Can a snapshot become a parent subvolume when its parent is deleted?

Can a snapshot become a parent subvolume when its parent is deleted?

[Marc MERLIN]

So, I had btrfs_pool1 that was trashed/lost as discussed here recently.

I did btrfs send btrfs_pool2/root_ro.date | btrfs receive /mnt/btrfs_pool1

Then btrfs subvolume snapshot root_ro.date root

Now, after I delete root_ro.date on btrfs_pool1, shouldn't root become a
parent subvolume?

Right now, I have:
legolas:/mnt/btrfs_pool1# btrfs subvolume list -q . 
ID 390 gen 5954 top level 5 parent_uuid faea7df8-d51a-6b4b-994b-887d55267cce path root
legolas:/mnt/btrfs_pool1# btrfs subvolume list  -u .  |grep faea7df8-d51a-6b4b-994b-887d55267cce
legolas:/mnt/btrfs_pool1#

Is it possible not to have a parent?

If so, shouldn't you become a parent at that time?

Thanks,
Marc
-- 
"A mouse is a device used to point at the xterm you want to type in" - A.S.R.
Microsoft is to operating systems ....
                                      .... what McDonalds is to gourmet cooking
Home page: http://marc.merlins.org/                         | PGP 1024R/763BE901

Re: Can a snapshot become a parent subvolume when its parent is deleted?

[Chris Murphy]

On May 14, 2014, at 7:50 AM, Marc MERLIN <marc@merlins.org> wrote:

> So, I had btrfs_pool1 that was trashed/lost as discussed here recently.
> 
> I did btrfs send btrfs_pool2/root_ro.date | btrfs receive /mnt/btrfs_pool1
> 
> Then btrfs subvolume snapshot root_ro.date root
> 
> Now, after I delete root_ro.date on btrfs_pool1, shouldn't root become a
> parent subvolume?

There's no explicit hierarchy like this. There's no state "parent" or "child". 

If a subvolume is created by snapshotting another subvolume, the new subvolume refers to the uuid of its source as the parent uuid. And the original subvolume, at least in the UI, contains a list of its snapshots but I don't know that this metadata is literally associated with the subvolume rather than inferred from its snapshots, all of which contain the "parent" subvolume's uuid as a parent_uuid.


> 
> Right now, I have:
> legolas:/mnt/btrfs_pool1# btrfs subvolume list -q . 
> ID 390 gen 5954 top level 5 parent_uuid faea7df8-d51a-6b4b-994b-887d55267cce path root
> legolas:/mnt/btrfs_pool1# btrfs subvolume list  -u .  |grep faea7df8-d51a-6b4b-994b-887d55267cce
> legolas:/mnt/btrfs_pool1#
> 
> Is it possible not to have a parent?
> 
> If so, shouldn't you become a parent at that time?

No. The listed subvolume parent_uuid is the subvolume uuid for now deleted subvolume root_ro.date. Even though it's deleted, the subvolume created from it still retains the parent_uuid reference.

If you snapshot root, you could say it "becomes a parent" but it's only a metaphor it's not something that literally happens on disk. All I'm aware of that happens is the new subvolume gets its own uuid and its parent_uuid is set to match the subvolume uuid it was created from.




Chris Murphy

Re: Can a snapshot become a parent subvolume when its parent is deleted?

[Marc MERLIN]

On Wed, May 14, 2014 at 12:09:38PM -0600, Chris Murphy wrote:
> 
> On May 14, 2014, at 7:50 AM, Marc MERLIN <marc@merlins.org> wrote:
> 
> > So, I had btrfs_pool1 that was trashed/lost as discussed here recently.
> > 
> > I did btrfs send btrfs_pool2/root_ro.date | btrfs receive /mnt/btrfs_pool1
> > 
> > Then btrfs subvolume snapshot root_ro.date root
> > 
> > Now, after I delete root_ro.date on btrfs_pool1, shouldn't root become a
> > parent subvolume?
> 
> There's no explicit hierarchy like this. There's no state "parent" or "child". 
> 
> If a subvolume is created by snapshotting another subvolume, the new
> subvolume refers to the uuid of its source as the parent uuid. And
> the original subvolume, at least in the UI, contains a list of its
> snapshots but I don't know that this metadata is literally associated
> with the subvolume rather than inferred from its snapshots, all of
> which contain the "parent" subvolume's uuid as a parent_uuid.

Right. So basically the parent UUID can go orphanned, which I found a
bit surprising.

> > Right now, I have:
> > legolas:/mnt/btrfs_pool1# btrfs subvolume list -q . 
> > ID 390 gen 5954 top level 5 parent_uuid faea7df8-d51a-6b4b-994b-887d55267cce path root
> > legolas:/mnt/btrfs_pool1# btrfs subvolume list  -u .  |grep faea7df8-d51a-6b4b-994b-887d55267cce
> > legolas:/mnt/btrfs_pool1#
> > 
> > Is it possible not to have a parent?
> > 
> > If so, shouldn't you become a parent at that time?
> 
> No. The listed subvolume parent_uuid is the subvolume uuid for
> now deleted subvolume root_ro.date. Even though it's deleted, the
> subvolume created from it still retains the parent_uuid reference.

Yep. If you think about daemonized processed that get inherited by init,
shouldn't parent-less snapshots change their parent uuid to '-'
 
> If you snapshot root, you could say it "becomes a parent" but it's
> only a metaphor it's not something that literally happens on disk. All
> I'm aware of that happens is the new subvolume gets its own uuid and
> its parent_uuid is set to match the subvolume uuid it was created
> from.

You are correct. The blocks don't care who is the parent and child
subvolume, they're just references.
But for management purposes by us humans :) it's useful to know what's
going on.

Would a developer agree to making parent less subvolumes have their
parent become '-'?

Thanks,
Marc
-- 
"A mouse is a device used to point at the xterm you want to type in" - A.S.R.
Microsoft is to operating systems ....
                                      .... what McDonalds is to gourmet cooking
Home page: http://marc.merlins.org/                         | PGP 1024R/763BE901