Re: [PATCH] btrfs: Fix no space bug caused by removing bg

[Hugo Mills <hugo@carfax.org.uk>]

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On Tue, Sep 22, 2015 at 04:23:33PM +0200, David Sterba wrote:
> On Tue, Sep 22, 2015 at 01:41:31PM +0000, Hugo Mills wrote:
> > On Tue, Sep 22, 2015 at 03:36:43PM +0200, Holger Hoffstätte wrote:
> > > On 09/22/15 14:59, Jeff Mahoney wrote:
> > > (snip)
> > > > So if they way we want to prevent the loss of raid type info is by
> > > > maintaining the last block group allocated with that raid type, fine,
> > > > but that's a separate discussion.  Personally, I think keeping 1GB
> > > 
> > > At this point I'm much more surprised to learn that the RAID type can
> > > apparently get "lost" in the first place, and is not persisted
> > > separately. I mean..wat?
> > 
> >    It's always been like that, unfortunately.
> > 
> >    The code tries to use the RAID type that's already present to work
> > out what the next allocation should be. If there aren't any chunks in
> > the FS, the configuration is lost, because it's not stored anywhere
> > else. It's one of the things that tripped me up badly when I was
> > failing to rewrite the chunk allocator last year.
> 
> Yeah, right now there's no persistent default for the allocator. I'm
> still hoping that the object properties will magically solve that.

   There's no obvious place that filesystem-wide properties can be
stored, though. There's a userspace tool to manipulate the few current
FS-wide properties, but that's all special-cased to use the
"historical" ioctls for those properties, with no generalisation of a
property store, or even (IIRC) any external API for them.

   We're nominally using xattrs in the btrfs: namespace on directories
and files, and presumably on the top directory of a subvolume for
subvol-wide properties, but it's not clear where the FS-wide values
should go: in the top directory of subvolid=5 would be confusing,
because then you couldn't separate the properties for *that subvol*
from the ones for the whole FS (say, the default replication policy,
where you might want the top subvol to have different properties from
everything else).

   Hugo.

-- 
Hugo Mills             | Putting U back in Honor, Valor, and Trth.
hugo@... carfax.org.uk |
http://carfax.org.uk/  |
PGP: E2AB1DE4          |

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