GEOM mentor request

[John-Mark Gurney]

Warren Block wrote this message on Fri, Nov 01, 2013 at 13:23 -0600:
> On Fri, 1 Nov 2013, symbolics at gmx.com wrote:
> 
> > + Implement new things. Some ideas I have had:
> >   + GEOM "ERASE" - Rewrite deletes into random writes.
> >   + GEOM "PLUG" - Persistent version of the connect/disconnect verbs
> >     where the flag sits in the class metadata. This might be a cleaner
> >     approach, rather than adding the verbs to all the existing
> >     providers.
> >   + GEOM "TAP" - Allow userspace processes to hook into the GEOM
> >     API. Intended for debugging and development.
> >   + GEOM "WCACHE" - Allow you to use small, fast provider as a buffer
> >     for a larger, slower provider.
> >   + GEOM DTrace provider. Provide GEOM specific probes to complement
> >     the IO provider.
> > + Probably other bits I can't remember right now.
> 
> How about an explicit geom retaste command?  "true > /dev/ada0" is
> misleading to the reader.
> 
> Also, a RAM-cached version of gmirror that would report writes finished 
> as soon as the faster drive finishes.  Kind of the opposite of the 
> WCACHE above.  This would permit creating mirrors of an SSD and hard 
> drive without performance loss, at least up until available write 
> buffer space runs out.  This one may not be so easy.

If you did that, you'd want to do something like set the SSD to prefer,
and only complete on that drive.. That way if there is ever a crash,
you mirror the second disk from the faster first...

Though this still has the possibility to leave your fs is a inconsistent
state as once the write for the faster drive completes, another write
could come in and complete on the second drive before the first IO
completes breaking the assumptions that UFS+S and ZFS about how/when
IO gets committed to the disk...

Also, depending upon the write load, it could be the HD is faster than
the SSD if there was a large enough volume of writes...

-- 
  John-Mark Gurney				Voice: +1 415 225 5579

     "All that I will do, has been done, All that I have, has not."