Btrfs devel - Oct 2010 - Can you please define "snapshot" and "subvolume"?

I have difficulties grabbing these two concepts.

As far as I can tell, a snapshot is an instant, synchronized,
photography of the filesystem at a given point in time; a subvolume is
a "subroot" to a btrfs filesystem.

While I fully understand (and use) the purpose of snapshots, I don''t
quite fathom the use case for subvolumes, apart from btrfs-convert...
Why has btrfs grown such a feature in the first place? Can someone
give me a use case for them?

-- 
Francis Galiegue, fgaliegue@gmail.com
"It seems obvious [...] that at least some ''business
intelligence''
tools invest so much intelligence on the business side that they have
nothing left for generating SQL queries" (Stéphane Faroult, in "The
Art of SQL", ISBN 0-596-00894-5)
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You may take a look at this article
this isn''t actually a explanation of why snapshots or subvolumes are
used, but maybe you can read the importance of the difference from the
context:
http://www.h-online.com/open/features/Snapshots-and-subvolumes-747029.html

On Thu, Oct 7, 2010 at 13:39, Francis Galiegue <fgaliegue@gmail.com>
wrote:
>
> I have difficulties grabbing these two concepts.
>
> As far as I can tell, a snapshot is an instant, synchronized,
> photography of the filesystem at a given point in time; a subvolume is
> a "subroot" to a btrfs filesystem.
>
> While I fully understand (and use) the purpose of snapshots, I
don''t
> quite fathom the use case for subvolumes, apart from btrfs-convert...
> Why has btrfs grown such a feature in the first place? Can someone
> give me a use case for them?
>
> --
> Francis Galiegue, fgaliegue@gmail.com
> "It seems obvious [...] that at least some ''business
intelligence''
> tools invest so much intelligence on the business side that they have
> nothing left for generating SQL queries" (Stéphane Faroult, in
"The
> Art of SQL", ISBN 0-596-00894-5)
> --
> To unsubscribe from this list: send the line "unsubscribe
linux-btrfs" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html


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Just to tell you one of my use case, I do compilations of OpenWRT on a
Btrfs filesystem, when I snapshot, I only snapshot the subvolume where
all the sources are instead of the whole root filesystem, and when I
need a snapshot of the root for backup purposes, only the root without
all the snapshot (which are used to compile for different models of
embedded devices) is snapshotted, which probably reduces the overhead
(I can''t tell for sure) and makes deletion instantaneous, the cleaner
just wipes behind the subvolume remove command.

On Thu, Oct 7, 2010 at 7:39 AM, Francis Galiegue <fgaliegue@gmail.com>
wrote:
> I have difficulties grabbing these two concepts.
>
> As far as I can tell, a snapshot is an instant, synchronized,
> photography of the filesystem at a given point in time; a subvolume is
> a "subroot" to a btrfs filesystem.
>
> While I fully understand (and use) the purpose of snapshots, I
don''t
> quite fathom the use case for subvolumes, apart from btrfs-convert...
> Why has btrfs grown such a feature in the first place? Can someone
> give me a use case for them?
>
> --
> Francis Galiegue, fgaliegue@gmail.com
> "It seems obvious [...] that at least some ''business
intelligence''
> tools invest so much intelligence on the business side that they have
> nothing left for generating SQL queries" (Stéphane Faroult, in
"The
> Art of SQL", ISBN 0-596-00894-5)
> --
> To unsubscribe from this list: send the line "unsubscribe
linux-btrfs" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
>
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On Thu, Oct 07, 2010 at 08:30:31AM -0400, Jérôme Poulin
wrote:
> Just to tell you one of my use case, I do compilations of OpenWRT on a
> Btrfs filesystem, when I snapshot, I only snapshot the subvolume where
> all the sources are instead of the whole root filesystem, and when I
> need a snapshot of the root for backup purposes, only the root without
> all the snapshot (which are used to compile for different models of
> embedded devices) is snapshotted, which probably reduces the overhead
> (I can''t tell for sure) and makes deletion instantaneous, the
cleaner
> just wipes behind the subvolume remove command.
BTW, it would be very useful to be able to turn existing directories
into subvolumes.

Mike
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+1 from me

On Thu, Oct 7, 2010 at 15:18, Mike Hommey <mh@glandium.org>
wrote:
> On Thu, Oct 07, 2010 at 08:30:31AM -0400, Jérôme Poulin wrote:
>> Just to tell you one of my use case, I do compilations of OpenWRT on a
>> Btrfs filesystem, when I snapshot, I only snapshot the subvolume where
>> all the sources are instead of the whole root filesystem, and when I
>> need a snapshot of the root for backup purposes, only the root without
>> all the snapshot (which are used to compile for different models of
>> embedded devices) is snapshotted, which probably reduces the overhead
>> (I can''t tell for sure) and makes deletion instantaneous, the
cleaner
>> just wipes behind the subvolume remove command.
>
> BTW, it would be very useful to be able to turn existing directories
> into subvolumes.
>
> Mike
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>


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On Thursday, 07 October, 2010, Francis Galiegue wrote:
> I have difficulties grabbing these two concepts.
> 
> As far as I can tell, a snapshot is an instant, synchronized,
> photography of the filesystem at a given point in time; a subvolume is
> a "subroot" to a btrfs filesystem.
> 
> While I fully understand (and use) the purpose of snapshots, I
don''t
> quite fathom the use case for subvolumes, apart from btrfs-convert...
> Why has btrfs grown such a feature in the first place? Can someone
> give me a use case for them?
By design in btrfs a snapshot is "a instant, synchronized, photography
of" a
subvolume.
In fact you can snapshot only a subvolume [*]. 

