Glossary

Acronyms and Terms

Term	Full Name	Description
ACE	Access Control Entry	A single entry in an ACL specifying permissions for a user or group
ACL	Access Control List	A table of ACEs controlling access to a ZFS object
ARC	Adaptive Replacement Cache	ZFS's in-memory read cache
COW	Copy On Write	ZFS's transactional write model; data is never overwritten in place
DMU	Data Management Unit	Layer that organizes blocks into objects and object sets
DSL	Dataset and Snapshot Layer	Manages relationships between datasets, snapshots, and clones
DVA	Data Virtual Address	A (vdev, offset) pair identifying a block's physical location
MOS	Meta Object Set	The pool-wide object set (type DMU_OST_META) pointed to by the uberblock; contains all pool metadata
SA	System Attributes	Modern replacement for the bonus buffer znode format
SPA	Storage Pool Allocator	Top-level layer managing the storage pool, vdevs, and I/O
TXG	Transaction Group	A group of writes committed atomically; identified by a monotonically increasing 64-bit number
ZAP	ZFS Attribute Processor	Module for storing name-value pair attributes in objects
ZIL	ZFS Intent Log	Records synchronous operations for replay after crash
ZIO	ZFS I/O Pipeline	The I/O scheduling and execution framework
L2ARC	Level 2 Adaptive Replacement Cache	Secondary read cache on a dedicated device (SSD)
MMP	Multi-Modifier Protection	Heartbeat mechanism preventing simultaneous pool import on multiple hosts
SLOG	Separate Log	A dedicated device for ZIL (intent log) writes
ZPL	ZFS POSIX Layer	Makes DMU objects appear as a POSIX-compliant filesystem
ZVOL	ZFS Volume	A logical block device backed by a ZFS object set

DMU Object Types

These constants identify the type of data stored in a dnode. Values are stored in the dn_type field of dnode_phys_t and in the type field of blkptr_t.

Value	Constant	Description
0	DMU_OT_NONE	Unallocated / free dnode
1	DMU_OT_OBJECT_DIRECTORY	MOS object directory (ZAP)
2	DMU_OT_OBJECT_ARRAY	Array of object numbers
3	DMU_OT_PACKED_NVLIST	Packed nvlist data
4	DMU_OT_PACKED_NVLIST_SIZE	Size of a packed nvlist object
5	DMU_OT_BPOBJ	Block pointer object (formerly BPLIST)
6	DMU_OT_BPOBJ_HDR	Block pointer object header
7	DMU_OT_SPACE_MAP_HEADER	Space map header
8	DMU_OT_SPACE_MAP	Space map
9	DMU_OT_INTENT_LOG	ZFS intent log
10	DMU_OT_DNODE	Array of dnodes (metadnode)
11	DMU_OT_OBJSET	Object set
12	DMU_OT_DSL_DIR	DSL directory
13	DMU_OT_DSL_DIR_CHILD_MAP	DSL child directory map (ZAP)
14	DMU_OT_DSL_DS_SNAP_MAP	DSL snapshot map (ZAP)
15	DMU_OT_DSL_PROPS	DSL properties (ZAP)
16	DMU_OT_DSL_DATASET	DSL dataset
17	DMU_OT_ZNODE	ZPL file metadata
18	DMU_OT_OLDACL	Legacy ACL data
19	DMU_OT_PLAIN_FILE_CONTENTS	Regular file data
20	DMU_OT_DIRECTORY_CONTENTS	Directory entries (ZAP)
21	DMU_OT_MASTER_NODE	ZPL master node (ZAP)
22	DMU_OT_UNLINKED_SET	Unlinked (delete queue) set
23	DMU_OT_ZVOL	ZVOL data
24	DMU_OT_ZVOL_PROP	ZVOL properties (ZAP)
25	DMU_OT_PLAIN_OTHER	Other uint8 data (test)
26	DMU_OT_UINT64_OTHER	Other uint64 data (test)
27	DMU_OT_ZAP_OTHER	Other ZAP object (test)
28	DMU_OT_ERROR_LOG	Persistent error log (ZAP)
29	DMU_OT_SPA_HISTORY	Pool history
30	DMU_OT_SPA_HISTORY_OFFSETS	Pool history offsets
31	DMU_OT_POOL_PROPS	Pool properties (ZAP)
32	DMU_OT_DSL_PERMS	DSL permissions (ZAP)
33	DMU_OT_ACL	ZFS ACL (v1)
34	DMU_OT_SYSACL	ZFS system ACL
35	DMU_OT_FUID	FUID table
36	DMU_OT_FUID_SIZE	FUID table size
37	DMU_OT_NEXT_CLONES	Next clones (ZAP)
38	DMU_OT_SCAN_QUEUE	Scan work queue (ZAP)
39	DMU_OT_USERGROUP_USED	User/group space used (ZAP)
40	DMU_OT_USERGROUP_QUOTA	User/group quota (ZAP)
41	DMU_OT_USERREFS	Snapshot user reference holds (ZAP)
42	DMU_OT_DDT_ZAP	Dedup table (ZAP)
43	DMU_OT_DDT_STATS	Dedup statistics (ZAP)
44	DMU_OT_SA	System attributes
45	DMU_OT_SA_MASTER_NODE	SA master node (ZAP)
46	DMU_OT_SA_ATTR_REGISTRATION	SA attribute registration (ZAP)
47	DMU_OT_SA_ATTR_LAYOUTS	SA attribute layouts (ZAP)
48	DMU_OT_SCAN_XLATE	Scan translations (ZAP)
49	DMU_OT_DEDUP	Deduplicated block
50	DMU_OT_DEADLIST	DSL deadlist map (ZAP)
51	DMU_OT_DEADLIST_HDR	DSL deadlist header
52	DMU_OT_DSL_CLONES	DSL directory clones (ZAP)
53	DMU_OT_BPOBJ_SUBOBJ	Block pointer object subobject

