache file. Because the kernel destroys and recreates this file when pools are added and removed, care should be taken when attempting to access this file. When the last pool using a cachefile is exported or destroyed, the file will be empty.

comment=text

A text string consisting of printable ASCII characters that will be stored such that it is available even if the pool becomes faulted. An administrator can provide additional information about a pool using this property.

compatibility=off|legacy|file[,file]…

Specifies that the pool maintain compatibility with specific feature sets. When set to off (or unset) compatibility is disabled (all features may be enabled); when set to legacy no features may be enabled. When set to a comma-separated list of filenames (each filename may either be an absolute path, or relative to /etc/zfs/compatibility.d or /usr/share/zfs/compatibility.d) the lists of requested features are read from those files, separated by whitespace and/or commas. Only features present in all files may be enabled. See zpool-features(7), zpool-create(8) and zpool-upgrade(8) for more information on the operation of compatibility feature sets.

dedup_table_quota=number|none|auto

This property sets a limit on the on-disk size of the pool's dedup table. Entries will not be added to the dedup table once this size is reached; if a dedup table already exists, and is larger than this size, they will not be removed as part of setting this property. Existing entries will still have their reference counts updated. The actual size limit of the table may be above or below the quota, depending on the actual on-disk size of the entries (which may be approximated for purposes of calculating the quota). That is, setting a quota size of 1M may result in the maximum size being slightly below, or slightly above, that value. Set to 'none' to disable. In automatic mode, which is the default, the size of a dedicated dedup vdev is used as the quota limit.

The dedup_table_quota property works for both legacy and fast dedup tables.

dedupditto=number

This property is deprecated and no longer has any effect.

delegation=on|off

Controls whether a non-privileged user is granted access based on the dataset permissions defined on the dataset. See zfs(8) for more information on ZFS delegated administration.

failmode=wait|continue|panic

Controls the system behavior in the event of catastrophic pool failure. This condition is typically a result of a loss of connectivity to the underlying storage device(s) or a failure of all devices within the pool. The behavior of such an event is determined as follows: wait

Blocks all I/O access until the device connectivity is recovered and the errors are cleared with zpool clear. This is the default behavior.

continue

Returns EIO to any new write I/O requests but allows reads to any of the remaining healthy devices. Any write requests that have yet to be committed to disk would be blocked.

panic

Prints out a message to the console and generates a system crash dump.

feature@feature_name=enabled

The value of this property is the current state of feature_name. The only valid value when setting this property is enabled which moves feature_name to the enabled state. See zpool-features(7) for details on feature states.

listsnapshots=on|off

Controls whether information about snapshots associated with this pool is output when zfs list is run without the -t option. The default value is off. This property can also be referred to by its shortened name, listsnaps.

multihost=on|off

Controls whether a pool activity check should be performed during zpool import. When a pool is determined to be active it cannot be imported, even with the -f option. This property is intended to be used in failover configurations where multiple hosts have access to a pool on shared storage. Multihost provides protection on import only. It does not protect against an individual device being used in multiple pools, regardless of the type of vdev. See the discussion under zpool create.

When this property is on, periodic writes to storage occur to show the pool is in use. See zfs_multihost_interval in the zfs(4) manual page. In order to enable this property each host must set a unique hostid. See genhostid(1) zgenhostid(8) spl(4) for additional details. The default value is off.

version=version

The current on-disk version of the pool. This can be increased, but never decreased. The preferred method of updating pools is with the zpool upgrade command, though this property can be used when a specific version is needed for backwards compatibility. Once feature flags are enabled on a pool this property will no longer have a value.

### 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 pools.

User property names must contain a colon (":") character to distinguish them from native properties. They may contain lowercase letters, numbers, and the following punctuation characters: colon (":"), dash ("-"), period ("."), and underscore ("_"). The expected convention is that the property name is divided into two portions such as module:property, but this namespace is not enforced by ZFS. User property names can be at most 255 characters, and cannot begin with a dash ("-").

When making programmatic use of user properties, it is strongly suggested to use a reversed DNS domain name for the module component of property names to reduce the chance that two independently-developed packages use the same property name for different purposes.

The values of user properties are arbitrary strings and are never validated. All of the commands that operate on properties (zpool list, zpool get, zpool set, and so forth) can be used to manipulate both native properties and user properties. Use zpool set name= to clear a user property. Property values are limited to 8192 bytes.