Re: Can't change partition table anymore

The "SPOILING" section of the geom(4) manpage sounds like it is relevant 
here.  From this, it sounds like attempts to open ad0 for writing should 
be denied if any ad0 partition is mounted, no matter read-only or 
read/write.

Perhaps the write protection was not actually enforced until recently. 
It doesn't appear to be enforced by a kernel from 4 October, but is 
enforced on my laptop with a kernel from 12 March.

- Brian

Quoting:

      SPOILING is a special case of orphanization used to protect against stale
      metadata.  It is probably easiest to understand spoiling by going through
      an example.

      Imagine a disk, "da0" on top of which a MBR geom provides "da0s1" and
      "da0s2" and on top of "da0s1" a BSD geom provides "da0s1a" through
      "da0s1e", both the MBR and BSD geoms have autoconfigured based on data
      structures on the disk media.  Now imagine the case where "da0" is opened
      for writing and those data structures are modified or overwritten: Now
      the geoms would be operating on stale metadata unless some notification
      system can inform them otherwise.

      To avoid this situation, when the open of "da0" for write happens, all
      attached consumers are told about this, and geoms like MBR and BSD will
      selfdestruct as a result.  When "da0" is closed again, it will be offered
      for tasting again and if the data structures for MBR and BSD are still
      there, new geoms will instantiate themselves anew.

      Now for the fine print:

      If any of the paths through the MBR or BSD module were open, they would
      have opened downwards with an exclusive bit rendering it impossible to
      open "da0" for writing in that case and conversely the requested exclu-
      sive bit would render it impossible to open a path through the MBR geom
      while "da0" is open for writing.

      From this it also follows that changing the size of open geoms can only
      be done with their cooperation.

      Finally: the spoiling only happens when the write count goes from zero to
      non-zero and the retasting only when the write count goes from non-zero
      to zero.