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Subject: Re: any idea about auto export multiple btrfs snapshots?
To:     NeilBrown <neilb@suse.de>, "J. Bruce Fields" <bfields@fieldses.org>
Cc:     Wang Yugui <wangyugui@e16-tech.com>, linux-nfs@vger.kernel.org
References: <162432531379.17441.15110145423567943074@noble.neil.brown.name>
 <20210622112253.DAEE.409509F4@e16-tech.com>
 <20210622151407.C002.409509F4@e16-tech.com>
 <162440994038.28671.7338874000115610814@noble.neil.brown.name>
 <20210623153548.GF20232@fieldses.org>
 <162448589701.28671.8402117125966499268@noble.neil.brown.name>
From:   Patrick Goetz <pgoetz@math.utexas.edu>
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Date:   Thu, 24 Jun 2021 16:58:56 -0500
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On 6/23/21 5:04 PM, NeilBrown wrote:
> On Thu, 24 Jun 2021, J. Bruce Fields wrote:
>> Is there any hope of solving this problem within btrfs?
>>
>> It doesn't seem like it should have been that difficult for it to give
>> subvolumes separate superblocks and vfsmounts.
>>
>> But this has come up before, and I think the answer may have been that
>> it's just too late to fix.
> 
> It is never too late to do the right thing!
> 
> Probably the best approach to fixing this completely on the btrfs side
> would be to copy the auto-mount approach used in NFS.  NFS sees multiple
> different volumes on the server and transparently creates new vfsmounts,
> using the automount infrastructure to mount and unmount them.

I'm very confused about what you're talking about.  Is this documented 
somewhere? I mean, I do use autofs, but see that as a separate software 
system working with NFS.


>  BTRFS
> effective sees multiple volumes on the block device and could do the
> same thing.
> 
> I can only think of one change to the user-space API (other than
> /proc/mounts contents) that this would cause and I suspect it could be
> resolved if needed.
> 
> Currently when you 'stat' the mountpoint of a btrfs subvol you see the
> root of that subvol.  However when you 'stat' the mountpoint of an NFS
> sub-filesystem (before any access below there) you see the mountpoint
> (s_dev matches the parent).  This is how automounts are expected to work
> and if btrfs were switched to use automounts for subvols, stating the
> mountpoint would initially show the mountpoint, not the subvol root.
> 
> If this were seen to be a problem I doubt it would be hard to add
> optional functionality to automount so that 'stat' triggers the mount.
> 
> All we really need is:
> 1/ someone to write the code
> 2/ someone to review the code
> 3/ someone to accept the code
> 
> How hard can it be :-)
> 
> NeilBrown
>