How to convert a directory to a subvolume

How to convert a directory to a subvolume

[John Center]

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.

Thanks!

    -John 

Sent from my iPad

Re: How to convert a directory to a subvolume

[Hugo Mills]

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.

-- 
Hugo Mills             | UNIX: British manufacturer of modular shelving units
hugo@... carfax.org.uk |
http://carfax.org.uk/  |
PGP: E2AB1DE4          |

Re: How to convert a directory to a subvolume

[John Center]

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.
>

Re: How to convert a directory to a subvolume

[Chris Murphy]

On Sun, May 1, 2022 at 8:02 PM John Center <jlcenter15@gmail.com> wrote:
>
> 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'

This is the mistake. Since @ is mounted at /, using /@opt actually
means /@/@opt, thus it's been nested.

>
> 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

@opt was created inside @, that's why it looks like this. Thus, @opt
is nested within @. Thus, @ and @home follow a flat layout. And @opt
follows a nested layout. To continue with the flat layout for @opt
you'd need to create the @opt subvolume in the top-level of the file
system alongside @ and @home rather than within one of them. To create
it in the top-level you need to mount the top-level somewhere, e.g.

btrfs subvolume get-default /

Assuming this refers to ID 5, then you just

mount /dev/sdXY /mnt
btrfs subvolume create /mnt/@opt

And the fstab entry should be subvol=@opt

If get-default reports a value other than 5, then you need to
explicitly mount the top level:

mount -o subvol=/ /dev/sdXY /mnt       OR
mount -o subvolid=5 /dev/sdXY /mnt    OR
mount -o subvolid=0 /dev/sdXY /mnt

The top-level subvolume was assigned ID 5 during initial creation of
the file system. Soon after, ID 0 was set up as an alias for 5 because
maybe it'd be easier to remember.

Note: The top-level nomenclature used by btrfs subvolume list is
considered confusing, and I think it's going away in btrfs-progs
soonish? The "top-level" of the file system is considered equivalent
to subvolume ID 5, which has no name, cannot be renamed or removed,
but can be snapshot, and is the subvolume created at mkfs time. Above,
it sounds like there's multiple "top-level" subvolumes, e.g. 5 and
256. But in practice it's not how anyone is using the term "top
level", which these days is intended to refer to just ID 5. The
subvolume with no (explicit) name.



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

Yep, / is still the @ subvolume here, because @ is bind mounted to /


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

It'd be neat if we had a way to create subvolumes in the top level
directly without having to mount the top level. The FD variant of the
C API found in libbtrfsutil suggests it's possible. And also we have a
way to delete subvolumes by subvolid, even when they aren't locatable
via the mounted file system hierarchy, using the --subvolid flag.




-- 
Chris Murphy

Re: How to convert a directory to a subvolume

[John Center]

Hi Chris,

Thanks for responding!  Well, I made some progress:

root@Mariposa:/# mount /dev/sda3 /mnt

root@Mariposa:/# btrfs subvolume create /mnt/@opt
Create subvolume '/mnt/@opt'

root@Mariposa:/# btrfs subvolume list /
ID 256 gen 11732 top level 5 path @
ID 257 gen 11733 top level 5 path @home
ID 268 gen 11733 top level 5 path @opt

Once I unmounted /mnt, I had to create the /opt directory again:

root@Mariposa:/# umount /mnt

root@Mariposa:/# mkdir opt

Then I mounted /opt & it didn't complain:

root@Mariposa:/# mount /opt
root@Mariposa:/# btrfs subvolume list /
ID 256 gen 11751 top level 5 path @
ID 257 gen 11751 top level 5 path @home
ID 268 gen 11733 top level 5 path @opt

So, is this correct?  It would have been good if there was a direct way 
to add the level that you wanted the subvolume created & mounted with 
the btrfs command.  So if I take a snapshot of @, /opt won't be 
included?  That'd my goal.  I've been using btrfs at a very basic level, 
added on top of an md raid1.  It worked well for a number of years, but 
then I needed to replace the disks, I thought I'd go all in on btrfs.

