Re: [PATCH] fs: allow cross-vfsmount reflink/dedupe

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On Thu, 17 Feb 2022, Wang Yugui wrote:
> Hi,
> 
> > On Thu, 17 Feb 2022, Wang Yugui wrote:
> > > Hi,
> > > Cc: NeilBrown
> > > 
> > > btrfs cross-vfsmount reflink works well now with these 2 patches.
> > > 
> > > [PATCH] fs: allow cross-vfsmount reflink/dedupe
> > > [PATCH] btrfs: remove the cross file system checks from remap
> > > 
> > > But nfs over btrfs still fail to do cross-vfsmount reflink.
> > > need some patch for nfs too?
> > 
> > NFS doesn't support reflinks at all, does it?
> 
> NFS support reflinks now.
> 
> # df -h /ssd
> Filesystem              Type  Size  Used Avail Use% Mounted on
> T640:/ssd               nfs4   17T  5.5T   12T  34% /ssd
> # /bin/cp --reflink=always /ssd/1.txt /ssd/2.txt
> # uname -a
> Linux T7610 5.15.24-3.el7.x86_64 #1 SMP Thu Feb 17 12:13:25 CST 2022 x86_64 x86_64 x86_64 GNU/Linux
> 

So it does ..... ahhh, the CLONE command in NFSv4.2.....
This is used by the .remap_file_range file operation.  That operation
only gets called when the "from" and "to" files have the same
superblock.
btrfs has an ....  interesting concept of filesystem identity.  While
different "subvols" have the same superblock locally, when they are
exported over NFS they appear to be different filesystems and so have
different superblocks.  This is in part because btrfs cannot create
properly unique inode numbers across the whole filesystem.
Until btrfs sorts itself out, it will not be able to work with NFS
properly.

NeilBrown




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