Re: strange btrfs sub list output

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

[-- Attachment #1: Type: text/plain, Size: 2083 bytes --]

On Fri, May 27, 2011 at 12:30:10PM +0100, Stephane Chazelas wrote:
> 2011-05-27 10:45:23 +0100, Hugo Mills:
> [...]
> > > How could a "subvolume 285" become a "top level"?
> > 
> > > How does one get a subvolume with a top-level other than "5"?
> > 
> >    This just means that subvolume 287 was created (somewhere) inside
> > subvolume 285.
> > 
> >    Due to the way that the FS trees and subvolumes work, there's no
> > global namespace structure in btrfs; that is, there's no single data
> > structure that represents the entirety of the file/directory hierarchy
> > in the filesystem. Instead, it's broken up into these sub-namespaces
> > called subvolumes, and we only record parent/child relationships for
> > each subvolume separately. The "full path" you get from "btrfs subv
> > list" is reconstructed from that information in userspace(*).
> [...]
> 
> Thanks, I can understand that. What I don't get is how one
> creates a subvol with a top-level other than 5. I might be
> missing the obvious, though.
> 
> If I do:
> 
> btrfs sub create A
> btrfs sub create A/B
> btrfs sub snap A A/B/C
> 
> A, A/B, A/B/C have their top-level being 5. How would I get a
> new snapshot to be a child of A/B for instance?

   Hm. OK, that's not doing what I thought it was, then. I'll have to
look at the code to work out what that top-level output actually is,
then. (Won't be for a few hours, until I get home from work).

> In my case, 285, was not appearing in the btrfs sub list output,
> 287 was a child of 285 with path "data" while all I did was
> create a snapshot of 284 (path
> u6:10022/vm+xfs@u8/xvda1/g8/v3/data in vol 5) in
> u6:10022/vm+xfs@u8/xvda1/g8/v3/snapshots/2011-03-30
> 
> So I did manage to get a volume with a parent other than 5, but
> I did not ask for it.
> 

-- 
=== Hugo Mills: hugo@... carfax.org.uk | darksatanic.net | lug.org.uk ===
  PGP key: 515C238D from wwwkeys.eu.pgp.net or http://www.carfax.org.uk
  --- I hate housework. You make the beds, you wash the dishes, and ---  
           six months later you have to start all over again.            

[-- Attachment #2: Digital signature --]
[-- Type: application/pgp-signature, Size: 190 bytes --]