Re: BSD Fragmentation

Geoff Clare <[email protected]> wrote:
> Marco Moock wrote:
> 
> > ZFS should be compatible - at least with Solaris and Linux
> 
> For "Solaris" variants derived from Open Solaris (i.e. systems
> now built from the Illumos source), that should be true.  However,
> I think ZFS in actual Solaris (i.e. 11.4 from Oracle) might not
> be compatible with BSD and Linux any more, as I seem to recall
> reading that their ZFS and Open ZFS have diverged.

ZFS has 'feature flags' to manage this.  When you install an old disc in a
new machine, 'zpool status' asks if you want to upgrade your flags to the
current version.  If you upgrade you get access to more features but lose
backward compatibility with the OS the disc was originally used with.  If
you don't you retain backwards compatibility.

I believe the Illumos and OpenZFS people have different extensions
controlled by different sets of feature flags.  So I think you can keep
compatibility if you stick to a baseline set of features.  The same applies
when upgrading to a newer OpenZFS - if you allow new feature flags then you
can no longer use the disc with the older OpenZFS.


$ sudo zpool status pool

  pool: pool
 state: ONLINE
status: Some supported and requested features are not enabled on the pool.
        The pool can still be used, but some features are unavailable.
action: Enable all features using 'zpool upgrade'. Once this is done,
        the pool may no longer be accessible by software that does not support
        the features. See zpool-features(7) for details.


$ man zpool-features

ZPOOL-FEATURES(7)     BSD Miscellaneous Information Manual

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 an‐ other
     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 en‐ countered 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.

[snip]