generic name for volume and subvolume root?

generic name for volume and subvolume root?

[Ulli Horlacher]

As I am writing some documentation abount creating snapshots:
Is there a generic name for both volume and subvolume root?

Example:

root@fex:~# btrfs subvol show /mnt
ERROR: not a subvolume: /mnt

root@fex:~# btrfs subvol show /mnt/test
/mnt/test is toplevel subvolume

root@fex:~# btrfs subvol show /mnt/test/data
/mnt/test/data
        Name:                   data
        UUID:                   b32a5949-dfd6-ef45-8616-34ae4cdf6fb8
(...)

root@fex:~# btrfs subvol show /mnt/test/data/sw
ERROR: not a subvolume: /mnt/test/data/sw


I can create snapshots of /mnt/test and /mnt/test/data, but not of /mnt
and /mnt/test/data/sw

Is there a simple name for directories I can snapshot?


-- 
Ullrich Horlacher              Server und Virtualisierung
Rechenzentrum TIK         
Universitaet Stuttgart         E-Mail: horlacher@tik.uni-stuttgart.de
Allmandring 30a                Tel:    ++49-711-68565868
70569 Stuttgart (Germany)      WWW:    http://www.tik.uni-stuttgart.de/
REF:<20170909083551.GC22060@rus.uni-stuttgart.de>

Re: generic name for volume and subvolume root?

[Qu Wenruo]

On 2017年09月09日 16:35, Ulli Horlacher wrote:
> As I am writing some documentation abount creating snapshots:
> Is there a generic name for both volume and subvolume root?
> 
> Example:
> 
> root@fex:~# btrfs subvol show /mnt
> ERROR: not a subvolume: /mnt
> 
> root@fex:~# btrfs subvol show /mnt/test
> /mnt/test is toplevel subvolume
> 
> root@fex:~# btrfs subvol show /mnt/test/data
> /mnt/test/data
>          Name:                   data
>          UUID:                   b32a5949-dfd6-ef45-8616-34ae4cdf6fb8
> (...)
> 
> root@fex:~# btrfs subvol show /mnt/test/data/sw
> ERROR: not a subvolume: /mnt/test/data/sw
> 
> 
> I can create snapshots of /mnt/test and /mnt/test/data, but not of /mnt
> and /mnt/test/data/sw
> 
> Is there a simple name for directories I can snapshot?
> 
> 

Nope, subvolume (including snapshot) is not distinguished by its 
filename/path/directory name.

And you can only do snapshot on subvolume (snapshot is one kind of 
subvolume) boundary.

For user to determine where is the subvolume boundary, one should first 
determine there is the btrfs mounted and then use "btrfs subvol show" to 
determine the boundaries.

Or, in a btrfs test the directory inode number. Subvolume/snapshot in 
btrfs will always have the same inode number 256, and regular 
file/directories/special files will not use that magic number.

Thanks,
Qu

Re: generic name for volume and subvolume root?

[Ulli Horlacher]

On Sat 2017-09-09 (18:40), Qu Wenruo wrote:

> > Is there a generic name for both volume and subvolume root?
> 
> Nope, subvolume (including snapshot) is not distinguished by its 
> filename/path/directory name.
> 
> And you can only do snapshot on subvolume (snapshot is one kind of 
> subvolume) boundary.

So, I can name a btrfs root volume also btrfs subvolume?

I am talking about documentation, not coding!

I just want yo use the correct terms.


-- 
Ullrich Horlacher              Server und Virtualisierung
Rechenzentrum TIK         
Universitaet Stuttgart         E-Mail: horlacher@tik.uni-stuttgart.de
Allmandring 30a                Tel:    ++49-711-68565868
70569 Stuttgart (Germany)      WWW:    http://www.tik.uni-stuttgart.de/
REF:<48008a58-a82e-d9f7-327e-eeb905e18aa1@gmx.com>

Re: generic name for volume and subvolume root?