Thanks for your help!

     -John


On 5/2/22 9:16 AM, Chris Murphy wrote:
> On Sun, May 1, 2022 at 8:02 PM John Center <jlcenter15@gmail.com> wrote:
>> 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'
> This is the mistake. Since @ is mounted at /, using /@opt actually
> means /@/@opt, thus it's been nested.
>
>> 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
> @opt was created inside @, that's why it looks like this. Thus, @opt
> is nested within @. Thus, @ and @home follow a flat layout. And @opt
> follows a nested layout. To continue with the flat layout for @opt
> you'd need to create the @opt subvolume in the top-level of the file
> system alongside @ and @home rather than within one of them. To create
> it in the top-level you need to mount the top-level somewhere, e.g.
>
> btrfs subvolume get-default /
>
> Assuming this refers to ID 5, then you just
>
> mount /dev/sdXY /mnt
> btrfs subvolume create /mnt/@opt
>
> And the fstab entry should be subvol=@opt
>
> If get-default reports a value other than 5, then you need to
> explicitly mount the top level:
>
> mount -o subvol=/ /dev/sdXY /mnt       OR
> mount -o subvolid=5 /dev/sdXY /mnt    OR
> mount -o subvolid=0 /dev/sdXY /mnt
>
> The top-level subvolume was assigned ID 5 during initial creation of
> the file system. Soon after, ID 0 was set up as an alias for 5 because
> maybe it'd be easier to remember.
>
> Note: The top-level nomenclature used by btrfs subvolume list is
> considered confusing, and I think it's going away in btrfs-progs
> soonish? The "top-level" of the file system is considered equivalent
> to subvolume ID 5, which has no name, cannot be renamed or removed,
> but can be snapshot, and is the subvolume created at mkfs time. Above,
> it sounds like there's multiple "top-level" subvolumes, e.g. 5 and
> 256. But in practice it's not how anyone is using the term "top
> level", which these days is intended to refer to just ID 5. The
> subvolume with no (explicit) name.
>
>
>
>> john@Mariposa:~$ sudo btrfs subvolume delete /@opt
>> Delete subvolume (no-commit): '//@opt'
>>
>> john@Mariposa:~$ sudo btrfs subvolume create /opt
>> Create subvolume '//opt'
> Yep, / is still the @ subvolume here, because @ is bind mounted to /
>
>
>> So, what am I missing between what I'm seeing vs what I think I should
>> be seeing?
> It'd be neat if we had a way to create subvolumes in the top level
> directly without having to mount the top level. The FD variant of the
> C API found in libbtrfsutil suggests it's possible. And also we have a
> way to delete subvolumes by subvolid, even when they aren't locatable
> via the mounted file system hierarchy, using the --subvolid flag.
>
>
>
>

Re: How to convert a directory to a subvolume

[Chris Murphy]

On Mon, May 2, 2022 at 11:37 AM John Center <jlcenter15@gmail.com> wrote:
>
> Hi Chris,
>
> Thanks for responding!  Well, I made some progress:
>
> root@Mariposa:/# mount /dev/sda3 /mnt
>
> root@Mariposa:/# btrfs subvolume create /mnt/@opt
> Create subvolume '/mnt/@opt'
>
> root@Mariposa:/# btrfs subvolume list /
> ID 256 gen 11732 top level 5 path @
> ID 257 gen 11733 top level 5 path @home
> ID 268 gen 11733 top level 5 path @opt
>
> Once I unmounted /mnt, I had to create the /opt directory again:
>
> root@Mariposa:/# umount /mnt
>
> root@Mariposa:/# mkdir opt
>
> Then I mounted /opt & it didn't complain:
>
> root@Mariposa:/# mount /opt
> root@Mariposa:/# btrfs subvolume list /
> ID 256 gen 11751 top level 5 path @
> ID 257 gen 11751 top level 5 path @home
> ID 268 gen 11733 top level 5 path @opt
>
> So, is this correct?