Moreover the subvolumes have the following properties:

1) it is possible to mount a subvolume of a filesystem: if you execute the 
following commands:

# mount -o subvol=name-of-subvol /dev/sdxx /mn/test

the kernel will use the subvolume "name-of-subvol" of the btrfs
filesystem of
the partition /dev/sdxx. Pay attention: this work *only* if the subvolume 
"name-of-subvol" is under the root of the filesystem.

2) a subvolume may be deleted asynchronously by the command "btrfs subvol 
delete <path>". Pay attention that the deletion is performed not
instantaneous
but in background. In fact even though the subvolume disappear instantaneous 
the space is freed during the background removing.

3) If you have a subvolume into another one, and you snapshot the latter, in 
the snapshot you cannot see the nested subvolume. 
For example you have the root (''/'') under a subvolume and the
/home under
another subvolume. If you snapshot the root, this snapshot will not contain 
the subvolume /home. This may be useful if you want restore an old snapshot of 
the root filesystem without affecting the users homes directories.

In the future some attributes (raid mode, compression) may be set per-
subvolume basis.

Regards
G.Baroncelli


[*] The btrfsctl utility doesn''t return an error when you snapshot a 
directory. But instead of snapshotting the directory you get a snapshot of the 
subvolume which contain the directory.
> 
> -- 
> Francis Galiegue, fgaliegue@gmail.com
> "It seems obvious [...] that at least some ''business
intelligence''
> tools invest so much intelligence on the business side that they have
> nothing left for generating SQL queries" (Stéphane Faroult, in
"The
> Art of SQL", ISBN 0-596-00894-5)
> --
> To unsubscribe from this list: send the line "unsubscribe
linux-btrfs" in
> the body of a message to majordomo@vger.kernel.org
> More majordomo info at  http://vger.kernel.org/majordomo-info.html
> 

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On Thu, Oct 7, 2010 at 6:39 AM, Francis Galiegue <fgaliegue@gmail.com>
wrote:
> While I fully understand (and use) the purpose of snapshots, I
don''t
> quite fathom the use case for subvolumes, apart from btrfs-convert...
> Why has btrfs grown such a feature in the first place? Can someone
> give me a use case for them?
I''m new here; I trust that someone will correct me if wrong:

As I understand it, since snapshotting works on volumes, having
subvolumes allows a smaller thing that you can take a snapshot of. A
use case? One could give each user on a multi-user system their own
subvolume rather than their own directory, under /home/... and then
take snapshots of these home directories to implement regular backups
(1)without duplicating unchanged files and (2) with independence
between the users.

As to why they exist, I understand that they began as an
implementation detail of snapshots, rather than their creation having
been driven by the needs of a particular use case, and one could
legitimately criticize currently offered use cases (such as the one
above) as contrived.

I believe it is fair to consider a new subvolume as equivalent of a
snapshot of an empty file system.
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On Thu, Oct 7, 2010 at 8:18 AM, Mike Hommey <mh@glandium.org>
wrote:
> BTW, it would be very useful to be able to turn existing directories
> into subvolumes.
does a (link,unlink) move work across subvolumes?
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On Thursday, 07 October, 2010, David Nicol wrote:
> On Thu, Oct 7, 2010 at 8:18 AM, Mike Hommey <mh@glandium.org> wrote:
> > BTW, it would be very useful to be able to turn existing directories
> > into subvolumes.
> 
> does a (link,unlink) move work across subvolumes?
The link across subvolumes is not allowable.  In the beginning it was 
possible, but that was source of bugs. See the thread "Hard link across 
subvolumes"

http://www.mail-archive.com/linux-btrfs@vger.kernel.org/msg03286.html

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If I were to use the defrag option in btrfsctl,
$ btrfsctl -d /
would it also defragment the subvolumes under the root?
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On Thu, Oct 07, 2010 at 11:13:18PM +0200, Goffredo Baroncelli
wrote:
> On Thursday, 07 October, 2010, David Nicol wrote:
> > On Thu, Oct 7, 2010 at 8:18 AM, Mike Hommey <mh@glandium.org>
wrote:
> > > BTW, it would be very useful to be able to turn existing
directories
> > > into subvolumes.
> > 
> > does a (link,unlink) move work across subvolumes?
> 
> The link across subvolumes is not allowable.  In the beginning it was 
> possible, but that was source of bugs. See the thread "Hard link
across
> subvolumes"
> 
> http://www.mail-archive.com/linux-btrfs@vger.kernel.org/msg03286.html
But couldn''t cp --reflink be made to work?

Mike
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On Saturday, 09 October, 2010, you (Mike Hommey) wrote:
> On Thu, Oct 07, 2010 at 11:13:18PM +0200, Goffredo Baroncelli wrote:
> > On Thursday, 07 October, 2010, David Nicol wrote:
> > > On Thu, Oct 7, 2010 at 8:18 AM, Mike Hommey
<mh@glandium.org> wrote:
> > > > BTW, it would be very useful to be able to turn existing
directories
> > > > into subvolumes.
> > > 
> > > does a (link,unlink) move work across subvolumes?
> > 
> > The link across subvolumes is not allowable.  In the beginning it was 
> > possible, but that was source of bugs. See the thread "Hard link
across
> > subvolumes"
> > 
> > http://www.mail-archive.com/linux-btrfs@vger.kernel.org/msg03286.html
> 
> But couldn''t cp --reflink be made to work?
> 
I don''t know why, but I can confirm that in the btrfs kernel source it
is
checked the case that the "reflink" is preformed between two
subvolume. And in
this case this action is denied.

http://git.kernel.org/?p=linux/kernel/git/mason/btrfs-
unstable.git;a=blob;f=fs/btrfs/ioctl.c;h=9254b3d58dbef22974af3c7c61dbc9a4af7a33b6;hb=HEAD#l1489

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