On Tue, Jun 16, 2015 at 01:13:40PM +0000, Holger Hoffstätte wrote:
> 
> Forking from the other thread..
> 
> On Tue, 16 Jun 2015 12:25:45 +0000, Hugo Mills wrote:
> 
> >    Yes. It's an artefact of the way that mkfs works. If you run a
> > balance on those chunks, they'll go away. (btrfs balance start
> > -dusage=0 -musage=0 /mountpoint)
> 
> Since I had to explain this very same thing to a new btrfs-using friend
> just yesterday I wondered if it might not make sense for mkfs to issue
> a general balance after creating the fs? It should be simple enough
> (just issue the balance ioctl?) and not have any negative side effects.

   Mount, balance, unmount... You can't balance on an unmounted FS.
You'd also have to write something to the FS (see below).

> Just doing such a post-mkfs cleanup automatically would certainly
> reduce the number of times we have to explain the this. :)
> 
> Any reasons why we couldn't/shouldn't do this?

   On an empty filesystem, it used to throw away the RAID
configuration, which was identified from the existing chunks on the
FS. With no chunks remaining, the RAID configuration reverted to
single all round (IIRC). I don't know if that's been fixed since those
days, but it's something to be wary of, at the very least.

   I've added the question as an entry to the FAQ, anyway.

   Hugo.

-- 
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