Values 0x80 and above use the new-type encoding; see Chapter 3, Section 3.5.

Object Set Types

Value	Constant	Description
0	DMU_OST_NONE	Uninitialized
1	DMU_OST_META	Meta Object Set (pool-level, one per pool)
2	DMU_OST_ZFS	ZPL filesystem, snapshot, or clone
3	DMU_OST_ZVOL	ZFS volume

Checksum Algorithms

Value	Name	Algorithm	Added
0	inherit	Property value resolved at write time	—
1	on	Property value (defaults to fletcher4)	—
2	off	none	v1
3	label	SHA-256 (labels only, embedded)	v1
4	gang_header	SHA-256 (gang header only, embedded)	v1
5	zilog	fletcher2 (ZIL only, embedded)	v1
6	fletcher2	fletcher2	v1
7	fletcher4	fletcher4	v1
8	sha256	SHA-256	v1
9	zilog2	fletcher4 (ZIL only, embedded)	v1
10	noparity	none (RAID-Z parity)	v1
11	sha512	SHA-512	feature
12	skein	Skein-512/256	feature
13	edonr	Edon-R/256	feature
14	blake3	BLAKE3	feature

Compression Algorithms

Value	Name	Algorithm	Added
0	inherit	Property value resolved at write time	—
1	on	Property value (defaults to lz4 if enabled, else lzjb)	—
2	off	none	v1
3	lzjb	lzjb	v1
4	empty	Special: all‑zero blocks	v1
5	gzip-1	GZIP level 1	v5
6	gzip-2	GZIP level 2	v5
7	gzip-3	GZIP level 3	v5
8	gzip-4	GZIP level 4	v5
9	gzip-5	GZIP level 5	v5
10	gzip-6	GZIP level 6	v5
11	gzip-7	GZIP level 7	v5
12	gzip-8	GZIP level 8	v5
13	gzip-9	GZIP level 9	v5
14	zle	Zero-Length Encoding	v20
15	lz4	LZ4	feature
16	zstd	Zstandard	feature

