[Samba] vfs_shadow_copy2: unmount snapshot while user is restoring from it

[Jeremy Allison]

On Mon, Feb 08, 2016 at 06:33:25PM +0200, Alex Lyakas wrote:
> Greetings,
> 
> I am trying to use vfs_shadow_copy2 with samba samba-4.2.7.
> 
> I have a share exported at /export/smb400/. A snapshot of the share is on a
> separate block device, which is mounted at
> /export_shadows/volume-00000001/@GMT-2016.02.08-11.48.00/. Samba
> configuration for the share is:
> 
> vfs objects = shadow_copy2
> shadow:snapdir = /export_shadows/volume-00000001
> shadow:fixinodes = yes
> 
> (Note: I had to pull two patches by Uri Simchoni to make the snapshots
> mounted outside of the share working [1] and [2]).
> 
> Everything is working fine, and on Windows the “Previous version" tab allows
> to restore files from the snapshot.
> 
> However, when a large file (say 10GB) is being restored, during that time it
> is not possible to unmount the snapshot block device, because Samba is
> holding open file descriptors on its mount point. Question: is there a way
> to forcefully unmount the snapshot block device, such that all ongoing
> Restore operations will fail?
> 
> I did some debugging and saw that during the Restore process
> shadow_copy2_fstat() is being called a lot (by
> smb_vfs_call_copy_chunk_send). So I tried to return a failure in this
> function (-1 with errno=ENOENT). I saw that it indeed helps: Restore
> operation hits an error. But it takes about 10 seconds, until unmount of the
> snapshot finally succeeds.
> 
> My question is: is there any other operation in “vfs_fn_pointers” that I can
> implement or fail to make vfs_shadow_copy2 quickly close all open file
> descriptors, such that unmount of the snapshot succeeds?
> 
> Thanks,
> Alex.

Hmmm. Isn't this the equivalent of cancelling an async
op in the middle of the operation ? That's notoriously
hard to do...

Wouldn't this also cause the user to lose data ?

What is the exact use case for this ?