Re: any idea about auto export multiple btrfs snapshots?

[Wang Yugui <wangyugui@e16-tech.com>]

Hi,

> On 6/21/21 3:41 PM, Wang Yugui wrote:
> > Hi,
> >
> >> OK thanks for the information. I think they will just work in nfs-ganesha as
> >> long as the snapshots or subvols are mounted within an nfs-ganesha export or
> >> are exported explicitly. nfs-ganesha has the equivalent of knfsd's
> >> nohide/crossmnt options and when nfs-ganesha detects crossing a filesystem
> >> boundary will lookup the filesystem via getmntend and listing btrfs subvols
> >> and then expose that filesystem (via the fsid attribute) to the clients
> >> where at least the Linux nfs client will detect a filesystem boundary and
> >> create a new mount entry for it.
> >
> > Not only exported explicitly, but also kept in the same hierarchy.
> >
> > If we export
> > /mnt/test		#the btrfs
> > /mnt/test/sub1	# the btrfs subvol 1
> > /mnt/test/sub2	# the btrfs subvol 2
> >
> > we need to make sure we will not access '/mnt/test/sub1' through '/mnt/test'
> > from nfs client.
> >
> > current safe export:
> > #/mnt/test		#the btrfs, not exported
> > /mnt/test/sub1	# the btrfs subvol 1
> > /mnt/test/sub2	# the btrfs subvol 2
> >
> 
> What's the problem with exporting /mnt/test AND then exporting sub1 and sub2 as crossmnt exports? As far as I can tell, that seems to work just fine with nfs-ganesha.

I'm not sure what will happen on nfs-ganesha.

crossmnt(kernel nfsd) failed to work when exporting /mnt/test,/mnt/test/sub1,
/mnt/test/sub2.

# /bin/find /nfs/test/
/nfs/test/
find: File system loop detected; ‘/nfs/test/sub1’ is part of the same file system loop as ‘/nfs/test/’.
/nfs/test/.snapshot
find: File system loop detected; ‘/nfs/test/.snapshot/sub1-s1’ is part of the same file system loop as ‘/nfs/test/’.
find: File system loop detected; ‘/nfs/test/.snapshot/sub2-s1’ is part of the same file system loop as ‘/nfs/test/’.
/nfs/test/dir1
/nfs/test/dir1/a.txt
find: File system loop detected; ‘/nfs/test/sub2’ is part of the same file system loop as ‘/nfs/test/’

/bin/find report 'File system loop detected', it means that vfs cache(
based on st_dev + st_ino?) on nfs client side will have some problem?

In fact, I was exporting /mnt/test  for years too.
but btrfs subvols means multiple filesystems(different st_dev),  in theory, we
needs to use it based on nfs crossmnt.

Best Regards
Wang Yugui (wangyugui@e16-tech.com)
2021/06/23