Re: snapshot/subvol deletion

[Chris Mason <chris.mason@oracle.com>]

On Tue, Aug 25, 2009 at 10:38:01PM +0800, Yan, Zheng wrote:
> Hi,
> 
> I will send a series patches that add snapshot/subvol deletion soon.
> But the way to delete snapshot/subvol is far from people's expectancy.
> To delete a snapshot/subvol, we need four steps: 1) snapshot/subvol
> deletion ioctl or rmdir; 2) umount; 3) btrfsck; 4) mount the fs.
> 
> The reason for this is bug in root back & forward references. In simple
> terms, the bug prevents us from knowing how many places a snapshot/subvol
> is referenced. So it's unsafe delete corresponding fs tree immediately
> after a link to snapshot/subvol is removed.

Thanks for working on this, its a major feature.  The problem with the
forward/backward reference counting is that our links to a subvolume or
snapshot are really more like symbolic links than active references.

If a directory entry points to a subvolume and someone uses rm -rf to
delete the files inside that subvolume or snapshot, you get the same kind of
semantics as deleting the subvolume with the ioctl.

So, we should be able to delete the snapshot without the unmount step.
It may create an invalid reference but the code to follow snapshot
directory items will have to be changed to deal with that.

If we later on allow root ids to be reused, the directory item pointing
to a subvol will again be like a symbolic link.  You'll end up in the
new subvol instead of the old.

-chris