Re: [PATCH/RFC 0/4] Attempt to make progress with btrfs dev number strangeness.

Hugo Mills
2021-08-12T14:47:21Z

On Thu, Aug 12, 2021 at 09:54:54AM -0400, Josef Bacik wrote:
> On 8/11/21 6:13 PM, NeilBrown wrote:
> > On Wed, 11 Aug 2021, Josef Bacik wrote:
> > >
> > > I think this is a step in the right direction, but I want to figure out a way to
> > > accomplish this without magical mount points that users must be aware of.
> >
> > magic mount *options* ???
> >
> > >
> > > I think the stat() st_dev ship as sailed, we're stuck with that. However
> > > Christoph does have a valid point where it breaks the various info spit out by
> > > /proc. You've done a good job with the treeid here, but it still makes it
> > > impossible for somebody to map the st_dev back to the correct mount.
> >
> > The ship might have sailed, but it is not water tight. And as the world
> > it round, it can still come back to bite us from behind.
> > Anything can be transitioned away from, whether it is devfs or 32-bit
> > time or giving different device numbers to different file-trees.
> >
> > The linkage between device number and and filesystem is quite strong.
> > We could modified all of /proc and /sys/ and audit and whatever else to
> > report the fake device number, but we cannot get the fake device number
> > into the mount table (without making the mount table unmanageablely
> > large).
> > And if subtrees aren't in the mount-table for the NFS server, I don't
> > think they should be in the mount-table of the NFS client. So we cannot
> > export them to NFS.
> >
> > I understand your dislike for mount options. An alternative with
> > different costs and benefits would be to introduce a new filesystem type
> > - btrfs2 or maybe betrfs. This would provide numdevs=1 semantics and do
> > whatever we decided was best with inode numbers. How much would you
> > hate that?
> >
>
> A lot more ;).
>
> > >
> > > I think we aren't going to solve that problem, at least not with stat(). I
> > > think with statx() spitting out treeid we have given userspace a way to
> > > differentiate subvolumes, and so we should fix statx() to spit out the the super
> > > block device, that way new userspace things can do their appropriate lookup if
> > > they so choose.
> >
> > I don't think we should normalize having multiple devnums per filesystem
> > by encoding it in statx(). It *would* make sense to add a btrfs ioctl
> > which reports the real device number of a file. Tools that really need
> > to work with btrfs could use that, but it would always be obvious that
> > it was an exception.
>
> That's not what I'm saying. I'm saying that stat() continues to behave the
> way it currently does, for legacy users.
>
> And then for statx() it returns the correct devnum like any other file
> system, with the augmentation of the treeid so that future userspace
> programs can use the treeid to decide if they want to wander into a
> subvolume.
>
> This way moving forward we have a way to map back to a mount point because
> statx() will return the actual devnum for the mountpoint, and then we can
> use the treeid to be smart about when we wander into a subvolume.
>
> And if we're going to add a treeid, I would actually like to add a
> parent_treeid as well so we could tell if we're a snapshot or just a normal
> subvolume.

Can I make a request to call it something other than a
"parent". There's at least three different usages of "parent" for
three different concepts related to subvolumes in btrfs(*), and it'd
be nice to avoid the inevitable confusion.

(*) 1. "subvolume containing this one",
2. "subvolume that was snapshotted to make this one", and,
3. at least informally, "subvolume that was sent/received to make this one"

Hugo.

[snip to end]

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