Feature Flags
ZFS on-disk formats were originally versioned with a single number, which increased whenever the format changed. The numbered approach was suitable when development of ZFS was driven by a single organisation.
For distributed development of OpenZFS, version numbering was unsuitable. Any change to the number would have required agreement, across all implementations, of each change to the on-disk format.
OpenZFS feature flags – an alternative to traditional version numbering – allow a uniquely named pool property for each change to the on-disk format. This approach supports:
- format changes that are independent
- format changes that depend on each other.
Compatibility
Where all features that are used by a pool are supported by multiple implementations of OpenZFS, the on-disk format is portable across those implementations.
Features that are exclusive when enabled should be periodically ported to all distributions.
Reference materials
ZFS Feature Flags (Christopher Siden, 2012-01, in the Internet Archive Wayback Machine) in particular: "… Legacy version numbers still exist for pool versions 1-28 …".
zpool-features(5) – illumos
zpool-features(7) – FreeBSD
Feature flags implementation
| Feature flag | FreeBSD | ZFS on Linux | OpenZFS OSX | OmniOS | |||
|---|---|---|---|---|---|---|---|
| 10.3 | 11.0 | stable/11 | r313645 | 0.6.5 | 1.4.5 | r151014 | |
| async_destroy | yes | yes | yes | yes | yes | yes | yes |
| empty_bpobj | yes | yes | yes | yes | yes | yes | yes |
| lz4_compress | yes | yes | yes | yes | yes | yes | yes |
| multi_vdev_crash_dump | yes | yes | yes | yes | no | no | yes |
| spacemap_histogram | yes | yes | yes | yes | yes | yes | yes |
| enabled_txg | yes | yes | yes | yes | yes | yes | yes |
| hole_birth | yes | yes | yes | yes | yes | yes | yes |
| extensible_dataset | yes | yes | yes | yes | yes | yes | yes |
| embedded_data | yes | yes | yes | yes | yes | yes | yes |
| bookmarks | yes | yes | yes | yes | yes | yes | yes |
| filesystem_limits | yes | yes | yes | yes | yes | yes | yes |
| large_blocks | yes | yes | yes | yes | yes | yes | yes |
| sha512 | no | yes | yes | yes | ?? | ?? | ?? |
| skein | no | yes | yes | yes | ?? | ?? | ?? |
Table legend
- no = not implemented
- yes = implemented
Source: OpenZFS Feature Flags Compatibility Matrix - VX weblog