ZIL Transaction Types

Value	Constant	Description
1	TX_CREATE	Create file
2	TX_MKDIR	Create directory
3	TX_MKXATTR	Create extended attribute directory
4	TX_SYMLINK	Create symbolic link
5	TX_REMOVE	Remove file
6	TX_RMDIR	Remove directory
7	TX_LINK	Create hard link
8	TX_RENAME	Rename file or directory
9	TX_WRITE	Write to file
10	TX_TRUNCATE	Truncate file
11	TX_SETATTR	Set file attributes
12	TX_ACL_V0	Set old-format ACL
13	TX_ACL	Set ACL
14	TX_CREATE_ACL	Create file with ACL
15	TX_CREATE_ATTR	Create file with attributes
16	TX_CREATE_ACL_ATTR	Create file with ACL and attributes
17	TX_MKDIR_ACL	Create directory with ACL
18	TX_MKDIR_ATTR	Create directory with attributes
19	TX_MKDIR_ACL_ATTR	Create directory with ACL and attributes
20	TX_WRITE2	Write via dmu_sync EALREADY (block already on disk)
21	TX_SETSAXATTR	Set SA-based extended attribute
22	TX_RENAME_EXCHANGE	Atomic swap via renameat2
23	TX_RENAME_WHITEOUT	Atomic whiteout via renameat2
24	TX_CLONE_RANGE	Clone a file range

Pool States

Value	Constant	Description
0	POOL_STATE_ACTIVE	Pool is active and in use
1	POOL_STATE_EXPORTED	Pool has been exported
2	POOL_STATE_DESTROYED	Pool has been destroyed
3	POOL_STATE_SPARE	Device is reserved as a hot spare
4	POOL_STATE_L2CACHE	Device is a level 2 ARC (cache) device

States 3 and 4 appear only in the labels of hot-spare and L2ARC devices; they are never the active state of a pool. Higher values (POOL_STATE_UNINITIALIZED, POOL_STATE_UNAVAIL, POOL_STATE_POTENTIALLY_ACTIVE) are in-memory software abstractions and never written to disk.

ZAP Block Type Identifiers

Value	Constant	Description
(1ULL << 63) + 3	ZBT_MICRO	Microzap block
(1ULL << 63) + 1	ZBT_HEADER	Fat ZAP header (first block)
(1ULL << 63) + 0	ZBT_LEAF	Fat ZAP leaf block

ZAP Leaf Chunk Types

Value	Constant	Description
251	ZAP_CHUNK_ARRAY	Name or value data chunk
252	ZAP_CHUNK_ENTRY	Attribute entry chunk
253	ZAP_CHUNK_FREE	Free chunk

Vdev Type Strings

Type String	Description
"root"	Root of the vdev tree
"mirror"	Interior vdev: mirror (N-way replication)
"raidz"	Interior vdev: RAID-Z (parity-based redundancy)
"draid"	Interior vdev: distributed-parity RAID
"replacing"	Interior vdev: temporary mirror during disk replacement
"spare"	Interior vdev: spare wrapper (contains the actual spare leaf)
"disk"	Leaf vdev: block storage device
"file"	Leaf vdev: file-backed storage
"dspare"	Leaf vdev: distributed spare within a dRAID group
"indirect"	Remapped vdev after device removal
"hole"	Placeholder for a removed top-level vdev slot
"missing"	Placeholder for a device not present at import

Note: log devices are marked with an is_log flag in vdev configs rather than a distinct vdev‑tree type string, and L2ARC/spares are stored in auxiliary config lists, not under vdev_tree.

Pool Versions

See Chapter 1, Section 1.6 for full descriptions of each version.

Version	Key Features
1	Initial ZFS format
2	Ditto blocks
3	Hot spares, RAID-Z2
4	Pool history
5	GZIP compression
6	Boot filesystem
7	Separate intent log devices (SLOG)
8	Delegated administration
9	FUID, refquota, refreservation
10	L2ARC
11	Next clones, origin, DSL scrub
12	Snapshot properties
13	Used space breakdown
14	Passthrough-x
15	User/group space accounting
16	STMF properties
17	RAID-Z3
18	User reference holds
19	Hole support
20	ZLE compression
21	Deduplication
22	Received properties
23	Slim ZIL
24	System attributes (SA)
25	Improved scrub/scan
26	Directory clones, improved deadlists
27	Fast snapshots
28	Multiple device replacement (last numbered version)
5000	Feature flags system

Feature Flags

Feature flags are identified by reverse-DNS GUIDs. See Chapter 1, Section 1.6 for the feature flag mechanism.

