Re: [PATCH 1/2] btrfs: protect snapshots from deleting during send

[Chris Mason <clm@fb.com>]

On 04/15/2014 10:41 AM, David Sterba wrote:
> The patch "Btrfs: fix protection between send and root deletion"
> (18f687d538449373c37c) does not actually prevent to delete the snapshot
> and just takes care during background cleaning, but this seems rather
> user unfriendly, this patch implements the idea presented in
>
> http://www.spinics.net/lists/linux-btrfs/msg30813.html
>
> - add an internal root_item flag to denote a dead root
> - check if the send_in_progress is set and refuse to delete, otherwise
>    set the flag and proceed
> - check the flag in send similar to the btrfs_root_readonly checks, for
>    all involved roots
>
> The root lookup in send via btrfs_read_fs_root_no_name will check if the
> root is really dead or not. If it is, ENOENT, aborted send. If it's
> alive, it's protected by send_in_progress, send can continue.

I'm worried about the use case where we have:

   * periodic automated snapshots
   * periodic automated deletion of old snapshots
   * periodic send for backup

The automated deletion doesn't want to error out if send is in progress, 
it just wants the deletion to happen in the background.

-chris