On 2020-12-30 17:56, john terragon wrote:
Hi.
I would like to maintain a tree-like hierarchical structure of
snapshots. Let me try to explain what I mean by that.
Let's say I have a btrfs fs with just one subvolume X, and let's say
that a make a readonly snapshot Y of X. As far as I understand there
is a parent-child relation between Y (the parent) and X the child.
Now let's say that after some time and modifications of X I do another
snapshot Z of X. Now the "temporal" stucture would be Y-Z-X. So X is
now the "child" of Z and Z is now the "child" of Y. The structure is a
path which is a special case of a tree.
Now let's suppose that I want to start modify Y but I still want to be
able to have a parent of Z which I might use as a point of reference
for Z in a
send to somewhere. That is I want to be able to still do a send -p Y Z
to another btrfs filesystem where there is previously sent copy of Y
(which, remember, as of this point has been readonly and I'm just now
wanting to start to modify it).
The only thing I think I can do would be to make a readonly snapshot
Y1 of Y and make Y writeable (so that I can start modify it).
You should simply make a 'read-write' snapshot (Y-rw) of the 'read-only'
snapshot (Y) that is part of your backup/send scheme. Do not modify
read-only snapshots to be rw.
At that
point the structure would be
Y1-Y
\
Z-X
(yes my ascii art is atrocious...) which is a "proper" tree where Y1
is the root with two children (Y and Z), Z has one child (X) and Y and
X are leaves.
Now, my question is, would Y1 still be usable in send -p Y1 Z, just
like Y was before becoming writeable and being modified? I would say
that Y1 would be just as good as the readonly original Y was as a
parent for Z in a send. But maybe there is some implementation detail
that escapes me and that prevents Y1 to be used as a perfect
replacement for the original Y.
I hope I was clear enough.
Thanks
John
