On Thu, Dec 31, 2020 at 05:08:57PM +0100, john terragon wrote:
> On Thu, Dec 31, 2020 at 8:05 AM Andrei Borzenkov <arvidjaar@gmail.com> wrote:
> >
> 
> > >
> > > OK, but then could I use Y as parent of the rw snapshot, let's call it
> > > W, in a send?
> >
> > No
> >
> 
> Of course I didn't mean to use Y as a parent of W itself but to a
> readonly snapshot of W whenever I want to send it to the second FS.
> 
> And I just tried the following steps and they worked:
> 
> 1) created subvol X
> 2) created readonly snap Y of X
> 3) sent Y to second FS
> 4) modified X
> 5) created readonly snap X1 of X
> 6) sent -p Y X1 to second FS
> 7) created readwrite snap Y1 of Y
> 8) modified Y1
> 9) created readonly snap Y1_RO of Y1
> 10) sent -p Y Y1_RO to second FS
> 
> So, as you can see,
> 
> -in 6) I've used the RO snap Y of X as the parent of X1 (and X) to
> send X1 to the second FS
> 
> -in 10) I did the opposite, Y is still used as the parent but this
> time I've sent the RO snap of a subvol that is a snap of Y.
> 
> So it seems to work both ways

I think your confusion is that you are thinking of these as a tree.
There is no tree, each subvol is an equal peer in the filesystem.

"send -p A B" just walks over subvol A and B and sends a diff of the
parts of B not in A.  You can pick any subvol with -p as long as it's
read-only and present on the receiving side.  Obviously it's much more
efficient if the two subvols have a lot of shared extents (e.g. because
B and A were both snapshots made at different times of some other subvol
C), but this is not required.