[Qu Wenruo]

On 2017年09月09日 18:48, Ulli Horlacher wrote:
> On Sat 2017-09-09 (18:40), Qu Wenruo wrote:
> 
>>> Is there a generic name for both volume and subvolume root?
>>
>> Nope, subvolume (including snapshot) is not distinguished by its
>> filename/path/directory name.
>>
>> And you can only do snapshot on subvolume (snapshot is one kind of
>> subvolume) boundary.
> 
> So, I can name a btrfs root volume also btrfs subvolume?

Yes, root volume is also a subvolume, so just call "btrfs root volume"
a "subvolume".

> 
> I am talking about documentation, not coding!
> 
> I just want yo use the correct terms.

If you're referring to the term, I think subvolume is good enough.
Which represents your original term, "directories one can snapshot".


For the whole btrfs "volume", I would just call it "filesystem" to avoid 
the name "volume" or "subvolume" at all.

Thanks,
Qu

Re: generic name for volume and subvolume root?

[Hugo Mills]

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On Sat, Sep 09, 2017 at 06:58:38PM +0800, Qu Wenruo wrote:
> 
> 
> On 2017年09月09日 18:48, Ulli Horlacher wrote:
> >On Sat 2017-09-09 (18:40), Qu Wenruo wrote:
> >
> >>>Is there a generic name for both volume and subvolume root?
> >>
> >>Nope, subvolume (including snapshot) is not distinguished by its
> >>filename/path/directory name.
> >>
> >>And you can only do snapshot on subvolume (snapshot is one kind of
> >>subvolume) boundary.
> >
> >So, I can name a btrfs root volume also btrfs subvolume?
> 
> Yes, root volume is also a subvolume, so just call "btrfs root volume"
> a "subvolume".

   I find it's best to avoid the word "root" entirely, as it's got
several meanings, and it tends to get confusing in conversation.
Instead, we have:

 - "the top level" (subvolid=5)
 - "/"             (what you see at / in your running system)
 - "<top-level>/@" or similar names
                   (the subvolume that's mounted at /)

> >I am talking about documentation, not coding!
> >
> >I just want yo use the correct terms.
> 
> If you're referring to the term, I think subvolume is good enough.
> Which represents your original term, "directories one can snapshot".
> 
> 
> For the whole btrfs "volume", I would just call it "filesystem" to
> avoid the name "volume" or "subvolume" at all.

   Yes, it's a filesystem. (Although that does occasionally cause
confusion between "the conceptual filesystem implemented by btrfs.ko"
and "the concrete filesystem stored on /dev/sda1", but it's generally
far less confusing than the overloading of "root").

   Hugo.

-- 
Hugo Mills             | Well, you don't get to be a kernel hacker simply by
hugo@... carfax.org.uk | looking good in Speedos.
http://carfax.org.uk/  |
PGP: E2AB1DE4          |                                         Rusty Russell

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Re: generic name for volume and subvolume root?

[Hans van Kranenburg]

On 09/09/2017 01:06 PM, Hugo Mills wrote:
> On Sat, Sep 09, 2017 at 06:58:38PM +0800, Qu Wenruo wrote:
>>
>>
>> On 2017年09月09日 18:48, Ulli Horlacher wrote:
>>> On Sat 2017-09-09 (18:40), Qu Wenruo wrote:
>>>
>>>>> Is there a generic name for both volume and subvolume root?
>>>>
>>>> Nope, subvolume (including snapshot) is not distinguished by its
>>>> filename/path/directory name.
>>>>
>>>> And you can only do snapshot on subvolume (snapshot is one kind of
>>>> subvolume) boundary.
>>>
>>> So, I can name a btrfs root volume also btrfs subvolume?
>>
>> Yes, root volume is also a subvolume, so just call "btrfs root volume"
>> a "subvolume".
> 
>    I find it's best to avoid the word "root" entirely, as it's got
> several meanings, and it tends to get confusing in conversation.
> Instead, we have:
> 
>  - "the top level" (subvolid=5)
>  - "/"             (what you see at / in your running system)
>  - "<top-level>/@" or similar names
>                    (the subvolume that's mounted at /)
> 
>>> I am talking about documentation, not coding!
>>>
>>> I just want yo use the correct terms.
>>
>> If you're referring to the term, I think subvolume is good enough.
>> Which represents your original term, "directories one can snapshot".
>>
>>
>> For the whole btrfs "volume", I would just call it "filesystem" to
>> avoid the name "volume" or "subvolume" at all.
> 
>    Yes, it's a filesystem. (Although that does occasionally cause
> confusion between "the conceptual filesystem implemented by btrfs.ko"
> and "the concrete filesystem stored on /dev/sda1", but it's generally
> far less confusing than the overloading of "root").

