Deleting subvolumes - there be dragons

Deleting subvolumes - there be dragons

[KC]

I just read this article here: https://lwn.net/Articles/579009/  which 
states:

---------------------------------
btrfs subvolume delete path

  For ordinary subvolumes (as opposed to snapshots, described below), 
the subvolume indicated by path must be empty before it can be deleted.
---------------------------------

So I gave it a try:

$ sudo btrfs subvolume create /home2
$ sudo mkdir /home2/lalala
< copy a random pdf file into /home2/lalala) >
$ sudo btrfs subvolume delete /home2

And boom! It's all gone!
Even though subvolume was not empty.

So apart from the fact that it looks like the article is wrong here, it 
seems very dangerous to me that one simple command can nuke an entire 
volume, even if it's full of files and folders.

Of course I'm new to btrfs so maybe I'm reading something wrong. If so, 
then please do set me straight.

Re: Deleting subvolumes - there be dragons

[Hugo Mills]

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On Wed, Jan 29, 2014 at 12:41:57PM +0100, KC wrote:
> I just read this article here: https://lwn.net/Articles/579009/
> which states:
> 
> ---------------------------------
> btrfs subvolume delete path
> 
>  For ordinary subvolumes (as opposed to snapshots, described below),
> the subvolume indicated by path must be empty before it can be
> deleted.
> ---------------------------------

   As you have found, that's not correct. In fact, there's a couple of
potentially misleading things in that article, just from skimming the
first few paragraphs...

> So I gave it a try:
> 
> $ sudo btrfs subvolume create /home2
> $ sudo mkdir /home2/lalala
> < copy a random pdf file into /home2/lalala) >
> $ sudo btrfs subvolume delete /home2
> 
> And boom! It's all gone!
> Even though subvolume was not empty.
> 
> So apart from the fact that it looks like the article is wrong here,
> it seems very dangerous to me that one simple command can nuke an
> entire volume, even if it's full of files and folders.

   You can do the same thing with rm -rf on any filesystem. So yes,
it's a dangerous command, and you need to be sure you want to run it.

> Of course I'm new to btrfs so maybe I'm reading something wrong. If
> so, then please do set me straight.

   No, I think the author of the article is just mistaken.

   Hugo.

-- 
=== Hugo Mills: hugo@... carfax.org.uk | darksatanic.net | lug.org.uk ===
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    --- You can't expect a boy to be depraved until he's gone to ---     
                             a good school.                              

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Re: Deleting subvolumes - there be dragons

[Chris Murphy]

On Jan 29, 2014, at 4:41 AM, KC <impactoria@googlemail.com> wrote:

> I just read this article here: https://lwn.net/Articles/579009/  which states:
> 
> ---------------------------------
> btrfs subvolume delete path
> 
> For ordinary subvolumes (as opposed to snapshots, described below), the subvolume indicated by path must be empty before it can be deleted.
> ---------------------------------

That's not correct, but maybe the author meant that a subvolume you intend to delete must not contain another subvolume. A subvolume in another subvolume will anchor the parent.


Chris Murphy