zfs discuss - Mar 2009 - usedby* properties for datasets created before v13

Hi,

The manpage says

          Specifically,  used  =  usedbychildren + usedbydataset +
          usedbyrefreservation +, usedbysnapshots.  These  proper-
          ties  are  only  available for datasets created on zpool
          "version 13" pools.

.. and I now realize that "created" at v13 is the important bit,
rather than "created pre v13 and upgraded", and I
see that for datasets created on a version prior to 13
show "-" for these properties (might be nice to note that
in the manpage - I took "-" to mean zero for a while).

Anyway, is there any way to retrospectively populate these
statistics (avoiding dataset reconstruction, that is)?
No chance a scrub would/could do it?

Thanks

Gavin

Gavin Maltby wrote:
> Hi,
> 
> The manpage says
> 
>          Specifically,  used  =  usedbychildren + usedbydataset +
>          usedbyrefreservation +, usedbysnapshots.  These  proper-
>          ties  are  only  available for datasets created on zpool
>          "version 13" pools.
> 
> .. and I now realize that "created" at v13 is the important bit,
> rather than "created pre v13 and upgraded", and I
> see that for datasets created on a version prior to 13
> show "-" for these properties (might be nice to note that
> in the manpage - I took "-" to mean zero for a while).
> 
> Anyway, is there any way to retrospectively populate these
> statistics (avoiding dataset reconstruction, that is)?
> No chance a scrub would/could do it?
In theory one could add code to calculate these after the fact.  A tricky 
part is differentiating between usedbydataset and usedbysnapshots for clones. 
  In that case you would need to examine all block pointers in the clone. 
Those born after the origin are usedbydataset, and usedbysnapshots is 
whatever''s left over.  Doing this while things are changing may be
nontrivial.

--matt