Looks correct. You need to include something like 'mount | grep btrfs'
to know for sure, that should show subvol and subvolid for each mount
point.



> It would have been good if there was a direct way
> to add the level that you wanted the subvolume created & mounted with
> the btrfs command.  So if I take a snapshot of @, /opt won't be
> included?

Btrfs snapshots via the btrfs-progs CLI tool are not recursive. So
whether the subvolumes are arranged in a nested or flat layout doesn't
matter. There is a non-atomic recursive snapshot (create and remove)
option in libbtrfs both for C API and Python bindings.


>That'd my goal.  I've been using btrfs at a very basic level,
> added on top of an md raid1.  It worked well for a number of years, but
> then I needed to replace the disks, I thought I'd go all in on btrfs.
>


-- 
Chris Murphy

Re: How to convert a directory to a subvolume

[John Center]

root@Mariposa:/# mount | grep btrfs

/dev/sda3 on / type btrfs 
(rw,relatime,space_cache=v2,autodefrag,subvolid=256,subvol=/@)
/dev/sda3 on /home type btrfs 
(rw,relatime,space_cache=v2,autodefrag,subvolid=257,subvol=/@home)
/dev/sda3 on /opt type btrfs 
(rw,relatime,space_cache=v2,autodefrag,subvolid=268,subvol=/@opt)

Thanks, Hugo & Chris!

     -John


On 5/2/22 12:15 PM, Chris Murphy wrote:
> On Mon, May 2, 2022 at 11:37 AM John Center <jlcenter15@gmail.com> wrote:
>> Hi Chris,
>>
>> Thanks for responding!  Well, I made some progress:
>>
>> root@Mariposa:/# mount /dev/sda3 /mnt
>>
>> root@Mariposa:/# btrfs subvolume create /mnt/@opt
>> Create subvolume '/mnt/@opt'
>>
>> root@Mariposa:/# btrfs subvolume list /
>> ID 256 gen 11732 top level 5 path @
>> ID 257 gen 11733 top level 5 path @home
>> ID 268 gen 11733 top level 5 path @opt
>>
>> Once I unmounted /mnt, I had to create the /opt directory again:
>>
>> root@Mariposa:/# umount /mnt
>>
>> root@Mariposa:/# mkdir opt
>>
>> Then I mounted /opt & it didn't complain:
>>
>> root@Mariposa:/# mount /opt
>> root@Mariposa:/# btrfs subvolume list /
>> ID 256 gen 11751 top level 5 path @
>> ID 257 gen 11751 top level 5 path @home
>> ID 268 gen 11733 top level 5 path @opt
>>
>> So, is this correct?
> Looks correct. You need to include something like 'mount | grep btrfs'
> to know for sure, that should show subvol and subvolid for each mount
> point.
>
>
>
>> It would have been good if there was a direct way
>> to add the level that you wanted the subvolume created & mounted with
>> the btrfs command.  So if I take a snapshot of @, /opt won't be
>> included?
> Btrfs snapshots via the btrfs-progs CLI tool are not recursive. So
> whether the subvolumes are arranged in a nested or flat layout doesn't
> matter. There is a non-atomic recursive snapshot (create and remove)
> option in libbtrfs both for C API and Python bindings.
>
>
>> That'd my goal.  I've been using btrfs at a very basic level,
>> added on top of an md raid1.  It worked well for a number of years, but
>> then I needed to replace the disks, I thought I'd go all in on btrfs.
>>
>

Re: How to convert a directory to a subvolume

[Chris Murphy]

On Sat, Apr 30, 2022 at 4:09 PM John Center <jlcenter15@gmail.com> 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.

https://btrfs.wiki.kernel.org/index.php/SysadminGuide#Nested

If you're using the nested layout, you won't need to use /etc/fstab.
When using the flat layout, subvolumes are mounted into position using
fstab.

-- 
Chris Murphy