Source: module/zcommon/zfeature_common.c

Feature Name	GUID	Description
async_destroy	com.delphix:async_destroy	Destroy filesystems asynchronously
empty_bpobj	com.delphix:empty_bpobj	Snapshots use less space
lz4_compress	org.illumos:lz4_compress	LZ4 compression algorithm
multi_vdev_crash_dump	com.joyent:multi_vdev_crash_dump	Crash dumps to multiple vdev pools
spacemap_histogram	com.delphix:spacemap_histogram	Spacemaps maintain space histograms
enabled_txg	com.delphix:enabled_txg	Record txg at which a feature is enabled
hole_birth	com.delphix:hole_birth	Retain hole birth txg for more precise send
zpool_checkpoint	com.delphix:zpool_checkpoint	Pool state can be checkpointed
spacemap_v2	com.delphix:spacemap_v2	Efficient space maps for large segments
extensible_dataset	com.delphix:extensible_dataset	Enhanced dataset functionality
bookmarks	com.delphix:bookmarks	zfs bookmark command
filesystem_limits	com.joyent:filesystem_limits	Filesystem and snapshot limits
embedded_data	com.delphix:embedded_data	Embedded-data block pointers
livelist	com.delphix:livelist	Improved clone deletion performance
log_spacemap	com.delphix:log_spacemap	Log-structured metaslab space maps
large_blocks	org.open-zfs:large_blocks	Blocks larger than 128KB
large_dnode	org.zfsonlinux:large_dnode	Variable on-disk dnode sizes
sha512	org.illumos:sha512	SHA-512/256 checksum
skein	org.illumos:skein	Skein checksum
edonr	org.illumos:edonr	Edon-R checksum
redaction_bookmarks	com.delphix:redaction_bookmarks	Bookmarks with redaction lists
redacted_datasets	com.delphix:redacted_datasets	Redacted datasets from redacted send
bookmark_written	com.delphix:bookmark_written	written#\<bookmark> property
device_removal	com.delphix:device_removal	Top-level vdev removal
obsolete_counts	com.delphix:obsolete_counts	Reduced memory for removed devices
userobj_accounting	org.zfsonlinux:userobj_accounting	User/group object accounting
bookmark_v2	com.datto:bookmark_v2	Larger bookmarks
encryption	com.datto:encryption	Dataset-level encryption
project_quota	org.zfsonlinux:project_quota	Project-based space/object accounting
allocation_classes	org.zfsonlinux:allocation_classes	Separate allocation classes (special vdev)
resilver_defer	com.datto:resilver_defer	Defer new resilvers during active resilver
device_rebuild	org.openzfs:device_rebuild	Sequential mirror/dRAID device rebuilds
zstd_compress	org.freebsd:zstd_compress	Zstandard compression
draid	org.openzfs:draid	Distributed spare RAID
zilsaxattr	org.openzfs:zilsaxattr	xattr=sa logging in ZIL
head_errlog	com.delphix:head_errlog	Per-dataset on-disk error logs
blake3	org.openzfs:blake3	BLAKE3 checksum
block_cloning	com.fudosecurity:block_cloning	Block cloning via Block Reference Table
vdev_zaps_v2	com.klarasystems:vdev_zaps_v2	Root vdev ZAP
redaction_list_spill	com.delphix:redaction_list_spill	Increased redaction snapshot arguments
raidz_expansion	org.openzfs:raidz_expansion	RAIDZ vdev expansion
fast_dedup	com.klarasystems:fast_dedup	Advanced deduplication
longname	org.zfsonlinux:longname	Filenames up to 1024 bytes
large_microzap	com.klarasystems:large_microzap	Microzaps larger than 128KB
block_cloning_endian	com.truenas:block_cloning_endian	Fixes BRT ZAP endianness on new pools
dynamic_gang_header	com.klarasystems:dynamic_gang_header	Dynamically sized gang headers
physical_rewrite	com.truenas:physical_rewrite	Preserve logical birth time during rewrite