Re: How to convert a directory to a subvolume

[John Center <jlcenter15@gmail.com>]

Hi Hugo,

Thanks for responding.  I guess what I don't understand, @home is a 
subvolume, but it appears as /home when it is mounted via fstab.  It has 
a top level ID of 5.  If I create a subvolume for opt, it has a top 
level of 256.  I've tried different variations of opt, /opt, & @opt, but 
they all appear as that variation under /:

john@Mariposa:~$ sudo btrfs subvolume create /@opt
Create subvolume '//@opt'

john@Mariposa:~$ sudo btrfs subvolume list /
ID 256 gen 5968 top level 5 path @
ID 257 gen 5968 top level 5 path @home
ID 259 gen 5966 top level 256 path @opt

john@Mariposa:~$ sudo btrfs subvolume delete /@opt
Delete subvolume (no-commit): '//@opt'

john@Mariposa:~$ sudo btrfs subvolume create /opt
Create subvolume '//opt'

john@Mariposa:~$ sudo btrfs subvolume list -t /
ID    gen    top level    path
--    ---    ---------    ----
256    5993    5        @
257    5993    5        @home
260    5993    256        opt


What I'm expecting is when I do the subvolume list, I would see 
something like this:

john@Mariposa:~$ sudo btrfs subvolume list -t /
ID    gen    top level    path
--    ---    ---------    ----
256    5993    5        @
257    5993    5        @home
260    5993    5        @opt

I would also think the fstab would look something like this:

UUID=ce05e908-2dce-4368-b864-2f29650185e8 /               btrfs   
defaults,space_cache=v2,subvol=@ 0 1
#
UUID=ce05e908-2dce-4368-b864-2f29650185e8 /home btrfs   
defaults,space_cache=v2,subvol=@home 0       2
#
UUID=ce05e908-2dce-4368-b864-2f29650185e8 /opt           btrfs 
defaults,space_cache=v2,subvol=@opt 0       2

I also thought I would have to mount the subvolume like a directory.

So, what am I missing between what I'm seeing vs what I think I should 
be seeing?

Thanks for your help!

     -John


On 4/30/22 4:14 PM, Hugo Mills wrote:
> On Sat, Apr 30, 2022 at 04:08:59PM -0400, John Center wrote:
>> Hi,
>>
>> I just installed Ubuntu 22.04 with a btrfs raid1 root filesystem.  I want to convert a directory, like /opt, into a subvolume. I haven’t been having much luck.  /opt is empty right now, so it’s a good candidate for conversion.  Could someone please explain how to do it?  I’ve been at a dozen different websites, & tried different variations of the “btrfs subvolume create” command, but nothing works when I go to mount it.  I think I’m missing something simple, but not sure what it is.
>     You can't convert a directory into a subvolume.
>
>     Since the directory in question is empty, just delete it and create
> a subvol there instead:
>
> # rmdir /opt
> # btrfs sub create /opt
>
>     If there's stuff in there, you need to create the subvolume with a
> different name, copy the contents of the directory into it (optionally
> with --reflink=always) and then delete the original directory and move
> the subvolume into its place.
>
>     Hugo.
>