git: Disable HAMMER live dedup, mark as experimental

[Thomas Nikolajsen]

> > Any special reason to turn vfs.hammer.live_dedup sysctl to R/O?
> > (I haven't noticed a tunable to turn it on; only way is to change code)
> >
> > Do we have any known problem with live_dedup?
> > (I haven't noticed a bug on this in our bug handling system)
> >
> > Live_dedup was off by default;
> > which seemed fine for an experimental feature.

> It can be enabled from the loader / loader.conf.

Are you sure?
I don't see a tunable in the code.

> Live dedup was suspected of data corruption (but not confirmed) --
> the thought was to make sure people really, really mean to turn it on
> for 3.0, it could be switched back to the way it was in master
> now that 3.0 has been branched.

Please switch back in master (& 3.0 IMO) &
add a note to HAMMER man pages that live dedup is experimental.

Please post a HEADS UP to users@
otherwise we might still have users running live dedup
(which didn't update to recent master).

Also it would be nice to open a bug to track this issue.

Is anybody working on confirming if live dedup can give data corruption?
(it would be rather sad if such a nice feature just rot because of false rumors)

Anyway, thanks for the info;
guess this has been discussed on IRC,
but this alone is rather volatile.

 -thomas