Yes, because every subvolume is a filesystem root! :-D

https://i.imgur.com/2VzmC.gif

-- 
Hans van Kranenburg

Re: generic name for volume and subvolume root?

[Hugo Mills]

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On Sat, Sep 09, 2017 at 10:35:51AM +0200, Ulli Horlacher wrote:
> As I am writing some documentation abount creating snapshots:
> Is there a generic name for both volume and subvolume root?
> 
> Example:
> 
> root@fex:~# btrfs subvol show /mnt
> ERROR: not a subvolume: /mnt
> 
> root@fex:~# btrfs subvol show /mnt/test
> /mnt/test is toplevel subvolume
> 
> root@fex:~# btrfs subvol show /mnt/test/data
> /mnt/test/data
>         Name:                   data
>         UUID:                   b32a5949-dfd6-ef45-8616-34ae4cdf6fb8
> (...)
> 
> root@fex:~# btrfs subvol show /mnt/test/data/sw
> ERROR: not a subvolume: /mnt/test/data/sw
> 
> 
> I can create snapshots of /mnt/test and /mnt/test/data, but not of /mnt
> and /mnt/test/data/sw
> 
> Is there a simple name for directories I can snapshot?

   Subvolume. If you can snapshot it, it's a subvolume. Some
subvolumes are also snapshots. (And all snapshots are subvolumes).

   The subvolume with ID 5 (or ID 0, which is an alias) is the "top
level subvolume", and has the unique property that it can't be
renamed, deleted or replaced, where all other subvolumes can be.

   Hugo.

-- 
Hugo Mills             | Well, you don't get to be a kernel hacker simply by
hugo@... carfax.org.uk | looking good in Speedos.
http://carfax.org.uk/  |
PGP: E2AB1DE4          |                                         Rusty Russell

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Re: generic name for volume and subvolume root?

[Peter Grandi]

> As I am writing some documentation abount creating snapshots:
> Is there a generic name for both volume and subvolume root?

Yes, it is from the UNIX side 'root directory' and from the
Btrfs side 'subvolume'. Like some other things Btrfs, its
terminology is often inconsistent, but "volume" *usually* means
"the set of devices [and contained root directories] with the
same Btrfs 'fsid'".

I think that the top-level subvolume should not be called the
"volume": while there is no reason why a UNIX-like filesystem
should be limited to a single block-device, one of the
fundamental properties of UNIX-like filesystems is that
hard-links are only possible (if at all possible) within a
filesystem, and that 'statfs' returns a different "device id"
per filesystem. Therefore a Btrfs volume is not properly a
filesystem, but potentially a filesystem forest, as it may
contain multiple filesystems each with its own root directory.

> Is there a simple name for directories I can snapshot?

You can only snapshot *root directories*, of which in Btrfs
there are two types: subvolumes (an unfortunate name perhaps) or
snapshots.

In UNIX-like OSes every filesystem has a "root directory" and
some filesystem types like Btrfs, NILFS2, and potentially JFS
can have more than one, and some can even mount more than one
simultaneously.

The root directory mounted as '/' is called the "system root
directory". When unmounted all filesystem root directories have
no names, just an inode number. Conceivably the root inode of a
UNIX-like filesystem could be an inode of any type, but I have
never seen a recent UNIX-like OS able to mount anything other
than a directory-type root inode (Plan 9 is not a UNIX-like OS
:->).

As someone else observed, the word "root" is overloaded in
UNIX-like OS discourse, like the word "filesystem", and that's
unfortunate but can always be resolved verbosely by using the
appropriate qualifier like "root directory", "system root
directory", "'root' user", "uid 0 capabilities", etc.